CI-121 Engine Coolant Temperature (ECT) Sensor Signal

Condition:





- Diagnostic trouble code (DTC) CI - 121 is stored if the combined instrument panel receives a signal from the engine control module (ECM) corresponding to a temperature below - 45°C (- 49°F) (low frequency) or higher than + 150°C (302°F) (high frequency).

Possible Source:

NOTE: An accessory immobilizer with separate plastic key can cause diagnostic trouble code (DTC) CI - 121 is the ignition is switched on before the immobilizer has been deactivated. This type of diagnostic trouble code (DTC) is not included in fault-tracing.

- Short-circuit to battery voltage in signal cable
- Short-circuit to ground in signal cable.
- Open-circuit in signal cable.
- Fault in fuel ignition/fuel injection system.
- Large contact resistance.
- Loose connections.

Fault Symptom(s):
- The engine coolant temperature (ECT) gauge constantly displays lowest temperature.

Start fault-tracing with step 1, below.

1) Checking Other Diagnostic Trouble Codes (DTCs):

Ignition On:

- Ignition on.





- Read off diagnostic trouble codes (DTCs) in fuel/ignition system.
- Is the engine coolant temperature (ECT) sensor diagnostic trouble code (DTC) stored?

Yes: Go to step 2, below.
No: Go to step 3, below.

2) Engine Coolant Temperature (ECT) Sensor Diagnostic Trouble Code (DTC):

- Fault-trace according to Appropriate ALLDATA section for the relevant fuel/ignition system in "Powertrain Management."

3) Status Check:

Ignition On:

- Ignition on.
- Go into scrolling values.





- Read off engine coolant temperature (ECT).
- Does engine coolant temperature (ECT) gauge display -40°C? (Negative 40°)

Yes: Go to step 4, below.
No: Go to step 10, below.

4) Checking Engine Coolant Temperature (ECT) Sensor Signal:

- Ignition off.





- Connect the breakout box to the engine control module (ECM). (LH 3.2, Fenix 5.2, Motronic 4.3, Motronic 4.4 or Diesel MSA 15.7) and check ground terminals according to the relevant ALLDATA section.





- Disconnect the engine control module (ECM).

Ignition On:

- Ignition on.
- Connect a voltmeter to the breakout box as follows:
- LH 3.2: # 53 (# B23) and # 20 (# A20)
- Other engine management system: # 23 (# B23) and # 28 (# B28)





- The voltmeter should read approx. 6 V.

If reading is OK, go to step 7, below.
If value is lower, go to step 5, below.
If value is higher, go to step 6, below.

5) Checking for Open-Circuit and Short-Circuit to Ground:





- Check cable between combined instrument panel # A26 and engine control module (ECM) # B23.
- On cars with ECC: Check cable between combined instrument panel # A26 and ECC control module # A23 for a short-circuit to ground. Permanent Fault
- Then continue with step 8, below.

6) Checking for a Short-Circuit to Supply Voltage:
- Check cable between combined instrument panel # A26 and engine control module (ECM) # B23.
- On cars with ECC: Check cable between combined instrument panel # A26 and ECC control module # A23 for a short-circuit to supply voltage. Permanent Fault
- Then continue with step 8, below.

7) Checking for Contact Resistance and Oxidation:
- The diagnostic trouble code (DTC) was caused by loose connections in the engine control module (ECM) connector. Check connector for contact resistance and oxidation and remedy. Permanent Fault
- Then continue with step 8, below.

8) Verification:

Note: After carrying out the repair check that the fault has been remedied.

- Ignition off.

Connect Connector:

- Reconnect connectors, reinstall components.

Ignition On:

- Ignition on.
- Go into scrolling values.





- Read off engine coolant temperature (ECT).
- Does engine coolant temperature (ECT) gauge display - 40°C? (Negative 40°C)

Yes: Go to step 9, below.
No: The fault is corrected.

9) Fault-Tracing Information:
- The verification result shows that the fault persists. Do you want to quit fault-tracing?

Yes: The fault is not corrected!
No: Start over at step 1, above.

10) Faulty Signal, Intermittent Fault:









- Check combined instrument panel connector A and engine control module (ECM) connector for loose connections, contact resistance and oxidation.
- Check cable between combined instrument panel # A26 and engine control module (ECM) # B23 for an intermittent open-circuit. Also, check for an intermittent short-circuit to ground and an intermittent short-circuit to supply voltage.
- On cars with ECC: Check cable between combined instrument panel # A26 and ECC control module # A23 for an intermittent open-circuit.Also, check for an intermittent short-circuit to ground and an intermittent short-circuit to supply voltage.
- To perform above stated checks, Intermittent Fault
- Then continue with step 11, below.

11) Fault-Tracing Information:
- For intermittent faults fault-tracing is not followed by a verification because the fault is not present at this moment.
- If you want to repeat fault-tracing, go back to step 1, above.