DTC 4-2-5

4-2-5 REAR HEATED OXYGEN SENSOR REGULATING

Preconditions for DTC
^ The Rear Heated Oxygen Sensor (HO2S) constantly ensures that the front HO2S adjusts the integrator's value in a way which corresponds to the exhaust gas's oxygen content. If the integrator's reading deviates, the rear HO2S fine-tunes the integrator.
^ During engine operation, this adjustment will be both positive and negative. If the adjustment is only positive or negative, the control module will interpret this as damage to the rear HO2S and DTC 4-2-5 will be set.

Substitute Value
^ Fuel trim shut off.

Fault Sources
^ Uneven compression.
^ Air leakage in the intake system.
^ Air leakage in the exhaust system.
^ Faulty fuel and residual pressure.
^ Defective rear HO2S.

Fault Symptoms
^ May result in impaired performance.

TESTING PROCEDURE





Checking the Compression
^ Carry out a compression test.

If the compression is OK:
- Check for air leakage in the intake system.
If the compression in any cylinder deviates:
- Rectify as necessary.





Check for Air Leakage in the Intake System
^ Check for air leakage in the intake system.

If there is no air leakage:
- Check for air leakage in the exhaust system.
If there is any air leakage:
- Rectify as necessary.





Checking for Air Leakage in the Exhaust System
^ Check for air leakage in the exhaust system.

If there is no air leakage:
- Check the fuel and residual pressure.
If there is any air leakage:
- Rectify as necessary.





Checking the Fuel and Residual Pressure
^ Check the fuel and residual pressure.

If the fuel and residual pressure are OK:
- Test with a new rear HO2S.
If the fuel and residual pressure deviate:
- Rectify as necessary.