C3 (2004) - Wiring diagram - Car won't start, multiple fault codes

[wheeler]

Will try it in a bit, but I'd have thought powering the F15 fuse would also power F2 (and the coil for the right-side relay)?

Yes you are right indeed as it looks like they are joined internally.
Do you have battery voltage at Fuses 8 & 16?

[ozvtr]

The fuses under the BSM are maxi fuses (labeled MFx in the diagram), but there are no fuses between the battery lead and that relay. So most likely scenario is burnt out track or bad relay contacts. My bet is bad contacts in the relay. The pre-2005 BSM’s can be repaired if you have the skills and the relays can be found in places like RS components and element 14, even Ebay and Aliexpress. However, the BSM’s can be found on Ebay at reasonable prices.
The other relay (the 'right side relay' as you called it) powers the exhaust injection air pump. I doubt it is fitted on your car, but even if it was, it’s irrelevant.
If the internal track in the BSM is OK then powering F15 should also power F2. I don’t see why that track would have burnt out, so it should be intact.
Does applying power to fuse F15 clear the Diagnostic Trouble Codes? If the car won’t start I might assume that your connection is unsatisfactory. If you can clear the DTC’s then you do have power at those components and we will need to look elsewhere. But check those DTC’s again.

[mowgli]

@wheeler - voltage is available at fuse 8, fuse 16 is for diesel cars and isn't present on my fusebox.

@ozvtr - your suspicions were right, thank you!
I've taken the BSM apart and discovered a burnt pin on our suspect relay The burnt one is pin 5, the normally open switch pin.
The relay is still functioning - I tested this by applying 12V across the two coil pins and heard the relay click.

In the meantime I've ordered a fuseboard for £25 on ebay, but I'll attempt to fix this board as I have it out.

Applying power to F15 doesn't clear the codes. It does power F2, so they're still connected. I'm not sure yet if the tracks around the burnt contact are dead, its a bit confusing to work out how things are connected from looking at the board. Will update as I progress.

Do you think I could have an issue past the 30A fuse which could have caused the relay contact to burn?

[mowgli]

Interestingly/annoyingly, after cleaning up the burnt pin, it still seems to be connected to the things it should be. Plus the relay still works.

[ozvtr]

The heat required to de-solder that joint will be coming from the contact inside the relay. The solder joints at the PCB might have continuity but the relay contacts will be fried!
The boards are double sided through-hole-plated. So you'll need to trace it under components.

[ozvtr]

Applying power to F15 doesn't clear the codes.
Do you think I could have an issue past the 30A fuse which could have caused the relay contact to burn?

But not blow the fuse? That tends to indicate a high load but not a short circuit. You also can not clear the DTC's. Meaning that you can power up the fuse but still not get power to all of the components!
The next logical place to look would be the electrical connector pins and connections BUT, there are 4 different circuits on 4 different pins. NOT PROBABLE that there is a heavy load on all 4 at once. You might burn out one pin on that one overloaded circuit, but then you would just loose that circuit, the others would be OK.
So the problem must be inside the BSM, between the fuse and the electrical connector, 16v NR (the 16 pin black connector) but not at the connector.
My best guess is the fuse holder. Study the solder joints of the fuse F15. Wiggle it, test it mechanically!!
Next, look for a damaged circuit track between F15 and 16VNR. Check for continuity between every part of fuse F15 and pins 1,9,16 and 10 of the connector 16VNR.

As far as an overload goes, best guess would be one of the Oxygen sensor heaters. Bad heaters wont effect the performance of the engine.
If the engine wasn't running well, maybe the coil pack breaking down. Maybe an injector. Water leaking from one of the washer nozzles is known to drip onto the No4 Injector and fry it!!
Unlikely to be the purge valve.
Not a short circuit, so not likely to be a damaged wire contacting bare metal.

[mowgli]

Thanks again for having a think and letting me know yours thoughts.

Good news on the car front - the second-hand fusebox came today so I swapped out the boards and, queue Ace ventura, SHE'S ALIIIIVE! All codes gone, job's a goodun. I'll keep in mind your advice about possible overload causes incase it happens again.

Before it arrived however I did have one more go at fixing the board. I found that the 0V side of the coil on the relay (or at least the connection on the underside of the board) wasn't receiving 0V when the "left-hand" relay was powered, which I believe it should based on my understanding of the diagram. So I tried giving it 0V directly from the 0V side of the "left-hand" relay and this did seem to provide 12V to F15 and the 4 pins on the 16way NR connector. When I hooked it up to the car, however, connecting the battery caused a strange noise to instantly start (no key in ignition). I wasn't sure what it was and didn't want to leave it connected for long enough to work it out. As the new board was arriving today I didn't want to try the ignition just in case I destroyed something in the process.

So in conclusion, this may have been the fault and my fix may have worked, but it also was causing something else to happen and as the car is now working I don't think I'll bother to ever find out :')
Either way, the relay and or its connections were certainly dodgy and everyone in this forum has been extremely helpful in providing me with a solution, so thanks for that!

[ozvtr]

You would need to fool the BSI into thinking that you have the ignition on, before you would get 0V at the two relays. The two relays cascade (for reasons).

Anyway as long as you have your car running again.