[Desertnate]

I had an odd TMPS alert of the weekend and I wonder if I'm expecting too much of the BMW TPMS compared to less sophisticated systems in other cars I own.

Here is what I experienced:

- This past weekend while out driving, I get a low tirepressure warning. A quick check of iDrive shows its my left front tire, but the actual PSI the car is reading is above the amount recomended on the door jam and normal for the current ambient temps and the fact I'd been driving for a while.

- The other front tire was actually showing a LOWER PSI than the one causing the alert, but again not outside normal conditions and withing specs

- The rest of the day the reading didn't change while driving, so it didn't indicate a slow leak

- I have not adjusted the air pressure in any of the tires for roughly two weeks.

- When I checked my tire cold PSI it was exactly the same as I set it two weeks prior.

- Temperatures are unseasonbly hot, so a cold snap can't be blamed for having low pressures.

Does TPMS not use the PSI readings it's getting from the sensor? In my case the tire thowing the alarm was showing 39 on the dispay and the other front tire was showing 38. Cold they are set to 35 (what is shown on the door jam), so the higher reading was above spec and not uncommon for the conditions

If any tire would have thrown an alarm, I would have expected the one with the lower pressure to cause the alarm, not the higher one.

I verified the pressure on all four tires while cold and reset the system to clear the alert. At this point I'm just trying to figure out why the alert was tripped in the first place since nothing was out spec.

[visionaut]

I suspect a battery going bad in the ‘identified’ tire sensor. Like it’s losing comm with the car. Might explain why it’s pressure didn’t change too while driving. What year are the TPMS from?

[Desertnate]

Originally Posted by visionaut

I suspect a battery going bad in the ‘identified’ tire sensor. Like it’s losing comm with the car. Might explain why it’s pressure didn’t change too while driving. What year are the TPMS from?

The car is a 2016 and a lease turn in I bought CPO. I hadn't thought about the sensor battery. How long are they expected to last? I assume you have to replace the entire sensor and not simply do a battery swap.

I may not have been too clear. The pressure reading for the "bad" tire did change, but it only showed the normal increase you'd see from the tire warming up. At the time the warning first came on, I thought there might be a slow leak and the decrease triggered the warning, but the pressure held steady on the display. I later verified the tire was exacty at the correct pressure, so a slow leak wasn' the cause.

[visionaut]

TPMS batteries can start failing at 3-4 years (depending on environment), though can also go to 10 years... Yeah, it would be a whole sensor change.

It’s my understanding the TPM system triggers a low pressure warning when it senses a drop in pressure from the original pressure set at TPMS reset. It’s not absolute (like it’s below the door plaque tire pressure) but a delta from ‘original pressure to current pressure’. If I reset with with a tire at 36psi, it might trigger at 33 - but if I reset at 33 then it may not trigger until 30...

If all’s fine, and you did a reset with your proper pressures, and you’re getting readings from all tires now, maybe it was just a hiccup, and it won’t happen again?

[Desertnate]

Thanks for the info on the battery life. All of my previous TMPS equipped cars didn't use in-wheel sensors, so this is something new I'll have to remember. I just hope one doesn't die before I have to replace the tires.

Since the pressure change was a mild increase and not a decrease, I'm hoping it was a odd fault in the TMPS and it won't happen again.