2005 GMC Envoy check engine light codes p0128 and p0410.
So far a coolant temperature sensor has been replaced.
Question = Before the switch just light on. After the switch truck started running rough with the heat
on. Now it cuts off if the heat or air is on.  I can drive it as long as heat or air is not on.


Answer:
Code P0128 is described as- Coolant temperature below threshold. The most common cause for this
code on the GMC Envoy and Chevy Trailblazer is not the coolant temp sensor, but rather the
thermostat. They tend to stick open, so the antifreeze is constantly running through the radiator and
getting cooled, not allowing the engine to get hot enough. The computer looks at this and sets the
code because the engine did not get warm enough as expected as compared to amount of time it has
been running. This code will not cause the truck to stall.

P0410 is for the
Secondary Air Injection system (A.I.R). This is an emission system. It pumps air into
the exhaust. This will also not cause your truck to die out. A common problem for this on your vehicle is
either the pump has sucked up water and has failed, or the air diverter valve on the side of the engine
has failed.

For the running rough and dying out- most likely just a throttle clogged with carbon. With A/C on the
compressor puts a load on the engine, and the computer cannot compensate to raise the idle a bit.
Many times when you have the heat on, the compressor is also on. A
cleaning of carbon from the
throttle body will cure this problem. You some Seafoam or carburetor spray on a rag and wipe down
the throttle and blade. Also spray it rel good. Then take it for a good ride doing some hard acceleration
to sort of Blow It Out. If the idle is now too high, try letting it idle for a few minutes, shut key off, restart
and drive again. If it is still too high, an idle relearn with a diagnostic scanner will be needed.


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2005 GMC Envoy Codes P0410 and P0128
Chevrolet Trailblazer Thermostat- 4.2L Engine