Quite often an intermittent fault makes a brief appearance then goes away, leaving the driver wondering if they’re imagining things. inevitably, it makes a reappearance but almost never when the vehicle is taken into the garage and  there is a technician waiting to diagnose the problem. This is where fault codes come in.

As a general rule, recent cars have a systems design architecture such that each micro-controller, the chip inside each ‘black box’ controlling a set of functions such as engine, ABS or air conditioning, continuously monitors the signals from a range of sensors and makes judgements as to whether they are within a normal range or not. If they are outside expected limits appropriate actions will follow. In some cases a value, or a set of values, for the suspect input is substituted, and in others a set of actions is inhibited, or warnings of some sort issued to the driver. In almost all cases the exceptional input will be logged in memory. This is often the vital information needed to diagnose a fault, especially one that has cleared itself.

In the case of the A4 roof, with its complex electro-hydraulic system, the driver reported an increasing frequency of intermittent operation that could usually be restored by cycling the ignition off/on and shaking the roof.  We read a code that indicated the output from an angle sensor buried in the tangle of struts, levers and rams under the folds of fabric was outside an acceptable value range. When we deleted the code and tried again, the roof still didn’t work but no codes were logged, either the original or any other. After further investigation, we found there was no hydraulic pressure in the ram systems. and that the pump was not taking any current. The pump and its associated solenoid valve system is buried deep in the bowels of the the boot and was removed for examination after some effort. The hefty 12V motor is integral with the pump so it got a bit messy. We then found that one of the carbon brushes was sticking in its guide periodically. It was the work of minutes to ease it (and, to be on the safe side, its mate too) within its guides and full operation of the motor was restored. But not roof operation. The original fault code reappeared and we found that tapping the angle sensor could restore operation, once the code was deleted. We dug the small sensor out and, after a quotation from Audi, decided to see if we could fix it ourselves.  So we cut the plastic moulding open. It was just as well we did as it turned out that Audi seem to make them badly and we might have had the same problem again; all that was wrong was some electrical connections were completely devoid of solder. They had worked for several years simply by the pressure of one conductor on another. Time, vibration and corrosion had taken its toll eventually.

So there we have it – not one, but two intermittent faults intent on confusing and confounding us.