VW T6 cranking but not starting due to failed ECU output

The Problem

A VW T6 Transporter (2.0 TDI — confirm your exact engine code, e.g. CXHA/CXFA/DFSC, as fuel metering valve wiring colours can differ between variants) was cranking but not starting, with the fuel rail only building 12–15 bar instead of the expected operating pressure. For these common-rail engines the rail must build to roughly 200–300 bar during cranking before the ECU will allow the engine to fire (verify the exact cranking and idle target pressure for your engine code with VCDS measuring blocks or VW dealer data — 12–15 bar is far too low to start). This followed water ingress through the EGR and subsequent replacement of the HPFP, fuel rail regulator/sensor, and cambelt/water pump, and VCDS was reporting low system voltage and multiple databus faults despite a new battery.

Solution

Note on model year: this procedure was carried out on a pre-facelift T6 (approximately 2015–2019). The T6.1 (2019 onwards) uses a revised fuse and relay layout, so the SB/SA fuse box references below may not map cleanly to later build years — confirm your van's fuse box layout against the correct VCDS wiring diagram for your VIN before tracing circuits.

1. Using a wiring diagram for your specific engine variant, identify the two wires on the fuel metering valve: Blue/Red (direct ECU feed) and Blue/Black (feed from fuse SB12, a 10A fuse in the engine bay fuse box near the front headlight). The Blue/Red and Blue/Black colour references apply to the 2.0 TDI as fitted to this pre-facelift T6 — verify the colours against the diagram for your own engine code, as they may differ between units. Check continuity on the Blue/Black wire first, as this was found to be open in this case.

   • The SB fuse box sits near the front headlight; the SA fuse box is under the battery — do not confuse them. (This SB/SA arrangement applies to pre-2019 T6; confirm for your build year.)

2. Disconnect the fuel pressure regulator connector from the harness and apply a 9V battery directly to the regulator terminals to confirm whether the regulator itself is functional. If fuel pressure rises to the correct operating level (around 200–300 bar at cranking — verify the target for your engine) immediately, the fault lies upstream in the supply circuit or at the ECU output.

3. With the regulator confirmed good, verify that the ECU is outputting voltage on the negative (switching) side of the fuel pressure regulator circuit. In this case the ECU was outputting no voltage, pointing to an internal ECU fault rather than a wiring issue.

4. Send the ECU to a specialist for bench testing and repair of the failed internal driver/output channel responsible for controlling the fuel pressure regulator.

   • In this case the repair was carried out under warranty.

5. Refit the repaired ECU, start the engine, and clear all fault codes, then recheck which codes return. A persistent code 24840 / P1007 (torque difference cylinder 4) after repair points to a potentially stuck injector on cylinder 4, which needs separate diagnosis via an injector circuit check or bench test.

   • Connect a second battery or charger during all diagnostic sessions to keep voltage above 12V; the multiple databus and communication faults seen here were caused by low cranking voltage (11.7V) and will clear once voltage is stable.