how to fix po128 code
on this van on 2002 GMC Safari

I am trying to determine how to diagnose a PO128 error code on a 2002 GMC Safari. I would appreciate any info anyone might have on this

Asked by for the 2002 GMC Safari
Here's the obd code:
I would suggest if the coolant level and strength are good, it is probably a bad thermostat that needs replaced.
thank you.. I will try that