I have a 2008 Chevy Trailblazer 4x4, when I go over bumps my temp gauge drops to 40'C and my A/C goes warm air blowing. It's intermittent. Going over another bump will return it to normal. I took it into the GM dealer and they pulled a code P0128 and changed the ECT sensor. A few days later it happened again. I have to take it back to them, any ideas as to where I should have them look. I have told them it seems like a wire is loose, but how is the temp gauge and A/C related. I drove 2 hours in sweltering heat without them working and the vehicle did not overheat. Any ideas appreciated. Tks.
Your engine running cooler than normal, 40C(104F), may be the ecm disabling ac operation. As a member in one vehicle forum covering several models, a recurring P0128 error code periodically appears in several models. My personal experience along with other forum members has shown the coolant sensor isn't the problem. The thermostat is the problem. The best way to determine coolant temperatures would be using an OBD II reader plugged into the OBD II port under the steering column, capable of displaying coolant temperatures. Two measurements are made; one with a cold engine before startup and a second measurement after warmup. Cold engine temps should display temps close to ambient local temps to verify the coolant sensor is operating as designed and not faulty otherwise a gross reading would imply either a faulty sensor or wiring issue. With a good temperature displaying local temps, a warmed up engine should display operating temps, anywhere between 175F(79C) to 200F(93C). Lower temps would suggest a faulty thermostat.
Vehicle ac systems rely on cooling systems operating within a certain range. If the engine computer detects incorrect cooling system temps, some may have programming to disable ac operation because the ac system generates heat that's cooled off in the condenser coil, in front of the radiator. The cooling fan(s) are used whenever ac runs since a hot ac condenser coil has its air flowing into the radiator. Protecting the the cooling system is more important than occupants in a compromised engine/xmission.