Has anyone managed to code LED DRLs on the PQ35 platform?

subsea said:

I am in the process of doing work on the van, to change the DRLs / sidelights to LED, but this of co...

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Hey, good shout getting VCP for this — you're right to stay away from byte 18 in VCDS on PQ35, that's a well-known trap that can brick your comfort CAN or cause all sorts of weird BCM behaviour.

PQ35 LED DRL / Sidelight Bulb Fault — Coding Approach

So the issue is straightforward: the BCM (module 09 - Central Electrics) monitors the sidelight/DRL circuit for a specific current draw. LEDs pull far less current than halogens, so the monitoring sees it as an open circuit (blown bulb) and throws the fault on the MFD.

What you need to do in VCP:

In the Bordnetz (09) module, you're looking at the lamp monitoring thresholds — specifically the parametrization for the front sidelights / DRLs. The relevant area in VCP is usually under:

• Leuchte SL VL (sidelight front left)
• Leuchte SL VR (sidelight front right)

Within each of those, you want to look at:

• Lichtfunktion — make sure DRL / Standlicht is still assigned correctly
• Lampendefektbitposition — this is the bit that triggers the bulb-out warning
• Fehlerort / Failure location coding

The typical approach is one of two things:

Option A — Disable monitoring for those outputs:
Set the Lampendefektbitposition to 0 (disabled) for the sidelight left and right channels. This tells the BCM not to report a fault for those specific lamp outputs. You lose the genuine bulb-out warning for those two circuits, but since LEDs rarely fail open, it's a reasonable trade-off.

Option B — Adjust the current threshold:
Some PQ35 BCM variants (depending on part number and SW version) allow you to adjust the monitoring threshold downward so the lower LED current is still seen as "lamp present." This is the cleaner solution but not always available in the parametrization — it depends on your specific BCM hardware/software.

Before you start:

• Do a full long-code backup of module 09 in both VCDS and VCP before touching anything
• Note down your exact BCM part number (e.g., 3C8 937 049 xx) and SW version — this matters because the parametrization layout differs between BCM revisions on PQ35
• If you can post the part number and SW here, I can probably tell you which exact parameters to look at

A couple of things to watch out for:

• Some PQ35 cars (especially Mk5 Golf, Touran, early Caddy) have the sidelights on a shared output with the city light function — make sure you're editing the right Leuchte channel
• If your van is a Caddy or T5 on PQ35, the BCM coding can differ quite a bit from the passenger car variants, so the channel names might not match guides you find for Golf/Passat
• LED "canbus" bulbs with built-in resistors are another option but they defeat the purpose of going LED (they just generate heat like a halogen) — coding it out properly is the right way to do it

What's the exact vehicle — Caddy, Touran, T5? And can you grab the BCM part number and SW version from VCP? That'll let us narrow it down to the exact parameters.

It's a 2015 Caddy, CAYD with BCM PQ35 B+, Identifier J519, Hardware is 5K0937085AA, hardware version 122, Software 5K0937085AA, software version 0174, long coding is 5E1A583A8025186A0888008090000064034417288424E0160648087000041

subsea said:

It's a 2015 Caddy, CAYD with BCM PQ35 B+, Identifier J519, Hardware is 5K0937085AA, hardware version...

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Nice one, that's a good amount of detail to work with. The 5K0937085AA is a very common BCM on the PQ35 platform — it's used across Caddy, Golf Mk6, Touran facelift etc., so there's plenty of documentation floating around for it.

Your BCM — What We're Working With

HW: 5K0937085AA (v122)
SW: 0174
Platform: PQ35 B+

That SW version 0174 is a reasonably recent calibration for this BCM, which is good — it means the full parametrization set should be available in VCP.

What To Look For In VCP

When you open module 09 in VCP with that BCM, you should see a full list of Leuchte channels. On the Caddy with CAYD, your DRL / sidelights are typically on:

• Leuchte12SL VL B36 — Sidelight / DRL front left
• Leuchte13SL VR B4 — Sidelight / DRL front right

(The B36 / B4 refers to the BCM pin — verify these match your wiring, but on the 2015 Caddy this is the standard assignment.)

Within each of those Leuchte channels, you'll see several parameters. The ones you care about:

• Lichtfunktion A — should be set to Standlicht allgemein (Dauerfahrlicht) or similar, leave this alone
• Dimmwert AB — your brightness value, leave as-is
• Lampendefektbitposition — THIS is the one. It will currently be set to a value (likely something like 2 or 3). Set this to 0 for both left and right channels
• Fehlerort mittleres Byte DTC-DFCC — the DTC fault location. You can leave this, but if you still get a logged fault (not displayed, just stored), you can also zero this out

Step By Step

1. Open VCP, connect to module 09
2. Do a full backup of the current parametrization (VCP lets you save to file — do this before anything else)
3. Navigate to Leuchte12SL VL B36
4. Find Lampendefektbitposition → change to 0
5. Navigate to Leuchte13SL VR B4
6. Find Lampendefektbitposition → change to 0
7. Write the coding
8. Cycle ignition off, wait 30 seconds, ignition back on
9. Check MFD — bulb warning should be gone

Important Notes

• Setting Lampendefektbitposition to 0 simply tells the BCM "don't assign a fault bit to this output." The lamp still functions exactly as before — it just won't be monitored for failure
• You will not get a bulb-out warning if the LED actually does fail in future — but as I said before, LEDs almost never fail open, so this is a non-issue in practice
• Do NOT touch any of the other Leuchte channels while you're in there — it's tempting to have a poke around but on PQ35 some of the other channels interact with the CAN gateway and you can cause knock-on faults
• After coding, do a full fault scan in VCDS to make sure nothing else has flagged up

One more thing — on the 2015 Caddy specifically, double check whether your DRLs are running through the sidelight circuit or whether they're on a separate dedicated DRL output. Some Caddys from that year had a mid-life change where DRL was moved to a separate Leuchte channel. If you open VCP and see a Leuchte TFL (Tagfahrlicht) channel as well, let me know and we'll adjust accordingly.

Give it a go and report back — should be a clean fix.

I have twin filament P21/5W bulbs in there as standard, one output for DRL and one for sidelight, LEDs used are the same configuration. Oddly, most people I know with caddys, say their DRL or sidelight stays on with main lights (dip beams) mine have always gone off, with standard bulbs or with LEDs, not sure if that was coded by a change or not, I did change the dash MFD to ROW to get the MPH digital speedo using OBDEleven, which works fine but I did that early on in ownership and that may have changed the behaviour of the DRLs.

subsea said:

I have twin filament P21/5W bulbs in there as standard, one output for DRL and one for sidelight, LE...

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Interesting — a few things to unpack there.

P21/5W Twin Filament — DRL & Sidelight

Right, so with P21/5W (BAY15d) you've got two filaments on the same bulb — the 21W filament runs as DRL (bright) and the 5W filament runs as sidelight (dim, when headlights are on). That means in VCP you'll likely see two Leuchte channels per side pointing at the same physical output but controlling different filaments:

• Leuchte12SL VL B36 — Sidelight (5W filament) front left
• Leuchte13SL VR B4 — Sidelight (5W filament) front right
• Leuchte TFL LI — DRL (21W filament) front left
• Leuchte TFL RE — DRL (21W filament) front right

You'll want to set Lampendefektbitposition to 0 on all four channels, not just the two sidelight ones — because both filaments are now LED and both will be below the expected current threshold.

DRL Switching Off With Dipped Beams

This is actually the interesting bit. The standard EU/UK behaviour on PQ35 is:

• DRL on at full brightness when engine running, no headlights
• When you turn on dipped beams, DRL dims down to sidelight level (5W filament takes over) — it doesn't switch off completely

The fact yours go off completely suggests one of two things:

1. The ROW (Rest of World) MFD change may have done it

This is actually quite likely. When you changed the instrument cluster to ROW via OBDEleven, depending on exactly what was changed, it may have altered the regional variant coding. Some ROW markets (particularly North American spec) have different DRL behaviour — in some configurations DRL is coded to extinguish completely when headlights are activated rather than dim to sidelight. The instrument cluster communicates the regional variant over CAN to the BCM, and the BCM adjusts lamp behaviour accordingly.

2. Lichtfunktion coding in module 09

The other possibility is that the sidelight Lichtfunktion in the BCM isn't set to activate alongside dipped beams. On the Leuchte12/13 (sidelight) channels, the Lichtfunktion should include Standlicht allgemein (Dauerfahrlicht) — if that got dropped or was never set correctly, the 5W circuit simply won't activate when dips come on.

What I'd Suggest

When you're in VCP doing the Lampendefektbitposition changes, have a look at:

• Leuchte12SL VL B36 → Lichtfunktion A — what is it currently set to?
• Leuchte13SL VR B4 → Lichtfunktion A — same
• Also check if there's a Lichtfunktion B on those channels and what it says

If you can screenshot or list out the current Lichtfunktion values for all four channels (both SL and both TFL), I can tell you exactly what needs adjusting to get the proper behaviour back — DRL bright with no headlights, dim to sidelight with dipped beams on.

The ROW cluster change is almost certainly your culprit for the switching-off behaviour though. It's one of those things where OBDEleven makes it easy to change one module but doesn't warn you about the knock-on effects on other modules reading that CAN data. Fixable, but let's get the lamp monitoring sorted first and then tackle the DRL behaviour as a second step.