Fix P0128: Mechanic’s solutions that WORKED

The P0128: Cooling System Performance/Rationality – Coolant Thermostat points out faults in your cooling system caused in most cases by either the stuck thermostat or faulty engine coolant temperature sensor.

P0128 quick overview

Meaning	Cooling System Performance/Rationality – Coolant Thermostat.
Is it serious?	Generally not an immediate emergency but requires attention.
Possible causes	– Faulty thermostat. – Engine coolant temperature (ECT) sensor issues. – Problems with the cooling fan operation. – Potential ECM/PCM (engine control module/powertrain control module) concerns.
How to diagnose?	Thermostat Check: Inspect and possibly replace the thermostat if it’s not functioning properly. ECT Sensor Inspection: Check the engine coolant temperature sensor and replace if necessary. Cooling Fan Operation: Verify proper cooling fan operation; repair or replace if needed. ECM/PCM Inspection: Check for Technical Service Bulletins (TSBs) related to ECM/PCM replacement. If suggested, consider upgrading or replacing the ECM/PCM. Temperature Verification: Use an infrared temperature gun to compare readings with the scanner. Ensure the radiator fan isn’t running continuously. Connector and Wiring Check: Inspect connectors for corrosion. Check the wires between the sensor and ECU for any issues. System Ground Check: Clean the computer ground to ensure proper grounding. Recheck for Codes: After any repairs or replacements, clear the codes and monitor for recurrence.
What is ECT?	ECT stands for Engine Coolant Temperature, meassured by ECT sensor.

How to fix P0128 – learn from mechanics

I looked into P0128 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: Chrysler

Vehicle Information:
2006 Chrysler 300 Limited
Engine: Hemi 3.5L

Issue + Repair History:

• The customer reported a MIL (Check Engine Light) with code P0128 (Thermostat Rationalization).
• The water level and cooling fans were checked and found to be fine.
• The code was cleared, but it reappeared after a short time.

Suggestions from the Community:

• Multiple technicians suggested checking and replacing the thermostat, emphasizing that a stuck or sticking thermostat could cause the temperature not to change as expected.
• Recommendations included verifying readings from temperature sensors and performing diagnostic steps accordingly.

Final Fix:
A new thermostat was installed, resolving the issue. The vehicle is now working fine.

Case #2: Volvo

Vehicle Information:
2001 Volvo S40
Engine: 1.9L

Issue + Repair History:

• Code P0128 was present.
• The engine reached normal temperature without delays.
• The thermostat and ECT sensor were replaced twice with genuine Volvo parts.
• The cooling fan comes on at startup and runs briefly, then turns off.

Suggestions from the Community:

• Recheck connections at the ECT sensor.
• Check sensor values through data stream information.
• Look up TSBs for possible ECM reprogramming.
• Ensure a good connection at the sensor and correct voltage.

Final Fix:
The customer took the car to the dealer to check for ECM reprogramming, as suggested by several technicians.

Case #3: Chevrolet

Vehicle Information:
2007 Chevrolet Malibu LT
Engine: 2.2L

Issue + Repair History:

• Code P0128 was present.
• Symptoms included the temp gauge falling, the fan kicking on, and the AC becoming inoperative.
• The instrument cluster was replaced with a new one.

Suggestions from the Community:

• Strong recommendation to replace the thermostat, as it was considered a fail-safe mode.
• Emphasis on using an OEM or high-quality thermostat.
• Clear codes and reset the PCM after thermostat replacement.

Final Fix:
The thermostat was found to be functioning properly while driving, but the rubber seal in the middle was allowing coolant to pass by. The thermostat was replaced, fixing the gauge issue.

Case #4: Mercedes

Vehicle Information:
2002 Mercedes-Benz C230 Kompressor
Engine: 2.3L

Issue + Repair History:

• Code P0128 (Coolant Temp Below Thermostat Regulating Temp) was present.
• The vehicle came from another shop with a previously replaced ECT (Engine Coolant Temperature) sensor.
• Scanner reading showed 195°F (90°C) when fully warmed up.
• Inspection found an open thermostat.

Suggestions from the Community:

• Strong recommendation to replace the thermostat, especially if it doesn’t close at ambient temperature.
• Emphasis on using an OEM thermostat.

Final Fix:
The thermostat was confirmed as the issue and was replaced with an OEM part costing $150 from the dealer.

Case #5: Honda

Vehicle Information:
2000 Honda Accord EX 2.3L

Issue + Repair History:

• The check engine light was on with code P0128 (Cooling thermostat).
• The thermostat was replaced initially, but the code persisted.

Suggestions from the Community:

• Check the cooling fan for proper operation.
• Replace the engine coolant temperature sensor.
• Check for voltage in the coolant and ensure little to no voltage is present.
• Inspect wires between the sensor and ECU; perform a wiggle test.
• Clean the computer ground.
• Verify that the radiator fan is not running continuously.
• Use an infrared temp gun to compare coolant temperature readings with the scanner.
• Check connectors for corrosion.
• Explore the possibility of an ECM replacement based on TSB 01-022.

Final Fix:
The engine coolant temperature sensor was replaced, resolving the issue. The system returned to normal operation.