I encountered an odd problem while driving to work two days ago. Early in my trip I had made a quick stop when someone ran through a stop sign in my neighborhood. The stop was sudden enough to cause the ABS to kick in. Although it didn't see it at the time, the sudden stop caused the cell phone that I placed on the passenger seat to slide forward until it was roughly in the middle of the seat. (I usually leave it on the seat, but I lay it next to the seatback.)

Later in my trip I stopped fairly quickly for traffic ahead of me and the SRS/Drive-to-Workshop warning displayed on the dash. It went away when I started up again, but it turned on and off a few times during the final 10 minutes of my commute. I also noticed that yellow light in the middle of the dash that indicates the status of the passenger airbag was flickering.

When I removed the cell phone from the seat, the seat occupancy light stopped flickering and SRS warning light stayed off. I searched the Internet and found a couple of postings on forums for non-Mercedes vehicles that indicated that a cell phone may affect the occupancy sensor. Here's an example on an Audi forum.

I kept my cell phone in the door pocket for my trip home yesterday and for both stages of my commute today. The SRS warning stayed off and the seat occupancy light didn't flicker. I had been contemplating a weekend spent with a multimeter hunting for a short in the wiring. I optimistic that the phone's interaction led to the SRS warning. I much prefer storing my phone in the door pocket or center console to tracking down a wire or sensor problem. I hope this post saves someone else time and/or money.

 

Interesting.

Bit of a dumb question, but would it simply be the weight of the phone we're talking about here? Are you able to trigger the same error if the phone is switched off? (fully off, not sleep mode)

The Audi thread also suggests using the cup holder.

 

Thanks for your comment.

It's possible that the weight of the phone was the culprit and I'm not ruling that out. According to some of the resources I've found on the Internet (and if it is on the Internet, it must be true) cell phones are a significant source of radio frequency energy (RF.) I found some warnings that they can potentially interact with some electronic medical devices such as pacemakers. This type of interference is called electromagnetic interference (EMI). The phones also contain magnets as discussed in this link: iPhone information

Our cars have technology in them that is 20-years old, and I don't think smart phones were as ubiquitous when the car was designed as they are now. Applications like CarPlay and Android Auto didn't exist when the cars were designed. I am almost certain that engineers didn't design the occupancy sensor with the consideration that drivers would lay their cell phone on them. I'm speculating, but I think the magnet or the EMI caused the sensor to rapidly flip back and forth between registering that the seat was occupied and unoccupied. This may have led to an "implausible" stream of data to the SRS system which triggered the warning.

I've driven the car about 150 miles since the incident and haven't had a problem. While I'm tempted to try to recreate the conditions that caused the warning, I decided to let sleeping dogs lie and keep the phone off the seat.

I'm not planning to use the cupholder to hold the phone. One of the flaws of the "facelift" cars is that the chrome finish on a plastic trim ring flakes off fairly easily and makes the cupholder look shabby. Another board member gave me a nice cupholder and I'm not going to let the phone damage it.

 

That just isn't how any of this works.Think Correlation doesn't indicate causation. What's more likely - the electronics and sensors are very old in your car or some new computer chip radio frequency technology has tapped into the SRS sensor system?

 

I should have followed up on Diamond Dave's theory a few months ago. The problem started to occur more frequently and lasted longer each time it occurred. I became concerned that the SRS was actually deactivated for longer and longer stretches. However, my scan of the SRS didn't show any codes. About a month ago, a couple of new symptoms appeared. The volume on the infotainment system (an aftermarket Kenwood unit) would suddenly drop and then the system would not respond to the controls on the steering wheel. The system would also respond erratically to my attempts to skip tracks in Spotify.

This led me to suspect that the interface module I installed to integrate the steering wheel controls with the head unit might be the source of the problem. Or, as Diamond Dave said, "or some new computer chip radio frequency technology has tapped into the SRS sensor system?" I replaced the interface and I have driven about 65 miles without seeing a warning message. The interface I had originally installed was a Crux Model # SWRBM57C. I replaced it with is an Axxess ASWC-1.

Thanks for the replies.