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Fault criteria

Fault criteria

The diagnosis was improved on M97 and has now been further improved to prevent the Check Engine lamp from going on when there is no fault. The major revision on M98 is that the diagnosis is performed twice during the same driving cycle, if the coolant temperature is below 40°C (104°F) when the engine is started. Both the cold start diagnosis and the regular diagnosis must be performed and a leak indicated during both diagnoses, before a DTC is generated.

The diagnosis is performed once each trip and is used to determine whether there is a leak corresponding to a hole of diameter 1 mm (0.040”) in the evaporative emission system.

The system includes a tank pressure sensor and a shut-off valve for the air supply to the EVAP canister.

When the conditions for the diagnosis have been fulfilled and the air supply to the EVAP canister has been stopped, the tank pressure will be controlled by the EVAP canister purge valve.

The diagnosis starts by storing the value from the tank pressure sensor.

Then, the PWM ratio of the purge valve is set to 15% and the EVAP shut-off valve closes. The purge is interrupted when the tank pressure has dropped by 0.6 kPa.

The rate at which the tank pressure increases is now monitored.

The fault criteria are fulfilled if a pressure drop of 0.6 kPa cannot be achieved within 23 seconds, or if the tank pressure increases more than 0.3 kPa during 9 seconds after the purge has been interrupted.

Conditions for cold start diagnosis


• Diagnosis not yet performed during this driving cycle.

• Diagnosis commenced more than 8 times during this driving cycle.

• Engine coolant temperature below 40°C (104°F) when engine was started.

• Idling speed

• Car stationary for more than 5 s.

• Max. one change of status of the brake light switch.

• Additional fuel from purge is less than 7 % of the engine's fuel requirements.


If the diagnosis is interrupted, the vehicle speed must exceed 4 km/h (2.5 mph) on one occasion before the diagnosis can be restarted.

While a diagnosis is in progress, the PWM of the purge valve is set to 15 % at the same time as the additional fuel from the purge is allowed to be 13 % of that required by the engine.

If the cold start diagnosis indicates a leak, the shut-off valve is activated 4 times in quick succession to remove any contamination present on the sealing surfaces of the valve.

The cold start diagnosis can be initiated with the diagnostic tool irrespective of what the coolant temperature was when the engine was started. At high temperatures and when the EVAP canister is saturated, it may be difficult to carry out the diagnosis as the additional fuel from the purge is then often more than 7 % of the engine's fuel requirements.

Conditions for the regular diagnosis


• Conditions for cold start diagnosis not fulfilled during this driving cycle or cold start diagnosis indicated a leak.

• Diagnosis commenced more than 8 times during this driving cycle.

• Approved transition diagnosis of oxygen sensor 1.

• PWM ratio to purge valve exceeds 40% at some time during this driving cycle (can be fulfilled by revving the engine to 3000 rpm).

• Idling speed

• Car stationary for more than 5 s.

• Max. one change of status of the brake light switch.

• Additional fuel from purge is less than 7 % of the engine's fuel requirements.


If the diagnosis is interrupted, the vehicle speed must exceed 4 km/h (2.5 mph) on one occasion before the diagnosis can be restarted.

While a diagnosis is in progress, the purge valve PWM ratio is set to 15 % at the same time as the additional fuel from the purge is allowed to be 13 % of that required by the engine.





1. Store reference value: - tank pressure


2. 15 % PWM to purge valve. Close EVAP shut-off valve.

3. Reduce tank pressure by 0.6 kPa within 23 s.

4. Stop pressure reduction.


5. Tank pressure increases more than 0.3 kPa during 9 s.

6. Revert to normal venting.

7. Fault criteria for evaporative emission fulfilled. Note that both the cold start diagnosis and the ordinary diagnosis must indicate leakage before a DTC is generated.