OK, Noobie alert!. I'm trying to understand how the fault code system of my car works.

The next paragraph is background; you can jump straight to the questions if you're easily bored!

I've just got a code reader. When I took my car to Stu for cruise control fitment in August he noticed a couple of fault codes on my car. One was to do with a brakelight switch malfunction & I think the other was 17705. I guess Stu cleared the faults because when I connected my new DTC reader, just prior to servicing the car I had one code present. It was 17705.

I cleared the fault code & serviced the car (Oil, filters, brake discs, pads & fluid). After the service I drove about two miles & checked for faults again. I had two: 17705 & 18??? (I forget, but it was to do with the left bank cam.) I cleared them & took the car for a brisk drive to bed the brakes.

15 miles later I checked to see whether my 17705 had returned. It hadn't but I had 11 faults (17525, 17633, 17634, 17635, 17636, 17697, 17829, 17833, 17843, 17938 & 17955). These all refer to different engine electrical components & end with 'Short to Earth'. I cleared them. That was two days & three drives ago & I still have no faults shown.

Questions:

Is it usual to get a bunch of faults after a service? I've had one 17705 since visiting Stu in August & then 13 faults in the 12 hours following my service.

Do fault codes 'stack up'. i.e. does the clearing of a 'bigger' fault allow you to then see the 'smaller' ones?

Does the car only record only one instance of each fault? i.e. if I had a hundred instances of 17705, would I only see one recorded.

Does a fault code registered disable or reduce the functionality of the relevant part? Does clearing a fault code restore functionality or is there just 'Limp Mode' which is either on or off?

Sorry for the novel; just trying to understand how it works.

Cheers

Darren

You can find out about some of the codes here -

17829/P1421/005153 - Ross-Tech Wiki

17843/P1435/005173 - Ross-Tech Wiki

17955/P1547/005447 - Ross-Tech Wiki

The length of time they remain for depends on the severity of the fault.

The length of time they remain for depends on the severity of the fault.

So, they self clear after a certain time? I've learnt something already.

Questions:

Is it usual to get a bunch of faults after a service? I've had one 17705 since visiting Stu in August & then 13 faults in the 12 hours following my service.

No. It is not usual. Perhaps you have a loose connection that was bumped during the service causing some intermittent faults to set. Without a full and proper scan, it's hard to say.

Do fault codes 'stack up'. i.e. does the clearing of a 'bigger' fault allow you to then see the 'smaller' ones?

No. One fault does not hide others. All faults that are set should be reported.

Does the car only record only one instance of each fault? i.e. if I had a hundred instances of 17705, would I only see one recorded.

Most likely yes - only one would show. Freeze Frame data may report more information if the module that has the fault supports Freeze Frame data.

Does a fault code registered disable or reduce the functionality of the relevant part? Does clearing a fault code restore functionality or is there just 'Limp Mode' which is either on or off?

It depends on the fault. some faults on the engine/tranny will cause the vehicle to drop into a "Limp Home" mode where there is little power available. It allows the car to move but is attempting to prevent damage.

I prefer not to think of them as fault codes but as symptoms of a fault. One example is when a lambda probe reports a mixture problem, if this was a fault code, then the logical thing is to replace the lambda probe, if you think of it as a symptom then you will be looking for reasons for the fault such as an air leak.

The 18?? code that you refer to after carrying out a service is a camshaft set point not reached. This is not a fault with the camshaft, but the effect of low oil pressure on the hydraulic chain tensioner, caused by the new oil taking time to circulate around the engine.

Only faults that affect the emissions bring the engine light on and if they do not reoccur after so many driving cycles the light will go out and the code will be downgraded to intermittent or sporadic status.

So, they self clear after a certain time? I've learnt something already.

I can only speak for the cars I've worked on (not VAG), but typically when a fault occurs it's "current", then when the fault is no longer detected it's "history". It will remain at "history" for 40 ignition cycles before clearing completely, unless the fault is detected again in the meantime...