"Resetting The ECU After Installing a New Thermostat in 2001 GMC Sonoma"

Black 2001 GMC Sonoma on Snow

Do I need to reset the ECU after installing a new thermostat in a ’01 GMC Sonoma? Replaced thermostat after getting a P0128 CEL code, and now I keep getting P0171 and P0174. These codes appeared after the thermostat replacement. I think the old thermostat was a lower temp one, could this have anything to do with it? Also, the codes don’t show up when I drive from point a to point b, but rather when I run the truck until it’s at operating temp, shut it off for a minute and restart it when it’s still warm.

Answer: The P0171 and P0174 are lean condition codes, Were these codes present before the thermostat replacement? If not, you have probably caused a lean condition(vacuum leak) during the thermostat repair or something. If these two codes were there with the P0128 code,then you have still got a lean condition that exists. The lean conditions is being determined by the Oxygen sensors and can have several reasons for having been set. One of the most overlooked reasons for a lean condition can be a clogged fuel filter causing a lack of fuel pressure especially at highway speeds. Unfortunately the only true way to determine the fault is to have the vehicle on a “scanner” to watch the fuel trims and the data for the oxygen sensors and the injector pulse width, this will determine if it is truly running lean or if the ecm is causing it to run lean. These types of tests will require the experience of a trained professional technican. If you are at the point of not being able to use a scanner yourself I suggest going to a trusted repair facility. Hope this helps.