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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 P0126: 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 DTCs are 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. Using scan tool, check ECT PID data. Ensure ECT PID data reads 210°F (99°C). Connect fused jumper wire 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 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 allowing 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 PID data. IAT PID data should be more than 14°F (-10°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-minute drive period. Test period depends on engine coolant temperature at engine start. The warmer the engine coolant, the less time required for test. For example, if engine coolant temperature is 14°F (-10°C), monitoring period is 38 minutes, but if engine coolant temperature is 86°F (30°C), monitoring period is only 8 minutes.
  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 25-62 MPH until TEST value changes. If measured value is not 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 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 & Fuel Pump Relay Harness Connector Terminals
G00020979Courtesy of MAZDA MOTORS CORP.
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