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Home >> Mazda >> 2002 >> Tribute LX, 4WD >> Repair and Diagnosis >> External Pages >> Different car >> Section 26 (Engine Controls - Self-Diagnostics - 2.0L) >> Diagnostic Tests >> DTC P0128: Coolant Thermostat Stuck Open >> Diagnosis & Repair Procedure

Diagnosis & Repair Procedure

WARNING: This page is about a different car, the 2002 Mazda 626. However, it is still accessible from the selected car via links, so may be relevant.
  1. Ensure FREEZE FRAME DATA has been recorded. Check for related service bulletins. Repair as necessary and go to next step.
  2. Ensure no other DTC have been set at this time. If any other DTC has been set, repair that problem first. If no other DTC is set, go to next step.
  3. Turn off HVAC and all electrical accessories. Remove No. 1 cooling fan relay located next to main relay box in left side of engine compartment. See Fig 1 . Using scan tool, check ECT PID data. Ensure ECT PID data reads 210°F (99°C). Connect a fused jumper sire between cooling fan relay harness connector terminals "A" (Black/Red wire) and "B" (Blue/Yellow wire). ECT PID data should indicate a continual decrease that stops at 176-183°F (80-84°C), indicating that thermostat is closed. If ECT PID data is not as specified, go to next step. If ECT PID data is as specified, go to step  5 .
  4. Remove coolant thermostat and ensure it is not stuck open. Replace as necessary and go to next step. If thermostat is okay, check ECT sensor operation. See SYSTEM & COMPONENT TESTING - MIATA, MILLENIA, MPV, PROTEGE, PROTEGE5 & 626 article. Replace ECT sensor as necessary and go to next step.
    5. NOTE: If workshop interior and outdoor temperature difference is significant, PCM might not operate thermostat monitor. In order for the PCM to operate thermostat monitor, manufacturer recommends to allow engine to cool with vehicle outside workshop.
  5. Reconnect all connections. Allow engine to cool. Turn ignition on. Clear DTCs. See CLEARING DIAGNOSTIC TROUBLE CODES  under SELF-DIAGNOSTIC SYSTEM. Using scan tool, check IAT and ECT PID data. IAT PID data should be more than 14°F (-10°C) and ECT PID data should be less than 88°F (31°C). Difference between ECT and IAT should be less than 6°C. If all PID data is as specified, go to next step. If any PID data is out of specification repeat this step.
    6. NOTE: This step requires driving vehicle. DO NOT use chassis roller for this test. Allow engine to cool with vehicle outside workshop. Test drive must performed with no more than 3 stops within a 5 minutes drive period.
  6. Start engine. Turn off HVAC and all electrical accessories. Using scan tool, access DIAGNOSIS MONITORING TEST RESULTS in GENERIC OBD II FUNCTIONS. Monitor TEST #10:30:00 or TEST #10:31:00. Drive vehicle at 25-62 MPH until TEST value changes. If measured value is more than minimum value, replace PCM and go to next step. See REMOVAL, OVERHAUL & INSTALLATION article. On vehicles equipped with immobilizer system, reprogram ignition key identification numbers. See COMPUTER RELEARN PROCEDURES - IMPORT article in GENERAL INFORMATION. If measured value is not more than minimum value, go to next step.
  7. Perform DTC confirmation procedure. See DTC CONFIRMATION PROCEDURE  under SELF-DIAGNOSTIC SYSTEM. Check for DTCs. See RETRIEVING DIAGNOSTIC TROUBLE CODES  under SELF-DIAGNOSTIC SYSTEM. If any other DTC is present, go to appropriate test.
Fig 1: Identifying Cooling Fan Relay & Condenser Fan Relay Location
G00032506Courtesy of MAZDA MOTORS CORP.
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