P0421

P0421

Diagnosis information - DME

Catalytic conversion

NOTE: The catalytic converter monitor compares the amplitude of the oxygen sensor downstream with the amplitude of a computed limit catalytic converter during defined air gulping and enrichment phases. The amplitudes allow conclusions to be drawn about the ability of the catalytic converter to store oxygen.

Diagnostic conditions
- Engine start temperature > -15 °C (5 °F)
- Catalytic converter temperature 385 .... 635 °C (725 .... 1175 °F)
- Charcoal canister load < 8
- Fuel trim upstream active
- Oxygen sensors downstream ready for operation
- 50 seconds [cumulative] within following load / speed range
- Engine speed 1280 .... 2880 rpm
- Engine load [relative filling] 22 .... 41%
- No fault oxygen sensors, tank vent, secondary-air pump relay, secondary-air valve relay, throttle adjusting unit, total mass air flow sensor, no misfire.
- The diagnostic conditions can also be established using the PIWIS Tester via the short test [Catalytic conversion]. It is not necessary to have the vehicle in motion for this.

Possible fault causes
- Oxygen sensors up and downstream are interchanged [Position in exhaust assembly]
- Valve lift error
- Aged oxygen sensor up or downstream
- Catalytic converter faulty [Pre-catalytic conversion rate too low]

If an aged oxygen sensor is also detected together with this fault:
- remedy the fault "Oxygen sensor signal downstream, dynamism" first,
- Then check if this fault - catalytic conversion - has been entered again.

IMPORTANT: If the primary catalytic converter is faulty, check the main catalytic converter for damage and also replace it if necessary.