P0172

DIAGNOSTIC TROUBLE CODES P0171, P0172
Adaptation too lean or too rich, rear cylinder bank (cylinders 1-3-5)

Fault symptoms
CHECK ENGINE lamp (MIL) on.
DRIVEABILITY may be affected.

Conditions.
More than 25% richer or leaner for longer than 15 seconds.
Driving under partial load.
Mass air flow sensor OK (no diagnostic trouble codes).

Diagnostic Procedure.
1. Check other diagnostic trouble codes in the Motronic system.
Connect the Scan Tool.
Obtain readouts of all trouble codes and save them.
If other diagnostic trouble codes are present, deal with them first.

2. Check the mechanical operation of the EVAP canister purge valve, using a pressure gauge and a pressure/vacuum pump.
Ignition switch in the ON position.
Disconnect the valve's vacuum hose from the engine and connect the pressure/vacuum pump. Build up negative pressure.
Select "ACTIVATE".
Select "EVAP VALVE".
Check the following:
- that the valve opens and closes without binding or leaking.
- that the valve is closed without current.





3. Check the operation of the mass air flow sensor.
Start the engine.
Select "READ FUNCTIONS".
Select "INTAKE AIR MASS".
Check that the Scan Tool shows:
- idling about 0.8 V ______ about 4 g/s
- at 2500 rpm (no load) about 1.2 V _____ about 12 g/s
Slightly higher values will be obtained with a cold engine.
Air flow rates that are too low primarily indicate air leakage.

4. Check the operation of the secondary air injection system.
Continue at "Fault diagnosis without diagnostic trouble codes": fault diagnosis, secondary air injection system, steps 5 and 6. Secondary Air System

5. Check that no air leakage occurs in the induction system after the mass air flow sensor.

6. Check the oxygen sensor's mounting for corrosion.
Measure the resistance between the oxygen sensor body and a grounding point on the engine.
- The resistance should be < 1 ohm.

If the resistance is > 1 ohm, unscrew the oxygen sensor and clean the threads.





7. Then check:
- that the fuel pressure is correct
- that air leakage does not occur in the exhaust system ahead of the oxygen sensor or at its mounting
- that the fuel tank is not empty
- that none of the injectors is clogged
- that the engine oil is not diluted with petrol.

8. Clear the diagnostic trouble code and drive the car to see whether the DTC is generated again.

If the diagnostic trouble code is generated again, continue at "Procedure Before ECM Replacement". Component Tests
If the trouble code is not generated again the remedial measure taken was correct, otherwise the fault is of intermittent type.