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DTC P0128: Thermostat Stuck Open: Testing

NOTE: Ensure freeze frame data has been recorded. Check for service bulletins and on-line information.
  1. Check for any other DTCs. If no other DTCs are present, go to next step. If any other DTCs are present, diagnose and repair before continuing.
  2. Turn off A/C and all accessories. Remove cooling fan relay No. 1, located next to main relay box. Start engine and warm until ECT PID reads 210°F (99°C). Using a jumper wire, connect cooling fan relay socket terminals "A" and "B". Fan should operate. If ECT PID decreases until thermostat closes at 184-190°F (84-88°C), go to next step. If ECT PID decreases to less than 184-190°F (84-88°C), replace thermostat. After repair, go to next step.
  3. Reconnect all components. Allow engine to return ambient temperature. Turn ignition on. Using scan tool, clear DTC. Ensure ECT PID is 20-86°F (-7-30°C). Start engine and warm to more than 160°F (71°C). Do not depress accelerator pedal. Check DIAGNOSTIC MONITORING TEST RESULTS 10:30:00. If value is less than maximum, go to next step. If value is at maximum, replace PCM and go to next step. PCM is mounted to passenger floor, behind glove box.
  4. Perform after repair procedure. See AFTER REPAIR PROCEDURE  . If no DTC is present, testing is complete. If any DTC is present, go to appropriate test for diagnosis and repair.
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