OK so this is my first car with a TPMS, I'm coming out of the stone age so be easy on me

My tire pressures were getting low according to the display screen in the car. They were reading around 31 PSI on each tire, with no warning light but the original pressure when I bought the car was 34 PSI so I decided to top off.
I used a digital JACO tire gauge to measure tire pressures. It confirmed the readings were close to 31 PSI on each tire before I started filling, which was in excellent agreement with the car's display.
I filled to 35 PSI per Lexus door sticker as measured by the Jaco Gauge.
I've run the car for several trips and the display consistency shows 32 PSI on each tire, ambient temperature is the same as when I filled them.

According to my owner's manual I should not have to reinitialize simply because I added air, and I have no idea what they mean by "Initialize the tire pressure warning system when the tire inflation pressure is changed when changing traveling speed".
According to Lexus website I should initialize the TPMS any time I add air.

1. Why does the car display reading now disagree with the Jaco Digital Gauge when they initially agreed? The tires needed a lot of air to reach 35 PSI on the Gauge but the car's display readings barely budged.

2. Why should I have to initialize TPMS simply because I added air and why is there a discrepancy between owner's manual and Lexus website?

Owner's Manual:


Lexus Website:

 

Could you post the specific URL of the Lexus website webpage where you found that?

There might be something in the surrounding context that may explain the discrepancy between the Owner's Manual and the you quoted Lexus website directions.

 

The sentences are confusing as f***.
and
What they really mean is that whenever you want to change the value of the normal tire pressure, you need to do the initialization to tell the system about this new value.

If you are going to do a lot of high speed driving --> add more pressure to your tires, teach the system this is your new desired pressure.
If you use different tires that requires a different pressure --> set them at that pressure, teach the system this is your new desired pressure.

There's a sentence explaining that:
You don't need to reinitialize when you just add air without changing the value of your target normal pressure.

• Now back to your issue: is this tire pressure discrepancy occuring when the tires are cold?
• Did you complete a tpms initialization? --> it wouldn't explain the wrong reading, but you might have programmed the system to a new desired normal pressure.
• Any chance you are far above sea level?

 

This error is probably because someone read the user manual, didn't had a clue what they meant, wrote something vaguely reassembling on the website.


 

https://www.iralexus.com/lexus-tire-...et-danvers-ma/

 

• Now back to your issue: is this tire pressure discrepancy occuring when the tires are cold?

Yes, I started the car after it had been sitting for more than 24 hours and the car display read 32 PSI on three tires and one was 31 PSI. The car had been driven several times before this and read pretty close to 32 PSI for each tire while driving. The ambient temperature when I checked it after sitting for 24 hours was about the same (45º F) as when I filled it. My underground garage's temperature hardly varies much during the Winter.

• Did you complete a tpms initialization? --> it wouldn't explain the wrong reading, but you might have programmed the system to a new desired normal pressure.

No, since there was conflicting guidance between the Lexus website and the Owner's manual, I did not perform the initialization. I also watched a Utube video on correcting a low warning TPMS light (which I never had) and it did NOT mention performing initialization.

• Any chance you are far above sea level

My sea level according to apple compass is 150 ft Elevation. This makes sense, I am not far from the Atlantic Ocean, can bike there!


All I did was add air, without performing initialization. I expected the Car's screen readings to agree with my JACO gauge after filling the tires, just as it did immediately before filling them. I am positive the tires took a lot of air based on knowing my compressor and the sound of the tire inflating. I also watched the compressor's gauge drop as the tires were filling. There is no way I only added 1 PSI in each tire as displayed by the car's screen.

I believe I added 4 PSI in each tire but the car display tells me I added 1 PSI in each tire.
I'm glad I'm not the only one that found the owner's manual confusing regarding "Initialize the tire pressure warning system when the tire inflation pressure is changed when changing traveling speed"
I think tomorrow I will double check the tire pressure with my Jaco gauge, but like I said I really think I inflated each tire to 35 PSI based on the compressor gauge, Jaco Gauge and sound of the tires filling.

 

Could it be that it's colder outside than where you inflated the tires?
I am imagining a warmish garage and snow/freezing outside. It could be that you put air that's warmer and then when you go outside, instead of the usual heating, you actually cool the tires?!
Since TPMS needs a couple minutes of driving to read the pressure.

 

We've been in a reasonably warmer trend the past week (encompassing the time period when I filled the tires), temps have been in the low to mid 30's out, The garage stays in the low to mid 40's pretty much all winter so not much temp difference garage vs. outdoors. I filled the tires in a 45 degree garage where the Jaco measured 35 PSI during the filling, and I just checked them again tonight with the car display after sitting 24+ hours in the garage, reading TPMS 32 psi 3 tires, one reading 31 psi.

I've read the pressures with the TPMS a few times during different drive cycles and after sitting in the garage, not much if any change from 32PSI. I don't drive far each time so the tires don't heat up much.

I'll do some more driving and keep my eye on the display and re-measure the pressure.

 

Also, the pressure when you initialize the tpms is not the pressure when the system alarms; that pressure is about 5-6 psi lower than “set point”. It is normal to see a 1 psi or even 2 psi difference; part of that is due to “round off error” and part may be due to parking with one side or tire in the sun.

You really have to drive a couple of blocks to get a sense of what the car is reading…when pumping up tires I use a pump with electronic pressure gauge the displays and shuts off to 1/10 th of a psi; after doing that I can still see 1 psi difference on the car display. Not to worry.

 

I did not initialize the TPMS system, my understanding is that is not necessary when you simply add air to an under inflated tire.
I have driven the car quite a bit since filling it, and the readings area still 3-4 PSI lower than what I expected after filling it using a digital tire gauge and monitoring with a separate gauge on the compressor which measures declining tank pressure.

It sound to me like the initialization procedure "teaches" the system what specific pressure you have filled, and this is the "benchmark" pressure referred to in the owner's manual.
So the TPMS sensors are actually reading "relative" pressure with respect to the "benchmark" setting provided by the user, as opposed to an "absolute" pressure that was calibrated internally in the sensor.
So you could fill a tire with say 50 PSI and command the system that it was 32 PSI.

 

No, that's not how it works.
The TPMS reads the absolute pressure. The initialization only tells the system what the normal operating pressure is.
It would not convert a 50 psi to 32 psi, but you could tell it "50 psi is my normal pressure, warn me at 40 psi".

What confuses me is that prior to inflating your TPMS and your gauge were agreeing, but they are not anymore.
It would have been very conceivable that the TPMS would read about -1psi and the gauge about +1psi.

Maybe deflate to a lower pressure and see if the TPMS still reads even lower?

 

Hmm Ok so it's an absolute measurement and not relative. I understand.
There must be a step during the initialization procedure which prompts the user to set the pressure value to trigger the warning. This was not clear in the user manual as shown below, there is no instruction to enter a pressure value.

Yes I am confused as well, I was pleased when I saw how perfectly my Jaco gauge matched (within 1 psi) with the initial low pressure readings displayed by the car menu before I filled them. Not so pleased after filling them.
Maybe that is a good suggestion to see if the sensors are tracking with a deflated tire. I assume the car does not have to be driven any distance for the readings to be accurate?

What exactly is the initialization procedure accomplishing? I know there is a separate procedure (not the initialization procedure) for registering sensor codes. The car has to be driven for the system to initialize properly according to the manual. I wonder why? Is the initialization process associating each sensor with a specific tire and what tells the system the correct tire after they are rotated?

 

1. The initialization is registering your current tire pressure as the normal operating pressure. Let's say you inflate to 36 psi, then you initialize: the car now knows 36 psi is your default pressure. And from there it can calculate at what pressure it should warn you that it's too low. I am not exactly sure how, I think it's 80% of the normal pressure - or maybe it's just a constant psi offset. So your issue should not have anything to do with initialization.
2. Yes, when driving, the car recognizes which tire has which sensor.

Also sensors go on sleep mode as much as they can to save battery.
When you do the initialization, the car forces the sensor to wake up so that it can read the value.
In normal condition the sensor is off when the car is not moving.
Possibly another way to force the sensor to wake up is to decrease the pressure in the tire: after all that's the main job of the TPMS, it is supposed to quickly alert the car when the pressure decreases fast (do that with the car on so that it can catch that signal)

Another thing to consider: the car might try to avoid showing small variations. It could be that it's what you are dealing with: the pressure before was 32 psi. Now the sensor reads 35 psi but the car think it's just a transient event that should be filtered out.

 

Another thing is that when you measure the pressure in a tire you are measuring gauge pressure. That is, the pressure with respect to ambient pressure (and why we call it “psig”, “g” for “gauge). The absolute pressure is the gauge pressure plus the ambient pressure, which at sea level is 14.7 psia (absolute). So a tire at 36 psig and sea level has a pressure of 36+14.7=50.7 psia. (note the “a”). If you then take that tire to, say 5,000 feet above sea level, the atmospheric pressure is 12.2 psia so the gauge pressure of the tire will be 50.7-12.2=38.5psig at the same temperature. This info is on the Tirerack website. I live at 3,000 ft above sea level. When I visit family who live on the coast my tires display a lower pressure by a couple of psi ( again when they are at the same temperature).

Of course you have been driving up that mountain so the tire temperatures will also be higher which also increases pressure…by about roughly 1 psi for every 10 deg. The complete explanation can be seen in Wikipedia or many engineering or physics sites.

Probably more than you wanted to know but it helps explain variances in your tire pressures. However, with all of those things affecting pressure readings it goes to show you should not fret over a couple of psi variation from time to time or tire to tire.

 

Thanks for clarifying that.

I looked up the accuracy of my Jaco tire gauge (FloPro) which is specified +/- 1% FS.
FS = 200 PSI so the accuracy is +/- 2 PSI
The Jaco gauge agreed within 1 psi of the car's display for all four tires prior to filling them.
After I filled them by adding 2-3 PSI according to the JACO, the car display only increased by about 1 PSI for each tire which is what confused me.
Perhaps there is a temperature factor I am not accounting for which explains the discrepancy, but I don't think so.