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Home >> Saab >> 1999 >> 9-3 Base, 2D Convertible, Standard >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Self-Diagnostics - TRIONIC T5 Fuel Control System >> Diagnostic Tests >> DTC P0125: Engine Coolant Temperature (ECT) Sensor Circuit

DTC P0125: Engine Coolant Temperature (ECT) Sensor Circuit

  1. If possible, measure freezing point of coolant in an environment below 32°F (0°C). If coolant concentration is too diluted to prevent freezing, replace with proper mixture of coolant. Allow engine to cool down enough to put your hand on the cylinder head without getting burned.
  2. With ignition off, connect TECH 2 scan tool to DLC. Contact TRIONIC 5 OBD II. Select TEST VALUE: COOLANT TEMPERATURE. Start engine and allow it to warm up at an idle. Feel upper radiator hose to determine when thermostat opens. Temperature should be about 192°F (89°C), and stay there for about a minute. If cooling system operation is as specified, go to step  4 . If operation is not as specified, go to next step.
  3. Replace thermostat and use scan tool to clear DTCs. Start engine and allow it to run until cooling fan starts. Recheck scan tool reading to confirm that coolant temperature levels off around 192°F (89°C).
  4. Start engine and allow it to idle. Note T-5 ECT coolant temperature for reference. Use scan tool to contact MAIN INSTRUMENT UNIT. Select TEST VALUE: COOLANT TEMPERATURE and note coolant temperature. The two ECT sensors should not give readings that vary by more than 9°F (5°C). If values are as specified, go to next step. If values are not as specified, go to step  6 .
    NOTE: T-5 ECT sensor is under upper engine cover, and has Yellow/White and Black wires running to it. MIU ECT sensor is right next to it, on driver's side. It has Yellow/Grey and Yellow/Brown wires running to it.
  5. Use scan tool to clear all DTCs. Start and run engine until radiator fan comes on. Unplug ECT sensor that goes to main instrument unit (MIU). Use scan tool to contact TRIONIC T 5 OBD II. Select TEST VALUE: COOLANT TEMPERATURE. Confirm that coolant temperature levels off around 192°F (89°C).
  6. Unplug connector to Trionic T-5 ECT sensor. Connect SST 86 11 857 (test light) or equivalent between positive battery terminal and ECT sensor harness connector terminal No. 2 (Black wire). See Figure . If test light illuminates, go to next step. If test light does not illuminate, check for faulty circuit between ECT sensor harness connector terminal No. 2 and splice connector J31 (Black wire) located about 2" from ECM harness connector. See appropriate wiring diagram in WIRING DIAGRAMS article. Repair as necessary and go to step  9 .
  7. Use scan tool to contact TRIONIC 5 OBD II. Select TEST VALUE: COOLANT TEMPERATURE. If scan tool display is about -40°F (-40°C), go to next step. If readings are not as specified, check for faulty circuit between ECT sensor harness connector terminal No. 1 and ECM harness connector terminal No. 68 (Yellow/White wire). ECM is next to glove box on right side. See Figure for ECM terminal identification. Repair as necessary and go to step  9 .
  8. Connect a jumper wire between ECT sensor harness connector terminals No. 1 and 2. Reading on scan tool should now be 257-266°F (125-130°C). If reading is as specified, replace ECT sensor. See ENGINE COOLANT TEMPERATURE (ECT) SENSOR under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION article. If reading is not as specified, check for faulty circuit between ECT sensor harness connector terminal No. 1 and ECM harness connector terminal No. 68 (Yellow/White wire). Repair as necessary. Go to next step.
  9. Use scan tool to clear DTCs. Drive vehicle for 5 minutes under varying loads and engine RPM. Recheck for any stored DTCs. If DTC P0125 is present, substitute known good ECM and retest system. See ENGINE CONTROL MODULE (ECM) under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION article.
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