About two months ago, I had a tire replaced because it had a flat that couldn't be repaired. The other tires are a couple years old, but they had about 5/32 of remaining tread so I did not replace them at the time. Immediately after replacing the bad tire, my TPMS light began coming on consistently after about three - five miles of driving. If I reset the TPMS system before driving, the light stays off for the trip, but will come on again next time I drive the car if I forget to do the reset procedure before starting off. I have tried all the standard fixes - I have checked the tire pressure in all four tires, and made sure that I drive at least 5 miles/10 minutes after resetting the TPMS system to make sure it has time to calibrate itself.

The weels have rubber valve stems so I know that I have an early production R56 with an indirect TPMS system (i.e. It runs on wheel speed instead of measuring the actual psi in each tire). As such, there are no TPMS sensors that could be failing. Since the new tire was approximately 3/8 of an inch taller than the older tire on the other side of the axle, I returned to the tire shop to have the tire on the other side of the axle replaced. That did not fix the problem.

I asked the dealer to look at the system when the car was in for service recently. They could find no fault codes in memory when I brought it in, but were able to replicate the issue on a road test. They found that the system reports a 'DSC Undervoltage' fault when the light comes on; but that the computer automatically clears the code when the engine is shut off. They hypothesized that it was the battery, which needed to be replaced anyway because it was six years old. I had them replace it. (Yes, it was expensive; but I got them to give me a good price because they had messed up another repair on the vehicle.) The new battery did not fix the issue.

I have read about various potential causes when multiple lights come on (e.g. When both the TPMS and DSC lights come on, it can be an indication of a faulty steering angle sensor). The odd thing here is that it is only the TPMS light that comes on for me.

My only remaining thought is that the system is thinking that both the front two tires are underinflated (since they are the older ones with 5/32 of tread remaining). I'm hesitant to replace the front tires, though, because that potential cause wouldn't address the DSC undervoltage fault. In addition, I thought that the reason car companies went to direct TPMS systems (that measures actual PSI) was because indirect TPMS systems, like the one on my car, can't detect when two tires on the same axle are simultaneously low on pressure.

Does anyone have any thoughts on what could be causing the problem described? Thanks much!

 

It is possible that having one tire with a smaller diameter will cause the indirect TPMS to think that the other wheel on that axle is low. The indirect system basically compares rotations of the tires to each other, and if it finds "enough" of a difference it will throw the warning light.

I can't think of a good diagnostic on this, frankly. You could try letting some air out of the newer tire, and/or adding more air to the other tires, to try to get the revs/mile to even out. But if the light still comes on, that could mean that you didn't guess right on the pressures, or it could mean that there is still some underlying problem.

Putting another new tire on is a good bit of money, but could solve the problem. Shaving the new tire down to the same tread depth as the old ones could also solve it if my thoughts above are right. Neither one of which is great as simply a test...

 

Letting some air out of the new tires to see if the light stays of is a good idea that I think I'll try. I agree that it could easily generate a false positive if I don't let out the right amount of air, but it's worth a try! Thanks much for the idea.