Help...gulp...bought a 2007 vrs tdi drove ok for about 100 miles then cut out on me, nown have flashing glow plug light and paranoid about driving it, bought a vcds and scanned it and got p3138 and p0642...cleared the faults and scanned again and just the p0642 is there, which means there's a few possibilities, can I pin point it using vcds, as I'm inexperienced with this tool !

I am trying to learn vcds as well. According to rosstech the p0642 is low voltage to a number of sensors but p3138 is intake manifold control which I think is one of the sensors. Check the wiring from ECU to inlet manifold for damage or corosion/looseness at terminals. Read common places for damaged wires are (taken from AI) Common culprits are shorts to ground from wiring/connectors (especially near turbo/intake/thermostat housing), a failing sensor (MAP, Boost, Fuel Rail Pressure, Throttle Position) Havent got VCDS yet but its on my list. Mate has just got it and is learning too.

Alasdair

I have just got vcds (big learning curve for me) with the p0642 circuit A (low voltage) is all the info you get, it will not pinpoint which sensor is down, if any as there are numerous sensors on this circuit...I've got to find out exactly and also a wiring diagram for the bcm which gives you the 5 voltage feed

Found this list but not sure if its the same on your car. Reccomendation is to unplug each individually with ignition off and then switch on and check if code is gone. The sensor that removes code is likely to be the problem or wiring to it. Not sure if it helps but worth a try. Check for dirt/moisture in connectors

• Manifold Pressure/Boost Sensor (G31)

• Intake Manifold Tuning Valve Position Sensor (G336)

• Low Pressure Fuel Sensor (G410)

• Fuel Rail Pressure Sensor

• Mass Air Flow (MAF) Sensor (often on this circuit)

• Throttle Position Sensor 

Engine code is the starting point for any wiring/circuit research.

Probably shown a few lines above the top of the section of your scan above, or on a sticker on the upper cambelt cover, or at the start of the engine number on V5c. Example shown below for a different engine type.

Edited 28 December, 202528 Dec by Breezy_Pete

4 hours ago, leopold72 said:

I have just got vcds (big learning curve for me) with the p0642 circuit A (low voltage) is all the info you get, it will not pinpoint which sensor is down, if any as there are numerous sensors on this circuit...I've got to find out exactly and also a wiring diagram for the bcm which gives you the 5 voltage feed

Have you carried out a load test on the battery?

No i have not, but the battery reads 12 volts and 14 with the engine running

12 volts is more than 50% discharged - what does the voltage fall to while the engine is being cranked over?

I will check tomorrow and let you know !

Just a note, the terminal voltage will register a little lower than true SOC if not all of the vehicles' control modules are in 'sleep mode' as they will be loading the battery by drawing current.

Fyi the first time the problem occurred was whilst driving, the engine cut out but restarted and drove as normal, no flashing glow plug light, I decided I needed to scan it so took the risk of driving 70 miles to pick up a vcds, which it seemed to do fine, then set off on my return journey and after quater of a mile the engine cut out again, it restarted and continued this time with a flashing glow plug symbol, after another 20 miles it cut out again, pulled over and restarted it and made it home, plugged in the vcds and got the p0642 code and haven't driven it since due to fear of breaking down....

Tested the battery... read 12.2v....ignition on 11.8....cranking flashed down to 9.? Engine running 14v

If you find the engine code I could look up the wiring relevant to the fault. Without it, I can't.

Engine no. BMN750071 cheers mate!

3 hours ago, leopold72 said:

Tested the battery... read 12.2v....ignition on 11.8....cranking flashed down to 9.? Engine running 14v

At the very least, the battery is failing.

Worth giving it a full charge, if possible, to confirm condition.

19 hours ago, leopold72 said:

Engine no. BMN750071 cheers mate!

I can't see anything in the wiring diagrams for a BMN that calls itself "Sensor reference voltage A" unfortunately. There are various 5V outputs from engine ECU to various individual sensors, and one that goes to two via a splice in the loom, G581 Position sensor for charge air pressure actuator and G212, the potentiometer of the EGR valve, both coming from pin 26 of the ECU, white/yellow wires to both sensors.

No idea if this is the specific supply that the fault code is referring to, or whether it is all of the 5V power outputs from ECU.

Not much help, sorry.

Thanks for trying, much appreciated, it's an odd one....most sensors I thought had a working range of + or - 0.2V and would still work at 4.8v...anyway on with the investigation!

The 11.3 volts indicated on the bottom line of that scan report is very likely an indication for the cause of fault. Strongly suggest a proper load test on the battery.

I put the battery on charge and it went up to 12.9 volts....with the ignition on i connected vcds cleared the fault but it remained...took it for a spin to recalibrate steering,traction and tyre pressure but within 200 meters it cut out, but restarted started OK another 400m and cut out but restarted this time with the p3138 returning....cleared the faults again and just the p0642 was back

If you keep getting P3138 I think its related to flaps in air intake/throttle body. They could be sticking or perhap failing or wiring fault. It may be the reason for the P0642 as I think its on the same circuit. Might be worth cleaning throttle body in case its carbon up. Try and unplug throttle body and remove P0642 and see if it comes back again.

Alasdair

19 hours ago, leopold72 said:

I put the battery on charge and it went up to 12.9 volts....with the ignition on i connected vcds cleared the fault but it remained...took it for a spin to recalibrate steering,traction and tyre pressure but within 200 meters it cut out, but restarted started OK another 400m and cut out but restarted this time with the p3138 returning....cleared the faults again and just the p0642 was back

12.9 volts on no load does not necessarily mean the battery is OK - after being charged, what does the voltage drop to now while the engine is being cranked?

Anything lower than about 10.5 volts while cranking means the battery is failing - the sensors and control modules will certainly malfunction if the supply voltage drops too low at any time.

Edited 31 December, 202531 Dec by Warrior193
added information

With the battery at 12.9v and ignition on cleared the faults but p0642 still remained low voltage surely the sensors would be receiving enough power, each time I ran a scan the voltage seems to fluctuate I've noticed 4.8-3,6v (sensor voltage?) and 12.6-11.4( battery voltage ?)

Falling to 11.4 volts, your battery SOC is less than 10% and considerably below the 'danger point' of discharge!

Get the battery properly tested.

Fyi....i decided that finding a fault like this is like finding a needle in a hay stack without a wiring diagram and pinout to the specifics of circuit 'A'...so this morning decided to take it to a mechanic, started the car and no flashing glow plug light ! But the cel light was on, drove really well, got it up to temp and drove it home, cleared the code which was the p0642 and the light went out....so far so good, thanks for all the input and suggestions 👍

Take a look at this: