Thread: P0194-27 Fuel rail pressure sensor intermittent , tdv8

Ok the investigations continues, so we've had a couple of times the engine actually die out at very low rpm, not driving and those time there has been some extra errors

P0407-21 (2C) Exhaust gas recirculation sensor B circuit low - general signal failure - signal amplitude minimum
P142D-21 (2C) Exhaust gas recirculation sensor D circuit low - general signal failure - signal amplitude minimum

P0642-16 (2C) Sensor reference voltage A - circuit low - General electrical failure - circuit voltage below threshold

Car still always starts right away again.

Have also had it twice where its like its in restricted performance mode but there is no error shown on dash.

From research and thoughts, it would seem 2 likely possibilities, wire chafe shorting or sensor fault shorting (another option would be faulty ECU)

I read that multiple sensors can share the 5v reference voltage circuit. Diagnostics even reports 2 separate 5v ref, A and B.

Looking at the wiring diagrams available its not clear which sensors share which circuit, ive seen some hints on forums, but wanted something a bit more specific to my model.

So I took the ECU out from behind the battery, located the fuel pressure sensor voltage supply line, and the used a multimeter to check for connection between that pin and any other pins on the ecu.

And then look up those pins in the wiring diagrams to see what the connect to.


First thing was to determine which plug on the ECU was connector A, B & C, I worked it out as such

Screenshot_2018-11-06_at_16.50.48__59580.jpg

CO872 = CONN-A
C0411 = CONN-B
C2518 = CONN-C

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I found the following pins all connected to the Fuel pressure sensor supply voltage:

Connector C:
C2518E-F1 - fuel pressure sensor
C2518E-H1 - EGR actuator sensor A & B
C2518E-E1 - TMAP/MAPT A (temp and manifold pressure)
C2518E-A1 - actuator vane sensor VGT (variable geometry turbo)
C2518E-B1 - TMAP/MAPT B (temp and manifold pressure)

Connector B:
C0411E-D4 - Sensor Differential Pressure (this is DPF exhaust sensor which I dont have)
C0411E-E4 - Inlet Throttle A & B Sensor

Connector A:
C0872L-E1 - Accelerator pedal position sensor

Note: Connector A & B have have different Letter/Number grid orientation to C , mirrored and inverted, (they are labeled in plug)

Originally Posted by haydent

Other errors happen too from abs but I believe they are due to reduced performance mode.

What makes you think that the ABS faults are caused by reduced performance mode? I suspect they have nothing to do with reduced performance but some insight into your reasoning might be helpful.

Originally Posted by Graeme

What makes you think that the ABS faults are caused by reduced performance mode? I suspect they have nothing to do with reduced performance but some insight into your reasoning might be helpful.

Generally by their description, but then also why would a fuel pressure sensor / signal reference voltage missing error be caused by the ABS ??

Buuut.... I will admit this issue did first happen not long after replacing the calliper seals and bleeding the brakes with the IID tool procedure. The first few times it happened I just put it down to not having bled the brakes of all the air completely (which was the case). But it keeps happening even after the brakes were good.

So far I cant make a connection, and feel it maybe cooincidence, we've had a few issues with the car in recent times that im putting down to it being very low km (100,000) and hardly being driven before we bought it to now being used all the time, driving on potholed, unsealed roads and towing a caravan.


U0401-68 ( AB ) Invalid data received from engine control module/power control module
U0421-68 ( 2B ) Invalid data received from ride level control module

My next approach is going to be to set up a dedicated 5v regulator for the fuel pressure sensor, and see if that reduces/eliminates the incidence of its error. And then if so, i will one by one transfer the shared sensors from their shared circuit to the new dedicated one, so see which one starts the error again.

Here is a thread on a similar topic, DISCO3.CO.UK - View topic - LPFP replaced

I realised i identified the pins in the ecu plugs wrong initially, this has been corrected in above post, and added note about orientation.

Also while I had the ECU out it tried to find the 2nd 5v ref circuit B,

not sure this is it but the main sensors left not in REF A is the 2 cam,

i found they share a supply line with the 2nd throttle pedal position sensor

C2518E-J1 cam sensor 2
C0411E-G4 cam sensor 1
C0872L-D2 Accelerator pedal position sensor 2

Originally Posted by haydent

U0401-68 ( AB ) Invalid data received from engine control module/power control module
U0421-68 ( 2B ) Invalid data received from ride level control module

These are canbus faults, with the 2nd from the suspension module rather than ABS. The canbus faults may indeed be connected with a fuel sensor 5v supply fault.

That the fault condition disappears when coasting suggests that a wiring harness is shorting when the engine/gearbox moves in its mounts when driving.
Whilst on an outback trip I attempted to assist a D3 owner whose engine would stall if reversing other than very gently, but I didn't have facilities to properly check for a chaffed harness. The vehicle couldn't reverse down a sandhill so had to be towed back far enough to have another attempt.

Chaffed throttle pedal sensor harness? When the fault clears after coasting, can you use cruise control to drive to see if the fault returns? If it doesn't then perhaps there's a fault with the pedal sensor itself.

When i say the error clears when coasting, its the reduced performance mode that clears, and its clears once you remove your foot off the accelerator and let the rpm get under 1000rpm, its like a fail safe, you have to back off before it clears and lets you accelerate properly again to stop you just powering on, ie "reduced performance" "limp mode"

Ive even had the error while braking with no accelerator. But sure like I said initially and linked a thread on that topic wire chaffe damage is a real possibility, and we have rats around, and even found the beginnings of a nest under the bonnet at one stage, though ive got more on top of them recently with bait and cold weather.

Now that I know all the sensors that are sharing the same circuit I will locate them tommorow and best inspect the wiring connecting them before cutting any sensor wires to split off to a separate 5v supply.

I dont believe the CAN bus has anything to do with the 5v circuit. The pedal sensor actually has dual sensors one off each 5v ref A & B for redundancy, so a mismatch in that would likely generate its own error.

Also just worked out, I dont have the exhaust differential potential sensor as no DPF, I thought it was for the axle diff initially!