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2009 Sienna LE
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I have some experience with the OBDLink MX+ and TPMS. Here is what I can tell you.

From my readings comparing the MX+ with my tire pressure gauge, the MX+ reads about 13psi high. So if it's reading 55psi, that tire would be around 42psi. When the sensors are not reading (i.e. just turned van on), they report 14.5psi if I recall correctly.

From my experience, if I turn the van ON (engine not running) and start up the OBDLink app, the TPMS values will change from 14.5psi to an actual reading within 1-2 minutes, without starting or moving the van. At least that was the case with the sensors in my van previously (I assume they were Toyota sensors). I just changed out all 4 sensors for Autel MX-Sensor. It remains to be seen if the Autel's are any different.
 

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2009 Sienna LE
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291 Posts
I would be interested to see what happens if you set the tire pressures differently, e.g. 35psi, 37psi, 39psi, and 41psi. I did something like that at one point to find out which tire was which so I could display them accurately in my phone app. That reminds me; I need to re-do that now that I have new TPMS sensors...
 

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2009 Sienna LE
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291 Posts
I suspect it's exactly a 14.7 psi difference, and the pressure read from the MX+ is displaying psia, not psig, as 0 psia = -14.7 psig, and 0 psig = 14.7 psia. When most people say "psi" they are referring to "psig", or gauge pressure, which is the pressure difference from atmosphere to what you are reading. Psia, or absolute pressure, is offset by 14.7 psi because it's the pressure relative to a vacuum, and standard atmospheric pressure at sea level is 14.7 psia.
Brilliant! Thanks for the info. I have an Autel TS-508, but I hadn't thought to compare its readings with the OBDLink MX+. I'll bet I will see the same as you. I will do that some time and report back.
 
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