DTC 1-2-3

1-2-3 ENGINE COOLANT TEMPERATURE SENSOR SIGNAL




Trouble Code Conditions
If the Engine Control Module (ECM) receives a signal from the Engine Coolant Temperature (ECT) Sensor which indicates a temperature lower than -45°C (-49°F) (high signal) or higher than +174°C (345°F) (low signal), it notes a fault and sets Diagnostic Trouble Code (DTC) 1-2-3.

Substitute value(s)
- Engine coolant fan started at low speed.
- Fuel trim inhibited.
- Idle Air Control (IAC) Valve adaption inhibited.

Earlier Models (before April 1991)
- The ECM assumes a substitute value corresponding to 0°C during start-up and 20°C after the engine has started.

Later Models (after April 1991)
- The ECM assumes a substitute value corresponding to 0°C during start-up and 20°C after the engine has started. Temperature is then assumed to rise by 1°C every 4 seconds.

Fault source(s)
Signal too high:
- Signal lead short circuit to live.
- Signal lead broken.
- Ground lead broken.
- Contact resistance in terminals.
- ECT sensor defect.

Signal too low:
- Signal lead short circuit to ground.
- ECT sensor defect.

Fault symptom(s)
- Engine will not start, or is hard to start.

TEST PROCEDURE




OTHER DTC's
- Readout of other DTCs.

If DTC 3-1-4, 4-1-1 (2-4-3), 4-3-3, 1-4-3 and/or 4-1-3 are stored:
^ Check ECT sensor, Camshaft Position (CMP) sensor, Throttle Position (TP) sensor, Knock Sensor (KS) and EGR-temperature sensor common terminal # 30 on ECM for contact resistance and oxidation, as well as break.

If none of the above DTCs are stored:
^ Status report. Proceed to STATUS REPORT section below.




STATUS REPORT
^ Permanent fault, signal too high:
- Proceed to: PERMANENT FAULT, SIGNAL TOO HIGH section below.

^ Permanent fault, signal too low:
- Proceed to: PERMANENT FAULT, SIGNAL TOO LOW section below.

^ Intermittent fault, signal too high:
- Proceed to: INTERMITTENT FAULT, SIGNAL TOO HIGH section below.

^ Intermittent fault, signal too low:
- Proceed to: INTERMITTENT FAULT, SIGNAL TOO LOW section below.

WARNING: Switch off ignition and wait until engine coolant fan stops, then disconnect it during fault tracing.




PERMANENT FAULT, SIGNAL TOO HIGH
Check ground lead
- Ignition off.
- Disconnect ECT sensor.
- Connect an ohmmeter between terminal 1 on ECT sensor connector and ground.

The ohmmeter should read approx. 0 ohms.

If value OK:
^ Check signal leads. Proceed to CHECK SIGNAL LEADS section below.

If value differs:
^ Check connector. Proceed to CHECK CONNECTOR section below.




CHECK CONNECTOR
- Ignition off.
- ECT sensor disconnected.
- Check ECM connector and 55-pin connector (A3/E) terminal 10 for contact resistance and oxidation.
- ECM connected.



- Connect an ohmmeter between terminal 1 on ECT sensor connector and ground.

The ohmmeter should read approx. 0 ohms.

If value OK:
^ Source of DTC is bad contact in connector.

If value differs:
^ Check lead between ECT sensors connector terminal 1 and ECM # 30 for break.




CHECK SIGNAL LEADS
- Ignition on.
- ECT sensor disconnected.
- Connect a voltmeter between ECT sensors connector terminal 1 and 2.

The voltmeter should read approx. 5 volts.

If value OK:
^ Test with new ECT sensor.

If voltmeter reads approx. 0 volts:
^ Check connector. Proceed to CHECK CONNECTOR section below.

If voltmeter reads over 5 volts:
^ Check lead between ECT sensor terminal 2 and ECM # 45 for short circuit to live.

- When fault is fixed, check ECT sensor signal. Proceed to CHECK ECT SENSOR SIGNAL section below.




CHECK CONNECTOR
- Ignition off.
- ECT sensor disconnected.
- Check ECM connector and 55-pin connector in central relay unit (A3/E) terminal 5 for contact resistance and oxidation.
- ECM connected.
- Ignition on.



- Connect a voltmeter between ECT sensors connector terminal 1 and 2.

The voltmeter should read approx. 5 volts.

If value OK:
^ Source of DTC is bad contact in connector.

If value differs:
^ Check lead between ECT sensors connector terminal and ECM # 45 for break.




CHECK ECT SENSOR SIGNAL
The ECT sensor can break down as a sequential fault resulting from a signal lead being short circuited to live.
Check ECT sensor signal as follows.

- Connect ECT sensor.
- Erase DTC.
- Start engine and allow to idle.

If DTC 1-2-3 is set:
^ Test with new ECT sensor (If ECT sensor is faulty, the DTC is set within 20 seconds).

PERMANENT FAULT, SIGNAL TOO LOW
Check signal leads
- Disconnect ECT sensor.
- Ignition on.



- Connect a voltmeter between ECT sensors connector 1 and 2.

The voltmeter should read approx. 5 volts.

If value OK:
^ Test with new ECT sensor.

If value differs:
^ Check lead between ECT sensors connector terminal 2 and ECM # 45 for short circuit to ground.




INTERMITTENT FAULT, SIGNAL TOO HIGH
Check leads and terminals
- Check lead between ECT sensors connector terminal 2 and ECM # 45 for intermittent break, as well as intermittent short circuit to live.
- Check lead between ECT sensors connector terminal 1 and ECM # 30 for intermittent break.
- Check ECT sensors, ECM connector and 55-pin connector in central relay unit (A3/E) terminal 5 and 10 for loose connection, as well as contact resistance and oxidation.




INTERMITTENT FAULT, SIGNAL TOO LOW
Check leads and terminals
- Check lead between ECT sensor connector terminal 2 and ECM # 45 for intermittent short circuit to ground.