Cybex Sirona M Review

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Cybex Sirona M

Cybex is a name well known to those in search of trendy, unique car seats.  The Cybex Aton and Aton Q are compact, colorful rear facing only car seats, and the Cybex Solution set of booster seats are well-featured, padded, colorful additions to the booster world. And now, Cybex has introduced a convertible car seat to the United States market!  We’ve had the chance to preview the Sirona M at the JPMA Baby Show and the ABC Kids Expo over the years, and now we’re so excited to see it available online and in stores. Let’s take a closer look at the new Sirona M.

CSFTL Quick Stats

  • Rear facing weight range: 5-40 pounds
  • Rear facing height range:  1 inch below the top of the headrest (see “Rear facing headroom” section for clarification on this requirement)
  • Forward facing weight range: 22-65 pounds
  • Forward facing height range: 49 inches or less
  • Shell height: 25 inches to top of fully extended headrest
  • Lowest harness position: 7.5  inches with insert (approximate based on contours of the insert)
  • Highest harness position: 17 inches
  • Lower anchor weight limit: 40 pounds forward facing
  • Expiration: 8 years


  • Width at widest point: approximately 21 inches at the top
  • Width at seat pan: 18 inches
  • Crotch buckle slots depth: 4, 5.5, 7 inches
  • Weight of car seat: 24.5 pounds


SensorSafe 2.0 — App Now Included!

Cybex Sensorsafe App

We’ve seen SensorSafe on some Evenflo car seats — it’s a chest clip and receiver designed to alert parents to the presence of children in the vehicle. Now, SensorSafe 2.0 comes to Cybex (Cybex and Evenflo are owned by the same corporate parent, Goodbaby International).  The 2.0 indicates that the SensorSafe on the Sirona M includes an app! We’ll take a closer look at this feature in our special SensorSafe section below.

Adjustable Linear Side Impact Protection System

Cybex Sirona Linear Side Impact Protection

Cybex Sirona M Linear Side Impact Protection

Like the Aton rear facing only seats, the Sirona M features Linear Side Impact Protection that should be extended on the side nearest to the vehicle’s door.

No Rethread Harness

The Sirona M features a no rethread harness with a 12-position headrest, which makes it easy to adjust the harness height. We’ll explain more about this later in the review.

Magnetic Buckle Tongue Holders

The Sirona M features magnetic buckle tongue holders to secure the buckle, to make it easier to put the child into the seat.

Cybex Sirona M Magnetic Buckle Tongue Holder


Cybex Sirona M Cupholder

The Sirona M comes with a cupholder which can be attached to the side of the seat. The cupholder has flexible rubber tips inside to better grip the cup.


Angle Adjustment and Its Challenges

My Cybex Sirona M install cheat sheet

The Sirona M features 10 available recline settings. Reclines 1 through 6 may be used rear facing, and reclines 5 through 10 may be used forward facing.

Unfortunately, these rules are not indicated on the car seat itself.  We mentioned this to Cybex, who acknowledged the feedback. We would urge them to indicate on the seat that only certain reclines may be used forward and rear facing, as the current design has great potential for misuse.  When I was installing the seat, I wrote the allowed reclines on a sticky note so I wouldn’t forget,  but we’d definitely prefer to see them on a label on the car seat.

Adjusting the Recline

Cybex Sirona M Recline indicator

To recline the Sirona M, pull a handle located under the front of the seat. The recline indicator can be very difficult to see. It’s a small triangle etched under the recline level ball.  It’s easiest to see at eye level but difficult to see when standing over the seat.

Cybex Sirona M Level Ball

A clearly marked level ball indicates when the seat is at the proper angle for rear and forward facing. There is a level ball on each side of the seat, one with English writing and one with Spanish.

Cybex allows the use of pool noodle(s) with the Sirona M if needed to recline it further, as long as the level ball is in the acceptable range of course.

The Sirona M does allow a recline forward facing, which may make forward facing children more comfortable if they fall asleep in the car, although head slump is not a concern at that age.

See more about the limitations of that feature below.

Installation: Rear Facing

Lower Anchors

Cybex Sirona M lower anchor connector

The Sirona M includes lower anchor connectors that are also on some other Evenflo car seats.  These are narrow and have a red loop to pull to detach the connector.

Cybex Sirona M fresh out of the box

The Sirona M is delivered with the lower anchor webbing sticking out of the belt path and ready for use.  That invites caregivers to try this installation method first.

Cybex Sirona M – Pull lower anchor strap through the seat to tighten

To install with the lower anchors, hook on these connectors and then pull the straps tight on both sides. The easiest way to pull the strap is to separate the cover and pull towards you across the seat.

Cybex Sirona M – tight lower anchor strap but still side to side movement

We ran into a problem, however, in many of the vehicles we used for installation. When all of the slack was removed from the lower anchor webbing on both sides of the webbing, the Sirona M could still move more than an inch side to side at the belt path.

The space between the car seat and the lower anchor was entirely filled with the lower anchor connector and adjuster. The lower anchor webbing itself is what provides the tension and tightness, but the webbing was pulled tight within the car seat.

In some vehicles, the recline of the Sirona M affected this. The recline determines where the belt path is located in relation to the lower anchor.

Changing the recline sometimes made it possible to get the seat tighter as more of the lower anchor webbing was exposed. In some cars, the use of latch guides (not included with the seat but available free from Cybex) helped stabilize the installation.

However, we have great news. We shared this feedback with Cybex, and they will be making a change to the lower anchor connectors and adjusters to correct this issue! They don’t have an exact date that this will be completed, but we will be sure to keep you posted!

Pro Tip

When removing the car seat, be sure to angle your hand away from the seat when pulling the red loop to release each lower anchor connector. The force when the connector releases can propel your hand into the side of the seat very easily. Take it from someone who stocks up on Band-Aids when she sees these types of lower anchor connectors!

Lower Anchor Connector Storage

Cybex Sirona M lower anchor connector storage

The Sirona M has a unique place to store the lower anchor connetors if the vehicle seat belt installation is used. Raise the headrest all the way and move the strap to the forward facing belt path. Insert each connector into the seat and hook it onto the storage hooks.

Vehicle Seat Belt

I’ll be honest — I had an extremely hard time installing this seat with the vehicle seat belt, as did another member of our team who tried to install the Sirona M in her vehicle.

First of all, a word about the manual. The manual mentions using the lockoff in one sentence after the instructions on how to route the belt through the belt path. We wish the manual had given more detail regarding this part of the installation, because it’s crucial to a correct installation yet very difficult to accomplish.

To install rear facing with the vehicle seat belt, route the seatbelt through the rear facing belt path and buckle. Then secure the shoulder portion of the vehicle seat belt through the lockoff on the side opposite the buckle.

Cybex Sirona M rear facing vehicle seat belt installation

After many attempts, I was finally able to get the vehicle seat belt into the lockoff. The motion that I used can best be described as “dental flossing.” I moved the seatbelt left and right while applying pressure to the lockoff opening. Once the seat belt was in the lockoff, I found it easy to pull the shoulder belt to tighten the installation, and I liked the one-way aspect of the lockoff.  A vehicle seat belt routed through the lockoff can get tighter but not looser.

We found that the lockoff did get easier to use after a few installations, because the lockoff got a tiny bit wider from previous use.

Note that as shown in the seatbelt installation photo above, Cybex has specifically approved the crotch buckle routing between the lap and shoulder belt, even though it’s not clear in the manual whether this routing may be used.

Crotch Buckle and the Vehicle Seat Belt

Cybex Sirona M rear facing

Make sure to arrange the vehicle seat belt around the crotch buckle slot that is in use.  Otherwise, the crotch buckle strap will be covered by the tight vehicle seat belt. Cybex has confirmed that arranging the vehicle seat belt around the crotch buckle slot is acceptable.

Installation: Forward Facing

Lower Anchors

Moving the lower anchors from the rear facing belt path to the forward facing belt path was incredibly easy. Simply grasp each lower anchor and slide the webbing to the other belt path.

Installation with the lower anchors forward facing is similar to the rear facing installation. Connect each lower anchor connector, and then pull back the seat pad and pull the lower anchor strap across the seat through the pad.

We had the same problem that we did with the rear facing lower anchor installation. Even when the lower anchor webbing was pulled tight completely within the car seat, we had more than 1 inch of movement side to side since the entire distance between the car seat and the lower anchor was filled with the lower anchor hardware and not the webbing.  Just like the rear facing installation, changing the Sirona’s recline angle sometimes improved this situation.  However, as mentioned above, Cybex will be correcting this issue! Stay tuned for more specifics on when the new version will be available.

Vehicle Seat Belt

Cybex Sirona M forward facing lockoff

The correct method of installation forward facing with the vehicle seat belt is to route the belt through the forward facing belt path and buckle the seat belt. Then, pull back the Sirona M’s cover to expose the forward facing lockoff. Pull the shoulder portion of the vehicle seat belt on the side opposite of the buckle through the lockoff.   Finally, pull the shoulder belt towards the retractor to tighten. The lockoff is a one-way lockoff, meaning that the belt will only pull in one direction but then will remain tight.

Cybex Sirona M forward facing vehicle seat belt installation

After I tried many, many times, I was finally able to get the vehicle seat belt into the forward facing lockoff. Cybex suggested angling the vehicle seat belt to enter the lockoff at a 45 degree angle. While this worked rear facing after considerable effort, it was even more difficult forward facing since one hand is inside the car seat and one hand is outside the car seat, and you need to wiggle the belt upwards while pulling it up.  A third hand would have made this a bit easier!

Cybex does not allow “bypassing the lockoff,” or not using the lockoff and locking the vehicle seat belt instead. The lockoff must be used with all lap-shoulder vehicle seat belt installations.

Lap Only Vehicle Seat Belt

To install the Sirona M with a lap only seat belt, route the belt under the seat cover and through the correct belt path. Tighten the lap belt. The lockoffs will not be used for a lap only seat belt installation.

Top Tether

The use of the top tether is required forward facing on the Sirona M (and always recommended on any forward facing seat), whether the vehicle seat belt or lower anchors are used. The Sirona M’s tether is easy to use and adjust and did not give us any difficulty.


Cybex Sirona M forward facing with overhang

Cybex, like many manufacturers, allows 20% of the bottom of the car seat to extend over the edge of the vehicle seat. We found that in vehicles with either a very upright vehicle seatback or a shallow seat pan, or both, the Sirona M had 20% overhang in the most upright position. If the Sirona M were reclined forward facing in these vehicles, the overhang would become greater than 20% and unacceptable. In some vehicles with very shallow seat pans, such as trucks, the Sirona M may not fit at all forward facing.



Cybex Sirona M headrest adjustment

The Sirona M features a no rethread harness. To move the harness straps up and down, pull up on a horizontal plastic bar behind the headrest. This is somewhat more difficult than it should be due to the fact that the backbone of the seat runs vertically where you should be pulling up on the plastic bar. I found it easiest to use two fingers spaced around the middle vertical pole.

Uneven Straps

Cybex Sirona M – uneven straps

Because this seat does not have a splitter plate, at times we found that the straps were uneven, as the stitched area joining the straps would get caught within the seat. Lifting the headrest all the way and then extending the straps all the way usually reset things.


Cybex SensorSafe receiver in OBDII port

The SensorSafe technology consists of a receiver that plugs into the vehicle’s OBDII port. The vehicle owners manual should give the location of this port in your specific vehicle. This receiver interacts with the chest clip of the car seat.

Cybex SensorSafe Warning

The receiver will sound an initial chime when the car is started (only once). After the car has been driven at least 5 mph, if the chest clip is buckled, a series of tones will play when the car is turned off. This is intended to remind the caregiver that a child is in the seat.

In addition, if the chest clip is unbuckled while the vehicle is on after it has been driven at least 5 mph, a series of tones will play.

The “2.0” in “SensorSafe 2.0” denotes the presence of a new app. This is designed to work in conjunction with the receiver and chest clip, to display alerts on the phone along with the chimes in the car. The app also displays the temperature in the car. The app was easy and intuitive to set up, though some parts, such as the Sensorsafe FAQ, are still being written.

I am also the owner of an Apple Watch, so alerts from my phone will display there as well.

Cybex SensorSafe Apple watch warning

I was blown away by how well all of this worked. I tested it (without a child in the car seat, of course!) and was given continual alerts on my watch, or my phone if the watch was not in use. We give Cybex a huge round of applause for this initiative. If it saves even one life, of course it’s worth it. We hope it saves many.

I did find one small glitch. When a child is removed from the car seat intentionally while the car is still running, and the driver does not intend to leave the car at that time, the app doesn’t quite know what to do.

For example, if my husband is home when I get my kids from school, sometimes he will come down to the curb and take them out of the car, and then I will continue on to run some errands. This would also come into play at some school dropoffs where a school staff member removes the child from the car. In this case, the alarm will keep going off because the chest clip is open. If the adult removing the child closes the chest clip, then alerts will go off when the driver eventually leaves the car. We mentioned this to Cybex, who acknowledged the feedback and mentioned updating the app to add some sort of “acknowledge” button to the alerts to shut them off.

As they said, it’s easier to update an app than a car seat!

Fit to Child

Crotch Buckle

Cybex Sirona M Crotch Buckle Adjustment

The Sirona M crotch buckle is unique in that as you move it outward, the strap lengthens. I am guessing that the first slot will virtually never be used, as most babies who would need the slot would have the infant insert in, and the second slot would then be in use (see below). I found it relatively easy to move outward, though moving it back in took some dexterity.

The crotch strap should be as close to the child as possible but not under. However, if reclines 5-7 are used forward facing, then it should be moved one slot further away from the child if available. In general we prefer that seats not have complex rules for the crotch strap, as it often leads to misuse. At the very least, we’d like to see the rule indicated somewhere on the seat near the crotch buckle.

Infant Insert

The infant insert is a plush, padded satin-covered insert which can be used for children from 5 to 11 pounds. I found that when the insert was in use, the crotch buckle must be in the second position. The crotch buckle was just too short in the first position with the insert, even on our 5 pound model.

When the headrest is on the lower positions, the seat pad behind the baby is folded in half. This helps support the back and prevent head slump when the child has outgrown the insert.

Strap Covers

Cybex Sirona M Strap Covers

The Sirona M strap covers are required for use with the seat. They are attached to each other through the seat by a strap that runs behind the seat cover. A friend compared this to the strap that connects a child’s mittens through a coat! As the strap covers are connected in this way, the only way to loosen the straps is by pushing the button to loosen and pulling the part of the strap that is not within the strap cover. This is not intuitive, and it took me a few tries to realize that the straps were not stuck.

Fit to Child

Rear Facing


Cybex Sirona M Newborn fit

The fit on our 5 pound, 19 inch model was, in a word, superb. In fact I would be hard-pressed to name another convertible car seat that fits a newborn as well. Although the lowest strap height is higher than we expect to see for newborns, the copious padding and contours of the insert help the seat fit correctly. The only quirk was that, as mentioned above, there is a very tiny area where one must pull to loosen the straps.

10 Months Old

Cybex Sirona M 10 month old fit

This model is 10 months old, weighs 20 pounds and is 30 inches tall. She fits very well in the Sirona M overall, though the short length of the crotch buckle means that the sides dig into her thighs if she puts her legs down.

Cybex Sirona M crotch buckle fit

This is the correct slot for her as per the manual. The headrest pushed her head forward slightly, but she has excellent muscle tone. This may be an issue, however, for infants who have outgrown the insert and folded pad (see above) but still do not have good head or torso control.

11 Months Old

Cybex Sirona M 11 month fit

This kiddo is 11 months old.  She weighs 22 pounds and is 27 inches tall. She fit very well in the Sirona M. The pad behind the crotch buckle shifted a bit, but Cybex confirmed for us that this is fine. That pad is optional and can be removed if needed.

14 Months Old

Cybex Sirona M toddler fit

At the time of this picture, my son was 14 months old, 19 pounds and 29 inches. He is very comfortable in the Sirona M, and it fits him well.

16 Months Old

Cybex Sirona M 16 month old fit

Here’s my son today, newly 16 months old, 20 pounds and and 29.5 inches. He still fits well in the Sirona M and seems very comfortable!

2 Years Old

Cybex Sirona M 2 year old fit

This young man is almost 3 years old and still rear facing. He weighs 25 pounds and is 33 inches tall. He fit the Sirona M well (ponytail notwithstanding!) with much room to spare.

3 Years Old

Cybex Sirona M 3 year old rear facing fit

This little is 3 years old, 38.5 pounds and about 40 inches. She will need to turn forward facing soon because of the 40 pound rear facing weight limit, but for now she’s still rear facing! She’s feeling a bit cramped rear facing in the Sirona M.

Rear Facing Headroom

We ran into an issue with some of our older rear facing kids. Since the headrest itself is short, sometimes the straps are at the correct setting for rear facing, but the top of the head is less than an inch from the top of the headrest. Cybex would like parents and caregivers to contact their customer service if this occurs, and they will decide on a case-by-case basis if the fit is acceptable.

Rear Facing Legroom

Cybex Sirona M rear facing legroom

Lack of rear facing legroom is not a safety concern, but we know parents and caregivers would like children to be as comfortable as possible in the car seat. We put our 16 month Huggable Images doll into the Sirona M rear facing to check the legroom.  When the Sirona M was at full recline, the doll was a bit short on legroom, which means that if the Sirona M were more upright, there would be less legroom. Again, this does not impact safety whatsoever, but we know it’s something parents and caregivers look for.

Fit to Child

Forward Facing

3 Years Old

Cybex Sirona M 3 year old forward facing fit

This is my 3 year old son, as he would sit if he were forward facing in the seat.  He’s 35 pounds and 39 inches. He found it quite comfortable!

5 Years Old

Cybex Sirona M 5 year old fit

This is my daughter. She’s 5 1/2 years old, 32 pounds and 42 inches. She fits with room to spare, but she kept complaining about the headrest. Her usual car seat, the Clek Fllo, has a more prominent headrest, so I found this interesting.  I think perhaps the “C” shape of the Sirona M makes her feel the headrest more.

Cybex Sirona M 5 year old fit

I was hoping that her experience would improve when we got in the car, but there she kept complaining about the strap covers. The rubber on the back seemed to bother her. This might be a consideration for some children as the strap covers are required.

The 17 inch highest harness position means that a child wearing a size 6 shirt or larger will probably have outgrown this seat by torso height.

Important Information — Where to Find

Cybex Sirona M expiration date label

Expiration: The Sirona M expires 8 years after the date of manufacture. The date of manufacture and expiration can be found on a label on the underside of the seat.

Cybex Sirona M FAA approval label

FAA Approval: The FAA approval can be found on a label on the underside of the seat. This location is a bit unwieldy if one has to show it at the gate. The Sirona M’s chest clip must be open for takeoff and landing because it is an electronic device. Cybex has confirmed that the seat is safe to use in this manner, for takeoff and landing only.

Cybex Sirona M manual storage

Manual storage: The manual is stored in a pocket on the back of the seat. We were very happy to see a card attached to the seat on delivery giving the manual location, as the caregiver may not find it otherwise.

Overall Thoughts

The Sirona M is a stylish addition to the Cybex lineup. The fit to children is excellent, and the SensorSafe technology and app is an exciting new frontier in child passenger safety. However, it is very hard for us to recommend a seat with such a difficult, and in some cases impossible, installation. At the least, we would suggest trying to install the Sirona M in your vehicle before purchasing it. We are thrilled that Cybex is using this feedback to make changes, and we look forward to taking another look at this seat when the lower anchor installation is improved!
Originally written by Lani Harrison, edits maintained by CSFTL