UK-POLOS.NET - THE VW Polo Forum

Been a long time since I've posted here, but I still have my Polo!

I've recently run into some strange issues after removing and reinstalling the rear lights to do some cleaning behind the lights. I have done this a couple of times before while owning this car. Every time that I do this it has resulted in a fault code, and I have always just plugged in my OBDeleven and cleared the fault code and everything has been fine after that. But this time it's different. The "03431 - Activation of left parking light Open circuit" fault code keeps reappearing after I reset it.

It sounds obvious: "if the open circuit code keeps popping up after reinstalling the rear light, obviously the light isn't plugged in properly, duh". But the light is plugged in (I have unplugged it and plugged it back in properly to make sure). And the light turns on and stays on as it should, I have tried shaking and knocking on the rear light, pushing on the connector, slightly flexing the cable etc. while the light is on to try and see if there is a loose connection. I am not able to provoke a loose connection, the connection seems to be solid and the light stays on. But still, when I drive the car the warning light in the dash will come on within a couple of minutes/kilometers, even when driving on smooth roads.

Another strange thing is the circumstances when the warning light comes on. I have found is that if I run with driving lights on (light switch set to dipped beam, which means that rear lights are ON) then I will NOT get the warning light in the dash while driving. However if I run with with the driving lights off (which means front DRL is ON, rear lights are OFF) then the warning light will come on within a couple of minutes/kilometers. This seems really odd to me, that the diagnostics are detecting the light circuit to be open while the rear lights are not even turned on.

I would very much appreciate some advice, this is driving me crazy!

Here is a fresh readout done today using OBDeleven:

09 Central Electrics
System description: BCM PQ25 Max4
Software number: 7H0937090B
Software version: 0712
Hardware number: 7H0937090
Hardware version: H74
Serial number: 0571 245130969
Long coding: 24B029BF08E1240093280F9176EB87D15AC0672F600D30C1200000030000
Subsystems:
System description: RLS 070813 05
Software number: 1K0955559AH
Software version: 0403
Hardware version: 54
Long coding: 07302D
System description: J386__TSG_FT
Software number: 6R0959801AT
Software version: 8620
Hardware version: 003
Serial number: KU0KU023081300012257
Long coding: 0000A0
System description: J387__TSG_BT
Software number: 6R0959802BE
Software version: 8620
Hardware version: 003
Serial number: KU0KU024081300011552
Long coding: 1000A0
System description: DWA-Sensor 00
Software number: 1K8951171A
Software version: 0103
Hardware version: 001
Long coding: 000000
System description: LIN BACKUP HO
Software number: 1K0951605C
Software version: 1501
Hardware version: H05
Long coding: 000000
System description: DWA-Sensor 00
Software number: 1K8951171A
Software version: 0103
Hardware version: 001
Long coding: 000000
System description: E221__MFL-TAS
Software number: 5K0959542C
Software version: 0037
Hardware version: H31
Serial number: 0174508
Long coding: 820000
Trouble codes:
03431 - Activation of left parking light Open circuit
Intermittent
Priority: 2
Frequency counter: 254
Driving cycle: 82
Mileage: 141618 km


I am also confused about the "frequency counter" value, I've only ever seen the values 254 and 255. The "driving cycle" have been showing varying values in the 70-80 range.

The car has OEM Auto light switch retrofitted using a harness from ciclo here on this forum. Fantastic product, I have been running with it in this car for many years, and it has always worked 100%. I do not think it has anything to do with my issue, I am just mentioning it in case someone is wondering why the long coding looks the way it does.

are you still using the original lights? might be a bad bulb. happens a lot.

iichel wrote: Mon Aug 12, 2024 5:22 pm are you still using the original lights? might be a bad bulb. happens a lot.

I am, actually. Haven't had to change a single bulb on this car apart from the xenons (after one of the original xenons started turning off intermittently after only 3 years or so).

But yeah, changing the bulb is probably the next step. It won't hurt and it's an easy thing to do.

As I mentioned, I have tried provoking a bad connection manually, but of course that's not a foolproof method.

Still I cannot understand how an intermittent connection would cause this behavior, that the warning light comes on when driving with the rear lights OFF but not when driving with rear lights ON...

Hi cheba, good to know you still have your WRC.
Hopefully we will have luck with the bulb change before thinking about other possible causes due to the diagnosis made by your BCM (which is correct).

Filament bulbs sometimes last a little longer than they should before dying (like me ).

ciclo wrote: Mon Aug 12, 2024 6:44 pm Hi cheba, good to know you still have your WRC.
Hopefully we will have luck with the bulb change before thinking about other possible causes due to the diagnosis made by your BCM (which is correct).

Filament bulbs sometimes last a little longer than they should before dying (like me ).

I got to hold onto it, they don't make the small cars quite like this anymore! (well, Toyota makes the GR Yaris but that is an exception...)

I'll go get a pair of new bulbs this week and we'll see.

Today when I got into the car after it had been sitting for more than 24 hours, I turned the ignition on (with driving lights OFF) and I got the warning light in the dash immediately, without any delay. I turned the driving lights on, and saw that the left rear light did not light up. I lightly tapped the outside of the light, and the light lit up. So there was indeed a bad connection, probably a bad bulb. I put a new bulb in, cleared the fault code and went for a short test drive. Hopefully it will be okay now.

So what I've learnt is that the light diagnostics are active also when the lights are turned off. Maybe not so surprising, it just has to measure the resistance in the circuit without applying a high enough voltage to light up the bulb. I just assumed that the diagnostics were very basic and were only active when the lights were actually turned on.

what you saw was probably 'cold' diagnostics when the car is checking the lights when they are off.

Even though you changed the bulb, keep an eye on the fault in routine/daily use and please let us know if it occurs again.
Thank you for keeping us informed.