Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
DTC P0128: Cooling System Malfunction ('04-05 models)
- Before you troubleshoot, record all freeze data and any on-board snapshot, and review the general troubleshooting information (see GENERAL TROUBLESHOOTING INFORMATION ).
- If the DTCs listed below are stored at the same time as DTC P0128, troubleshoot them first, then recheck for P0128.
P0112, P0113: Intake air temperature (IAT) sensor P0116, P0117, P0118, P0125: Engine coolant temperature (ECT) sensor
P0300: Random misfire
P0301, P0302, P0303, P0304: No. 1, No. 2, No. 3, or No. 4 cylinder misfire
P0335, P0339: Crankshaft position (CKP) sensor P2227, P2228, P2229: Barometric pressure (BARO) sensor
P2646, P2647, P2648, P2649: VTEC system P0506, P0507: Idle control system malfunction
DTC P0128 can occasionally set when the hood is opened while the engine is running.
- Turn the ignition switch ON (II).
- Clear the DTC with the HDS.
- Turn the blower switch OFF.
- Turn the A/C switch OFF.
- Check the FAN CTRL in the DATA LIST with the HDS.
Is it OFF?
YES -Go to step 6.
NO -Wait until the FAN CTRL is off, then go to step 6.
- Check the ECT SENSOR in the DATA LIST with the HDS, then check the radiator fan operation.
Does the radiator fan keep running when the engine coolant temperature is less than 158°F (70°C)?
YES -Check the radiator fan circuit (see RADIATOR FAN CIRCUIT TROUBLESHOOTING ), the radiator fan switch circuit for open (see RADIATOR FAN SWITCH CIRCUIT TROUBLESHOOTING (OPEN) ), the radiator fan switch circuit for short (see RADIATOR FAN SWITCH CIRCUIT TROUBLESHOOTING (SHORT) ), and the radiator fan switch (see RADIATOR FAN SWITCH TEST ). If the circuits and the switch are OK, go to step 21.
NO -Go to step 7.
- Start the engine. Hold the engine speed at 3,000 rpm without load (in Park or neutral) until the radiator fan comes on, then let it idle.
- Turn the ignition switch OFF.
- Turn the ignition switch ON (II).
- Do the RADIATOR FAN TEST in the INSPECTION MENU with the HDS for 20 minutes.
- Check the ECT SENSOR in the DATA LIST with the HDS.
Is about 151°F (66°C) or less, or 1.25 V or more indicated?
YES -Intermittent failure, the system is OK at this time.
NO -Go to step 12.
- Turn the ignition switch OFF.
- Let the engine cool until the coolant temperature (ECT SENSOR) is between 23°F (-5°C) and 95°F (35°C).
- Replace the thermostat (see THERMOSTAT REPLACEMENT ).
- Turn the ignition switch ON (II).
- Reset the ECM/PCM with the HDS.
- Do the ECM/PCM idle learn procedure (see ECM/PCM IDLE LEARN PROCEDURE ).
- Start the engine. Hold the engine speed at 3,000 rpm without load (in Park or neutral) until the radiator fan comes on, then let it idle.
- Check for Temporary DTCs or DTCs with the HDS.
Is DTC P0128 indicated?
YES -Check the cooling system, then go to step 1.
NO -Go to step 20.
- Monitor the OBD STATUS for DTC P0128 in the DTCs MENU with the HDS.
Does the screen indicate PASSED?
YES -Troubleshooting is complete. If any other Temporary DTCs or DTCs were indicated in step 19, go to the INDICATED DTCS TROUBLESHOOTING .
NO -If the screen indicates FAILED, check the cooling system, then go to step 1. If the screen indicates NOT COMPLETED, let the engine cool until the coolant temperature is between 23°F (-5°C) and 95°F (35°C), then go to step 18.
- Update the ECM/PCM if it does not have the latest software (see UPDATING THE ECM/PCM ), or substitute a known-good ECM/PCM (see SUBSTITUTING THE ECM/PCM ).
- Let the engine cool until the coolant temperature is between 23°F (-5°C) and 95°F (35°C).
- Start the engine. Hold the engine speed at 3,000 rpm without load (in Park or neutral) until the radiator fan comes on, then let it idle.
- Check for Temporary DTCs or DTCs with the HDS.
Is DTC P0128 indicated?
YES -Check for poor connections or loose terminals at the ECT sensor and the ECM/PCM. If the ECM/PCM was updated, substitute a known-good ECM/PCM (see SUBSTITUTING THE ECM/PCM ), then go to step 22. If the ECM/PCM was substituted, go to step 1.
NO -Go to step 25.
- Monitor the OBD STATUS for DTC P0128 in the DTCs MENU with the HDS.
Does the screen indicate PASSED?
YES -If the ECM/PCM was updated, troubleshooting is complete. If the ECM/PCM was substituted, replace the original ECM/PCM (see ECM/PCM REPLACEMENT ). If any other Temporary DTCs or DTCs were indicated in step 24, go to the INDICATED DTCS TROUBLESHOOTING .
NO -If the screen indicates FAILED, check for poor connections or loose terminals at the ECT sensor and the ECM/PCM. If the ECM/PCM was updated, substitute a known-good ECM/PCM (see SUBSTITUTING THE ECM/PCM ), then go to step 22. If the ECM/PCM was substituted, go to step 1. If the screen indicates NOT COMPLETED, go to step 22.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.