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Fault Criteria, Catalyst

Fault Criteria, Catalyst

The three way catalytic converter is diagnosed once per trip.

The three way catalytic converter's ability to store oxygen is proportional to its efficiency. If the ability of the three way catalytic converter to store oxygen is poor, the closed loop's natural fuel-air ratio oscillations will also be noticed after the three way catalytic converter. An oxygen sensor is also mounted after the three way catalytic converter (rear heated oxygen sensor).

When the conditions for diagnosis are fulfilled, the voltage changes emanating from the rear heated oxygen sensor are analyzed. If the voltage changes are excessive, having regard to engine load and speed, the three way catalytic converter is defective.

The voltage changes emanating from the rear heated oxygen sensor are corrected in regard to engine load and speed and totalled in 50 individual, approved periods for the front heated oxygen sensor 1 *). If the total exceeds a calibrated value, the fault criteria are satisfied.

*) An approved period is when the pulse ratio of the sensor voltage is 30-80%% (+).


Conditions for diagnosis

Diagnosis not yet performed this trip


Heating of the three way catalytic converter:

• Vehicle speed >0 km/h

• Open throttle

• Engine speed (rpm) x load (kPa) = >60 000

• Aggregate acceleration >15 sec.

• Above conditions fulfilled for 120 sec.

• No idling period >40 sec.

• No fuel shut-off >10 sec.



Evaluation of the three way catalytic converter

• Vehicle speed 30-100 km/h

• Open throttle

• Closed loop activated

• Engine speed (rpm) x load (kPa) = 60 000 - 260 000

• Coolant temperature >60°C (>140°F)

• Rear heated oxygen sensor >0.4 V at any time in this driving cycle.



1. Direction:

-front heated oxygen sensor voltage

-rear heated oxygen sensor voltage


2. During one oxygen sensor 1 period with duty cycle 30-80%%(+), oxygen sensor 2 voltage fluctuations are registered.

3. Correct voltage fluctuations on basis of load and rpm

4. Add voltage fluctuations in a memory

5. Voltage fluctuations from 50 periods added together

6. Total below threshold

7. Function monitored this trip

8. Fault criteria for catalyst fulfilled