03 Impala Diagnostic Code PO128

I have a 2003 Impala that the check engine light came on. A mechanic at my husband's work put a gizmo on it and told him the error code was PO128. I've been searching several sites and can't determine exactly what it is. What does this code mean? What part do I need? Can hubby fix it himself or do I have to take it to a mechanic? The car runs fine - there's no loss of heat - no dying out - no overheating, temperature cage reads normal, etc.

Thanks for your help.

Welcome to AF, replace the thermostat.

Max is dead on for that one. P0128....Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature). to answer your question about wether hubby can do it himself, i must ask a couple questions. what is the engine size? does he have some basic tools and a bit of patience?