P0128

DTC P0128 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

HINT: This DTC relates to the thermostat.

CIRCUIT DESCRIPTION

DTC Detection Condition:

This DTC is set when the Engine Coolant Temperature (ECT) does not reach 75 °C (167 °F) despite sufficient engine warm-up time.

MONITOR DESCRIPTION

Monitor Strategy:

Typical Enabling Conditions:

Typical Malfunction Thresholds:

Component Operating Range:

Monitor Result (Mode 06 Data):

The ECM estimates the coolant temperature based on the starting temperature, engine loads, and engine speeds. The ECM then compares the estimated temperature with the actual ECT. When the estimated coolant temperature reaches 75 °C (167 °F), the ECM checks the actual ECT. If the actual ECT is less than 75 °C (167 °F), the ECM interprets this as a malfunction in the thermostat or the engine cooling system and sets the DTC.

INSPECTION PROCEDURE

Step 1 - 2:

Step 3:

HINT: Read freeze frame data using the hand-held tester or the OBD II scan tool. Freeze frame data records the engine conditions when the malfunction is detected. When troubleshooting, it is useful to determine whether the vehicle was running or stopped, the engine was warmed up or not, the air-fuel ratio was lean or rich, etc. at the time the malfunction occurred.