CP911

Hi all,

Is it normal for the TPMS to display a reading that is a few PSI below what is measured by a mechanical pressure gauge? I have a 2007 C2S and it has been doing this since I bought it a few months ago. I thought it was the likely aging TPMS sensors, but I just replaced the wheels with new ones that had newly manufactured sensors, so that isn't it.

The temperature outside really dropped today into the low 40s and I thought my reading of 31psi/fronts and 39psi/rears was low as a result of that, but when I got home from work I measured 37psi/fronts and 44psi/rears, which is what I filled them to the other day.

The low tire pressure warning light never came on. It just seems to read consistently low and this bothers me a bit. The mechanical gauge I used has always been consistent what with our other cars display in their respective TPMS readouts. I believe it to be calibrated and working just fine.

Anyone else experience this and is there a remedy?

ogun228

Tires were warm when you got home. The colder, the lower the PSI will read when cold. After warmup it comes back to speck or close to it.

DC911S

Mine always read three pounds low compared to the hand gauge

ecostellodo

Mine too.

granprixweiss928

mine too

Supermode

Had the same problem a month ago. TPMS alerted my left rear psi was low but it wasnt. I reseted it and it went away. My PPI report said my TPMS had 84 months of life left so I knew it wasnt right..

OKB

when its normally hot, 85 deg, my sensors read exactly what the pressure machine is putting into them. I was in freezing weather and the pressure monitor went off, low press. the tires still had the 36lbs but said 24lbs. As I drove it went back normal. When i got back to 80 weather it was back at 40lbs. Never put air in or out.

Hella-Buggin'

Mine always read about 2 lbs lower than reality.

ru'

https://en.wikipedia.org/wiki/Boyle%27s_law

OKB

I think it has to do with the temp of the sensor housing body changing the pressure reading, rather than the air temp or air press

chuck911

Readings vary because, yes, pressure changes with temperature. But also, none of these are calibrated. TPMS sensors vary, tire pressure gauges vary. None of it matters though because as long as they're not crazy off then people are just going to run whatever pressure makes the car ride and handle the way they want. Which is by the way the correct approach.

So the point of bothering to use a hand-held gauge is not accuracy, but consistency. Even though your gauge may be off, using the same gauge on all wheels they are all off by the same amount, which you adjust anyway based on what you like, so being off doesn't matter.

CP911

Appreciate the input everyone!

This is pretty much exactly how mine behaves. When it's hot outside, the pressure displayed by the TPMS is usually at or +/- 1psi off from what I measure with a hand gauge. Yesterday when it was cold outside it was low by 5-6psi.


I think you are right. This is the most logical explanation I can think of.

3Series

How did whoever did you PPI detemrine the remaining life of your TPMS?

84 months seems like the TPMS is brand new. I'm under the impression these last 5-7 years and like any battery, could last 4 years or 8 years depending on conditions.

CP911

The sensors have a date of manufacture etched on them, so I imagine the only way to "guesstimate" how much life is left would be to take a tire off and check the sensor, but I just don't see many mechanics going through the trouble, even for a PPI. I wonder if it's possible for a mechanic to retrieve information from the DME that tells when the sensors were first learned by the vehicle.

nzskater

It is - the sensors read out battery life remaining. It can also be checked via a Durametric.

My sensors are also out by 2-3psi.