P0128
INFO
Thermostat below regulating temp
The truck's control module (ECU) reports a fault on: Thermostat below regulating temp.
P0128 means a truck control module (ECU) flagged a fault on thermostat below regulating temp, part of the Cooling system on heavy-duty diesel trucks. DERATE rates it INFO: it does not limit driving; have it checked at the next service. Part usually replaced: Thermostat, $60–$320 (aftermarket to dealer) plus 1.5–3 h labor.
Cooling
Can I keep driving?
Yes. It is informational and does not limit driving; have it checked at the next service.
Part usually replaced
Thermostat
$60–$320 (aftermarket to dealer)
Labor: 1.5–3 h
USD, part only. Labor is billed at the shop's hourly rate.
Data: DERATE verified base, updated October 2026
Related codes
Read it off your own dash
Take one photo and DERATE tells you whether you can keep driving.
Ask DERATE
DERATE assists. It does not replace inspection by a qualified technician.