DTC Advanced Diagnostics: P0128
DTC P0128:Cooling System Malfunction

General Description
The powertrain control module (PCM) monitors the temperatures before and after the thermostat to diagnose whether it is sticking in open position. If the coolant temperature after the thermostat is a specified value, the PCM detects a malfunction and stores a DTC.

Monitor Execution, Sequence, Duration, DTC Type
Execution
Once per driving cycle
Sequence
None
Duration
0.5 second or more
DTC Type
Two drive cycles, MIL on

Enable Conditions
Precondition for warm-up diagnosis and permanent diagnosis
Condition
Minimum
Maximum
Elapsed time after starting the engine
4.96 seconds
–
Engine coolant temperature (at engine) [ECT Sensor 1]
–
121.92 °F (49.96 °C)
[ ]: HDS Parameter

Warm-up diagnosis
Condition
Minimum
Maximum
Engine coolant temperature (at engine) increase since engine start
36 °F (20 °C)
–
State of the engine
At idle

Permanent diagnosis
Condition
Minimum
Maximum
Initial engine coolant temperature (at engine) [ECT Sensor 1]
–
121.92 °F (49.96 °C)
Engine coolant temperature (at engine) [ECT Sensor 1]
168.72 °F (75.96 °C)
–

Malfunction Threshold
Either of the conditions is met:
Warm-up diagnosis
The ECT sensor 2 output temperature is less than 168.72 °F (75.96 °C).
Permanent diagnosis
The difference between the ECT sensor 1 output temperature and the minimum ECT sensor 1 output temperature is more than 9 °F (5 °C) - 180 °F (100 °C)*.

*: Depending on engine coolant temperature (at radiator)

Possible Cause
NOTE: The causes shown may not be a complete list of all potential problems, and it is possible that there may be other causes.
Thermostat failure

Diagnosis Details
Conditions for setting the DTC
When a malfunction is detected during the first drive cycle, a Pending DTC is stored in the PCM memory. If the malfunction returns in the next (second) drive cycle, the MIL comes on and a Confirmed DTC and the freeze data are stored.

Conditions for clearing the DTC
The MIL is cleared if the malfunction does not return in three consecutive trips in which the diagnostic runs. The MIL, the Pending DTC, the Confirmed DTC, and the freeze data can be cleared with the scan tool Clear command.