Hoping someone here has fixed this in the past or has an idea because I don't know what to do. My heated seats will work on the drivers and passengers side. Both the back and seat gets warm and they work great. However, if I try to have both sides turned on at the same time, the button flashes and then both sides cut off. I don't see how it could be the elements because both sides work and heat up if they're on individually. Any one have a clue what to try?

 

Are you trying to heat them up while two people are siting in the seats ?
Mine would work as long as someone is in the seats, but would flash if the passenger seat is empty.
GM would only sell you an entire cushion for $350 to fix the problem. I bought this heater for $70 on ebay for a Tahoe

https://www.ebay.com/itm/2003-2006-Chevy-GMC-Driver-Passenger-Bottom-Replacement-Heater-Heating-Element/122663348864?ssPageName=STRK%3AMEBIDX%3AIT&_trksid =p2057872.m2749.l2649

I got under the seats and plugged the heater externally and tested it on both seats individually by unplugging the green connector under the seat and plugging the new heater. My problem was the passenger heater. I took the Hummer to an upholstery shop and they replaced the unit for me . all is good again.
You have a break in your heat pads somewhere that is manifesting itself either under load or without. Buy this heater, and test your seats. Good luck.

 

I've recently replaced the heating element in my drivers side so I'm familiar with taking the seat out and doing that. So even though if my passenger side will work fine with the drivers side off, and vise versa, it is still a heating element issue? It only seems to kick off if both drivers and passenger side are turned on. If I just turn the passengers side on, both the seat and back get warm. If I turn just the drivers side on, both the seat and back get warm. If I have the drivers side on and then try to turn the passengers side on, they both turn off. If I have the passengers side on and try to turn the drivers side on, they both turn off. Something about having them both on... but they work independent of each other. If it were say... the passenger side seat or back heater had gone bad, wouldnt it not heat up when I have the passenger side on? That is why this is so confusing

 

Thank you. You were correct, I replaced the passenger side heating element for the seat (not back) and it is working properly now. I didn't bother removing the OEM heating element sewn into the leather, I just yanked the plug out of it. Then I stuck the new one to the seat foam using the adhesive backing and ran the new wire. The extra foam from leaving the OEM one doesn't seem to affect the heat from the new one. I've now replaced the heating pad on both driver and passenger seat cushions, hopefully I don't need to do it any time soon for the back - that looks a lot more time consuming and difficult.

 

Glad you found the problem. To be honest, I am not sure the upholstery shop took mine out either.
I ordered one for a Tahoe on Ebay and it fit. They installed it for $100 I think. I was not about to take my seat apart.