P0128

DTC P0128 Thermostat Malfunction

DESCRIPTION

DTC Detection Condition:

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

MONITOR DESCRIPTION

Monitor Description:

Monitor Strategy:

Typical Enabling Conditions:

Typical Malfunctions Thresholds:

The ECM estimates the coolant temperature based on 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 will interpret this as a fault in the engine cooling system or thermostat and set a DTC.

Wiring Diagram:

Step 1-2:

Step 2(Continued)-3:

INSPECTION PROCEDURE

HINT: Read freeze frame data using the intelligent tester. Freeze frame data records the engine conditions when a malfunction is detected. When troubleshooting, freeze frame data can help determine if the vehicle was running or stopped, if the engine was warmed up or not, if the air-fuel ratio was lean or rich, and other data from the time the malfunction occurred.