Vauxhall Combo CDTI case study

Vauxhall Combo CDTI 2005
Mileage 86,000
Vehicle is in generally good all round condition.

Fault detail
Engine will not start after alternator failure.

Recorded DTCs
No DTC’s are recorded when scanned with generic OBD scan tool

Fault History
The vehicle was being driven normally.
The driver heard a mechanical  grinding noise and the alternator warning light came on.
The vehicle was driven for a few more minutes and the Check Engine light was illuminated and the engine stopped.
The vehicle would not restart. The vehicle did however, restart and drive to the workshop when a new charged battery was fitted.

Rectifying action
Upon examination the alternator drive pulley had failed and the alternator driven belt was not driving the alternator armature.
The alternator was replaced but the engine would not start.
When the DTC’s were scanned with a generic OBD scanner, no DTC’s were logged.
No Vauxhall diagnostic software was available so it was not possible to check if a fault code had been logged indicating the nature of the engine non start.
Although no DTC’s were present with the generic OBD scanner is does not mean that there weren’t any codes stored. The generic scanner may not get full access to the engine control module.

It was suspected that a diagnostic fault code had been logged by the engine module due to the failed alternator, and it was this fault that was causes the engine not to start.
As part of the repair action, now the alternator had been replaced the DTC codes should be cleared.
On our vehicle/scanner combination no DTC was detected so it is not possible to clear the codes.
In order to run the In order to run the ‘Clear all Fault codes’ routine a fault code must be present; a fault code was introduced into the engine management system by disconnecting the airflow meter while the ignition was switched on.
With a OBD fault code introduced, our generic OBD scanner will now run the ‘Clear all Fault codes’ routine; this will clear both the detected airflow code and any undetected codes which maybe present.
With the ‘Clear all Fault codes’ routine complete, the vehicle would start and run normally.

Summary
Some vehicle failures require a ‘Clear all Fault codes’ routine to be run as part of the repair action.
Many generic code scanner will only read a percentages of diagnostic codes; only expensive and manufactures own scanners will access all codes.
It is possible to force a code clear action by introducing OBD faults and then running ‘Clear all Fault codes’ routine.