My 2010 Octavia BKD has developed a strange issue when the fan sometimes runs and the engine idles at 1100 RPM even if the engine isn't particularly warm.

No DPF, so it's not doing a regen.

Aircon is off.

Checked the codes in VCDS, nothing coolant temperature related

Coolant flow and return values in VCDS were about 75 and 55 respectively last time it happened. Temp gauge is working normally.

Happens seemingly at random, it's been occurring for a couple of months in not particularly hot weather (I'm in the UK) so there should be no need for the fan to come on with the journeys I'm doing and temperatures the car is reaching.

Any clues?

Sounds like a failing sensor. Maybe Aircon pressure sensor even if AC is off. Next time it happens unplug it and see if fans stop.

Alasdair

Ok, so I've done a few scans and I've got the code 01598 - drive battery voltage 011 open circuit - intermittent. Apparently this can cause the fan on/high idle I'm seeing.

Putting the code into Google brings up a lot of info about bad earths and dead batteries. I actually changed the battery last month and I shined up the main battery earth while I was at it so that's ok, but the little wire beneath it is apparently the ECU earth point so will give that a good cleaning along with any others that I can find.

Check that the alternator connections etc is making good contact as well plus maybe check fuses near battery for corrosion or looseness.

Alasdair

Today I ripped into it - took out the air filter box, battery, ECU and both headlights. Then, armed with some 120 grit aluminium oxide paper and a wire brush, I shined up all the earth points I could find (two on the bulkhead by the battery, the gearbox/chassis earths, and one under each headlamp). I also shined up all the fuses and relay in the engine bay fusebox.

I inspected the ECU and wiring for signs of corrosion and damage, no smoking gun - the ECU connections looked pristine, and I measured the earth connections with a multimeter. The earth points all looked "ok" - no obvious problems, but they did all look like they were 16 years old on a car that had spent every single day outside I suppose!

I was bit annoyed that I hadn't found an obvious answer, but after re-assembling I drove it to my girlfriend's and back and the problem hasn't reared its head. Also, the voltage readout on my android head unit was a lot more stable than it had been previously. I won't call it fixed just yet, but it looks like it was worth doing!

Doesnt take much to make a bad connection and cause voltage drop. At least if it does it again you can rule out what you have done.

Alasdair

So I had about a month trouble free with nice stable voltage, then last week I saw the volts wobbling on my Android head unit, and then the fan started running again with a cold engine...

I took some inspiration from this thread: https://www.briskoda.net/forums/topic/459234-p01598-error-code-and-random-high-idle-solved/ and ran a couple of earth's from the alternator, one to the battery and a new one to bolt on the driver's side.

Problem is gone (for now at least). Must be a problem with the main engine/gearbox earth, but for the life of me I'm not sure what! No way I'm ripping out the battery and air box to get to it again, but I'll take another look when I'm next in that area (dsg oil change is next year).

The Mk 1 Octavia had a problem with corrosion at the battery end of the alternator cable. It is a crimp connection and subject to black corrosion, probably by water ingress. Your problem suggests the Mk2 may suffer the same way with time.