Powertrain, generic · Fuel and air metering

P0128 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

After a cold start the thermostat stays closed so the engine warms up quickly. CARB requires the coolant to reach a temperature at most 11 °C below the thermostat's rated value within a set time. GM models the heat put in (fuel energy minus losses to ambient, cabin heater and coasting) and fails if enough energy has gone in without coolant reaching about 65 °C (55 °C in cold weather). Ford compares the measured value with a modelled one, typically failing below 78 °C with an 89 °C thermostat. The test runs once per drive cycle, pauses with much idling or below about −7 °C, and needs two trips. Usually the thermostat is stuck open; a sensor reading too low is the other suspect.

Likely causes

Thermostat stuck open or opening too early
Coolant sensor reading too low, or corrosion and resistance in its circuit
Low coolant level or air in the system
Radiator fan running constantly (relay stuck, wrong control)
Driving pattern or conditions (much idling, extreme cold, strong heating)

Symptoms

  • Check engine light on
  • Temperature gauge stays low or drops while driving
  • Heater does not get properly warm
  • Higher fuel consumption

Repair estimate

DifficultyMediumDIY possible
Parts and labour€60–400
Time0.5–3 h

Typical range across vehicles; the real cost depends on what the testing finds.

What to check first

  1. Replace a thermostat that is stuck open
  2. Check fan control
  3. Verify ECT sensor reading

Parts involved

thermostatengine coolant temperature sensorcooling system

Related codes

P0125P0126P0116P0117P0118P2181

References: law.cornell.edu, gsi.ext.gm.com, newtonnet.co.uk, fordservicecontent.com.