cogley_efc

I have an A4 B9 Ultra SE 2.0 diesel and on startup I am intermittently getting some warning lights / system failure warnings. The most common are parking brake failure and cruise control system unavailable and I have also had start/stop system failure.

The frequency was initially every couple of weeks on startup but it now appears to be every other day. Once I turn the car off and restart the lights clear. There are no issues when driving and the systems that display the warning lights work fine. I’m assuming it is some sort of electrical fault and wondering if anyone had experienced anything similar before?

I also have a left heated mirror not working and issues with my phone connecting on CarPlay but not sure if these are related.

any help appreciated.

SMac770

Those dash messages are not specifically informative. You're going to have to scan for DTCs to see what the actual problems are.

A4Audi4Fun

What year is your A4? if one of the early years, maybe you are getting to a point where battery condition is having an impact.

cogley_efc

Hi both,

just some further context it’s a 2016 (65) reg.

I initially suspected the battery as it failed to start earlier in the year. The RAC came out and ran diagnostics and said the battery was possibly the cause. I have battery cover and they wouldn’t change it as they managed to get it going again and I was reluctant to pay £200 for a new battery since I had the cover. They will only change it if the battery is dead so I was only going down this route if it fully failed - but so far it hasn’t although it does struggle to start on a cold morning.

The diagnostics did throw up fault code U112300, CAN communication fault and U112100, CAN-Bus Failure. Would these potentially be caused by the battery?

SMac770

"it’s a 2016 (65) reg", UK eh? That's meaningless info from a car perspective. It would be more informative to provide the 7th-10th characters of the VIN. That tells anyone anywhere actual specifics about the vehicle (model type and model year).
The early B9 have a lot of random comms bus errors listed in scans all the time it seems. Is it worthwhile errors, or just noise? Hard to say.
People toss out the "could be the battery" all the time. Either it is or it isn't, but you use real information to decide that, not "well why not". But the charging system is computer controlled. And like having the scan tool so you can get the real error codes instead of the simplistic dash messages, you also need the scan tool to properly query the measuring values of the gateway module to see anything regarding the battery health. I advise Ross-Tech VCDS, but people use that OBDeleven thing too.

142 guy

Audis seem to be acutely sensitive to system voltage. Since this is a start-up related problem it is possible that during cranking of the starter motor the vehicle voltage is dropping enough to generate random errors. 25 + years ago before cars became heavily integrated devices with LANs, the electronics was concentrated in the power train control module. All the sensors typically ran off of a 5 volt or 3.3 volt sanctioned power supply in the power train control module and if the better OEMs used low drop out regulators. The mission critical electronics would typically operate down to about 6 - 7 volts, long after all the other stuff had crapped out. I expect that those days are now history.

Those U code faults on a B9 seem to be like freckles, everybody seems to get them and they don't necessarily mean anything.

If you had VCDS you could log the vehicle voltage during start up to see how low it drops. You can also do this with a stand alone voltage data logger. If you have a laptop, VCDS is likely cheaper and easier to use. If you don't want to get VCDS take the car to an automotive electrical specialist and have them do a load test on the battery. The old school giant toaster oven testers are the best. The handheld units that inject a high frequency signal into the battery to attempt to measure resistance can give misleading results. A correctly done load test will indicate whether the battery is near its end.

If this car was built / delivered in 2016 and it is still on the same battery, stop fiddling around and just purchase a new battery. 7 years on a battery is probably well passed the mean time to failure for a battery on a B9. If you do get a new battery, be aware that the charging system has to be adapted to the new battery. Just stick in a battery without doing the adaptation and you may find that the new battery has a relatively short life.