Hey guys my wifes 08 is having ac issues. Off and on The temperature gage will not register and ac will not blow cold. The dash gage and the digital readout will not register. It's throwing a P0128 code. If I reset then all works fine. Any ideas? Thermostat has been changed. Appreciate any input.

 

It is not an A/C issue.

A p0128 code indicates that the engine is not reaching the proper temperature fast enough, and blanks out the digital temp reading with "-- --", turns off the analogue temp gauge, and disables the A/C.

Typically that is caused by a thermostat that is sticking open due to a gasket on the internal valve jamming it open. But a temperature sensor may be giving a false reading, but I believe those have been very rare.

If the problem has persisted after changing the thermostat, the new thermostat may be suspect - so the question is: exactly what thermostat was used as a replacement? More than a few owners recommend that ONLY a genuine GM ACDELCO thermostat be used.

 

Wrong T stat! Get an AC Delco 131-158 !

 

Just to add. There have been instances of new thermostats getting jammed open by debris from the old one.
The rubber inner gasket would break loose from the old thermostat and float around in the coolant. Put the new T-stat in and that debris from the old one eventually wedges the new one open.

Anyhow. Your description points to thermostat problems. Pull it out and have a look. If it's a debris issue, have the cooling system flushed. If you're not using the AC Delco brand thermostat, get one.
Several members have reported problems with off brand thermostats.