After replacing the thermostat, why am I still getting a P0128 code?

Question

I have been dealing with a check engine light for 2 years. What started out as a P0300 ended up in a domino effect. I have changed the PCM, fuel pump, various sensors, plugs, wires, etc... and now the thermostat twice. I'm trying to get my car smogged but now I can't because of the P0128 code. What else do I need to check?

Answer 1

P0128 Thermostat rationality

To set this code the PCM uses a predicted value and compare to the actual coolant temperature,if the thermostat opens too soon the DTC code will set, If there is any other code set with P0128 do those code first.

Comments

I changed the thermostat in May and still got the P0128 code and P0171 came up later on. I took care of the 171 but not the 128. I just replaced the thermostat again last week and the car is still running cold. Is there a way to check the ECT and/or IAT sensors or should I just replace them? The fluid levels are fine.