TPMS is not 100% trustworthy

There is some previous posts here at this forum about the "Tire Pressure Monitor System" (TPMS) that GM added to its vehicle since 2008 or earlier. My first experience with TPMS was in a rented 2009 Chevy Impala that was reporting 26 PSI for the rear left tire (driver side) and 28 PSi for the others. I bought a mechanical tire gage and measured the air pressure in all four wheels, and none matched the TPMS readings on the dashboard display. The rear left tire had 29 PSI, and the others had 30 PSI, so I decided to add a bit of air pressure to the rear left until it was 31 PSI. The TPMS reading stayed at 26 and never went up. Since I trusted the gage, I ignores the TPMS readings. When I returned the Impala, I reported the problem, and the renter took the report and probably sent the Impala to its service area to fix it.

Now I have a 2009 Chevy Equinox (same year as the Impala), and its TPMS readings are always off by one or two PSI. So only trust the TPMS readings if the PSi drops quickly from one day to another. I prefer to read the pressures with the gage and check them weekly, then add pressure if the reading drops by 2 PSI. TPMS isn't trustworthy for exact pressure readings. I can only trust it as a warning for any prssure loss that is more than 2 PSI in any of the tires.

The Volt Owners Manual has a procedure to reset the TPMS reading per tire, but if that doesn't work, get your vehicle back to the dealer.

 

I have the same issue with my Volt and just accept the limitations of the technology and use a manual tire gauge I use on all of my older vehicles anyway. I use the Volt TPMS to note sudden changes only which would mean I have more pressing problems at that point then just the accuracy of the little buggers.

 

Has anyone figured out the best tire pressure to use? Mine came with 38 and I kept it there but when my car was at the dealer getting a blown tire replaced, they dropped them all to 34. I know too high gets better mileage, but it also wears the tires faster.

 

I'm running 41 psi now (by an accurate gage) and it gives better mileage. I'm watching the front tires in particular for any signs of uneven wear, and this seems to be about the right pressure for them to wear "flat" across the tire - no preference for center or edge wear that I can see so far - it takes awhile for symptoms of uneven wear to show up.

I've also tried dropping to the low 30's to see if the car rode smoother or got quieter. Not so you'd notice, and the mileage went down very much so you'd notice.

 

I don't know if the TPMS sensors are battery-powered or via induction, but when the car has not moved in a while the readings are off. As far as I know the sensors do not transmit when the wheels are not moving. The OnStar monthly diagnostics was taken when my Volt was just sitting and the tire readings are all different (38, 39, 40, 41 psi) even though my pencil gauge shows them to be all the same. For people with OCD it is really hard to look at these.

I suppose when the TPMS sensors are added to the computer there could be a calibration factor somewhere. Have you asked a service adviser?

 

I refilled with air this weekend. I noticed that the readings were about 2 psi below what my (I think reliable) gauge said on all tires. I filled them to what read as 37 on the TPMS, but 39 on my gauge.

Also, the sensor on the TPMS does read while the car is not in motion. (Though maybe not as accurate), cuz as I added air, the TMPS reading did increase.