I had the exact same problem on my 2003 Trailblazer. The check engine light came on, I took my vehicle to autozone and they told me the code was P0495 (cooling fan speed high)but they did't know how to fix the problem.
I called the dealer and they told me that it would cost me $89.00 to reprogram my PCM. I took my vehicle to the dealer and now my trailblazer is working like new. I hope it's that simple, the other thing also that could have gone bad would be the relay to your electric fan. The relay is a not like a normal relay but is more like a module that can regulate the speed of the electric fan which a tech2 scanner can determine if it's bad or not. Good luck
Thank you for using fixya.
This means that the engine's PCM detected that the engine has not reached the required temperature level within a specified amount of time after starting the engine. The intent of the P0128 code is to indicate a faulty thermostat. Similar codes: P0125
In determining the engine did not reach a "normal" temperature, it takes into account the length of time the vehicle has been running, the intake air temperature (IAT) sensor reading, the engine coolant temperature (ECT) sensor reading, and the speed of the vehicle.
A code P0128 may mean that one or more of the following has happened:
Past experience indicates that the most likely solution is to replace the thermostat. However here are some suggestions on troubleshooting and repairing a P0128 OBD-II code:
REFERENCE: http://www.obd-codes.com/p0128
I hope this helps a bit!
Take note of the blurb regarding NISSAN products below
A code P0128 may mean that one or more of the following has happened:
Past experience indicates that the most likely solution is to replace the thermostat. However here are some suggestions on troubleshooting and repairing a P0128 OBD-II code:
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