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DTC P0128: Cooling System Malfunction

CAUTION: When substituting or replacement of PCM is recommended, see PCM REMOVAL & REPLACEMENT  for important precautions.
NOTE: If DTCs P0112, P0113, P0116, P0117, P0118, P0300, P0301-P0306, P0335, P0339, P0385, P0389, P2227-P2229, P2646-P2649, P0506, or P0507 are stored, troubleshoot these DTCs first. DTC P0128 can also set when hood is opened while engine is running.
  1. Turn ignition on. Do the DTC CLEAR in the CLEAR menu with the Honda PGM tester or the HAS. Turn blower switch off. Turn A/C off.
  2. Check FAN CTRL in the DATA list with the Honda PGM tester or the HDS. If the fan is off, go to next step. If the fan is not off, wait until FAN CTRL is off Go to next step.
  3. Check the ECT sensor in the DATA list with the Honda PGM tester or the HDS, then check radiator fan operation. If the fan keeps running when engine coolant temperature reaches 158°F (70°C) check radiator fan circuits, fan relay circuits and the fan relay. If circuits are OK , go to step  10 . If the fan does not keep running when engine coolant temperature reaches 158°F (70°C), go to next step.
  4. Start engine. Hold engine at 3000 RPM with no load in (Park or Neutral) until the radiator fan comes on, then let engine idle. Turn ignition off.
  5. Turn ignition on. Do the RADIATOR FAN test in the DATA list with the Honda PGM tester or the HDS for 20 minutes. Check ECT SENSOR in the DATA list with the Honda PGM tester or the HDS. If about 140°F (60°C) or less, 1.25 volt or more is measured, problem is intermittent, system is OK at this time. If about 140°F (60°C) or less, 1.25 volt or more is not measured, go to next step.
  6. Turn ignition off. Allow engine to cool to between 19°F (-7°C) and 86°F (30°C). Replace thermostat.
  7. Turn ignition on. With the Honda PGM tester or the HDS, do PCM RESET in the CLEAR menu. Do the PCM idle learn procedure. See PCM IDLE LEARN PROCEDURE  .
  8. Start engine. Hold engine at 3000 RPM with no load in (Park or Neutral) until the radiator fan comes on, then let engine idle. Check for Temporary DTCs or DTCs in the DTC menu. If DTC P0128 is set, check the cooling system. Go to step  1 . If any other Temporary DTCs or DTCs are set, troubleshoot those indicated. If there are no Temporary DTCs or DTCs set, go to next step.
  9. Monitor the OBD STATUS for DTC P0128 in the DTCs menu with the Honda PGM tester or the HDS. If it PASSED, troubleshooting is complete. If it is FAILED, check the cooling system. Go to step  1 . If it is NOT COMPLETED, go to step  8 and recheck.
  10. Update the PCM if it does not have the latest software, or substitute a known-good PCM. Allow engine to cool to between 19°F (-7°C) and 86°F (30°C). Start engine. Hold engine at 3000 RPM with no load in (Park or Neutral) until the radiator fan comes on, then let engine idle. Check for Temporary DTCs or DTCs in the DTC menu. If DTC P0128 is set, check for poor connections or loose terminals at ECT sensor and PCM harness connectors. Go to step  1 . If any other Temporary DTCs or DTCs are set, troubleshoot those indicated. If there are no Temporary DTCs or DTCs set, PCM was updated, troubleshooting is complete. If PCM was substituted, replace original PCM.
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