Pressure Sensor for Brake Boost (G294): Implausible Signal Fault

Andrew said:
I don't know but the evidence points that way.

An initial peruse of forum posts including G294 over the years are all from FSI owners.

The wiring diagram shows you posted is identified as No 41/6, the start of section 41 clearly identifies it as relevant to the BAD engine code - For Model year 2004 and the only other place G294 pops up is section 3 which is the same for Model year 2003, at least in my copy of the wiring diagram.

Andy
Click to expand...
Andy,
I think the NOx sensor connection is coincidental.Two of the three G294 wires, (a & b), are the 5 Volt supply and Grnd for the G294, and also for several other sensor, including the NOx sensor. These sensors are active sensor ("electronic"), rather than simple resistive sensors.
So,( if it actually exists), the G294 is measuring a pressure, that the ECU uses, (or not).
The mystery is where, on an FSI, it is fitted, and what it does.
Mac.
 
I believe the g294 could be a red herring.
I had 17192 reoccur with 18322. Both at the same time…
Unplug the NOX… neither of those faults occur but obviously get replaced with an open circuit fault for the NOX sensor.
When my proboost ecu turns up it will be interesting to see if I see any of these codes again.
 
SportyGru said:
I believe the g294 could be a red herring.
I had 17192 reoccur with 18322. Both at the same time…
Unplug the NOX… neither of those faults occur but obviously get replaced with an open circuit fault for the NOX sensor.
When my proboost ecu turns up it will be interesting to see if I see any of these codes again.
Click to expand...
Certainly "fishy"
Mac.
 
I think it’s safe to conclude the G294 fault is indeed a glitch from the factory programming.

If you get this fault - it’s the NOX sensor or is associated wiring.
 
SportyGru said:
If the FSI schematic is correct, and the G294 wires are in the loom, but there's no G294 sensor, then the sensor output is connected to the ECU. That wire becomes an antenna, and will pick up electrical noise, fooling the ECU into logging a G294 DTC. The NOx sensor is not the only sensor sharing the 5 volt supply with the phantom G294. Mac.
Click to expand...
 
Ok - mystery continues …
I have installed the proboost ecu and I’m still getting the 17912 and 18322 codes…
So I’m guessing the NoX sensor I thought was mapped out is not the one causing me grief …
Running diagnostics nothing is looking untoward. Idle is actually a little lumpier after fitting the proboost ecu. From previous experience im leading to an o2 sensor but I realise there are two… and then the Nox one which i believe is the one just forward if the Cat under the car.

Any pointers much appreciated
 
Both of those codes are pointing to a vacuum leak. Check the oil filler cap, it has a vacuum pipe that runs to the maf sensor. It should be tight in its location and not able to rock back and forth.

The brake pressure may also be affected by the lack of vacuum as the servo relies on vacuum pressure to function.
 
Thanks Steve, I know the oil filler cap end is ok but I’ll check where it goes to..
I did have a spurious NOX fault come up at one point as well. Prior to changing the ecu.
After some reading I think the Nox sensor also does the downstream lambda reading as well. The exhaust pipe has been replaced here and I’m wondering if they damaged the wiring or the probe when it was removed.
I will see what values I get with blocks 30,32 and 32 and see if there’s an obvious abnormality
 
The Nox is combined with the lambda sensor on the main cat with the FSI. There's also a lambda sensor on the pre-cat just off the exhaust manifold.

The main nox sensor is redundant with the PoBoost, so if you are still getting an oxygen related fault then it's more likely to be the pre-cat one.
 
A2Steve said:
Both of those codes are pointing to a vacuum leak. Check the oil filler cap, it has a vacuum pipe that runs to the maf sensor. It should be tight in its location and not able to rock back and forth.

The brake pressure may also be affected by the lack of vacuum as the servo relies on vacuum pressure to function.
Click to expand...
Vacuum, on the A2, is used, for the brake servo, the ABS pressure, and in the FSI, also for the inlet flap actuator. That is why I wasn't surprised to see provision for the (phantom?) G294.
Since ProBoost doesn't use inlet flaps, the vacuum connection to the actuator could be, maybe should be, blanked of at the source.
One less vac leak to cause trouble. The actuator could be the leak of course.
Mac.
 
Whilst I had the vacuum and pcv parts removed from my manifold to clean, replace O rings and install correctly I did note a sensor on the brake vacuum line.
Could this be the G294?
I’ll see if I can find what I saw in a parts diagram
 
Part number
8Z2611931G
2B607335-BDE4-4802-86ED-9CDC3FFCB37D.jpeg
 
SportyGru said:
Whilst I had the vacuum and pcv parts removed from my manifold to clean, replace O rings and install correctly I did note a sensor on the brake vacuum line.
Could this be the G294?
I’ll see if I can find what I saw in a parts diagram
Click to expand...
Sounds promising, can't believe the wiring diag is duff. The FSI has a big load on the vacuum, drives the servo, and the abs pressure reserve, as well as the flap actuator.
Maybe Audi were concerned about the effects of the actuator diaphragm failing.
Just seen the proof, well spotted, another FSI mystery solved!
Mac.
 
Hi everyone..
Hopefully this will wrap this up.
The G294 is indeed installed on the brake vacuum line to the inlet manifold as pictured above.
Although replacement is not written in the a2 manuals. Other Vag models share the same item and it’s fitted in the same way.
The part is Bosch 0261230081. Not to be confused with a similar part that has little anti crush rings fitted in the bolt holes.

This type just clips on as per picture attached.

It is just a map sensor and the engine takes a reading in channel 08 block 4. I believe this sensor is also used as the G71 on the FSI as no other pressure sensor is fitted yet the management uses its reading.

Replacement is a little fiddly in situ and easiest on a ramp. You need to compress the clips that go through the sensor hole and pry it apart gently.

Hope this helps others with the G294 mystery.
 

Attachments

  • B47E716F-C53D-4041-AF7E-28F108A5FCD1.jpeg
    B47E716F-C53D-4041-AF7E-28F108A5FCD1.jpeg
    70.4 KB · Views: 109
You must log in or register to reply here.
Back
Top