I've got a 2011 2500 Suburban. After about 20 minutes of highway driving the TMPS warning comes on and the module looses connectivity to the tires.

Most of the time it loses connection to both front wheels at the same time - the sensor was replaced in at least one of the wheels last year.

Sometimes it loses connection to the front as well as one or both of the rear wheels.

Any idea what could be causing it?

 

99% of the time its bad sensors. time to replace the other ones

 

Does that explanation fit with the fact that it's usually after 20 minutes of driving and almost always 2 wheels at the same time, but occasionally 3 wheels and rarely 4. Then the next time it goes back to being fine for 20 minutes or so of driving?

 

it does if the batteries are near the end of life. have the dealer measure the battery life.
the sensors sleep until centrifugal forces wake the sensor.

 

I suspect batteries are depleted as my experience on several vehicles is they last about 5-6 years. BTW, you can find them a lot cheaper on the interweb than the dealer.

 

Is there anyway to confirm that the batteries are depleted without going to the dealership?

 

Not that I know of but they should check then for free. If they don't, then you have the wrong dealer.

 

I just noticed that I have metal valve stems on the front tires and rubber on the rear. Could that be effecting anything? I thought I read somewhere that metal valve caps cause problems for a TMPS system - presumably metal stems would as well?

 

100% no. caps r not the problem. the only issue with metal caps is seizing onto the metal valve stem

 

I replaced all four sensors. Same problem is occurring. Clearly not with the sensors. The front wheels have had two new sets of sensors in the past 16 months. What would be the next possible place to look? Could a new audio head unit and/or my phone be causing interference with the wireless connection between the sensors and the car's brain?