I have a 2006 Audi A4 Quattro 2.0T B7 chassis with an APR K04 S3 turbo and need to reach out to the community for tech support before I go about replacing a costly ECU and then having to ship it to APR to re-program it to my car.
After several days of troubleshooting, the diagnosis seems to point to an intermittently faulty ECU as everything else has been tested or replaced with the exception of the APR HP Fuel Pump solenoid (N276). I have pulled a VAG-COM log an attached it to the email and included additional information below. I have done output tests on the ECU and the J757 relay which supplies the circuit power to the N276 valve and N75 valve is not consistently responding (both relays and the N75 were replaced, see below).
The car starts rough with 4-5 cranks and throws the P codes immediately most times, if the codes are cleared it will drive fine and you can hear the solenoid responding on the fuel pump during idle. Typically, at startup it will throw the codes again, with it often going into limp mode and when in limp mode you don't hear the HP FP solenoid. When the car starts on first crank then there are typically no p-codes.
I believe this to be internal to the ECU as I recently tapped on the ECU plastic housing cover and the J757 relay began responding again to an output test, but that was a single occurrence that I have not been able to reproduce Please advise.
Summary of P Codes:
Primary Codes (most common, usually at engine start):
P0245- Solenoid Valve for Boost Pressure Control (N75): Short to Ground*
P2295-Fuel Pressure Regulator Valve (N276): Short to Ground*
Other common codes (associated with primary codes):
P2294-Fuel Pressure Regulator Valve (N276): Open Circuit
2293- Fuel Pressure Regulator Valve (N276): Mechanical Malfunction
P0243-Solenoid Valve for Boost Pressure Control (N75): Malfunction
Secondary Codes (typically follows the others):
P0135-Oxygen (Lambda) Sensor B1 S1; Heating Circuit: Malfunction
P0130-Oxygen (Lambda) Sensor B1 S1: Malfunction in Circuit
P0037-Oxygen (Lambda) Sensor B1 S2: Heating Circuit: Short to Ground
P0036-Oxygen (Lambda) Sensor B1 S2: Heating Circuit: Open Circuit
Ongoing efforts to resolve:
July-14 (5 HR): Replaced both 70 amp ECU power relays, the J757-Engine Component Current Supply Relay and the J271-Mototronic Current Supply Relay (these are typically replaced in pairs). Note, the J757 relay supplies power to the N75 wastegate solenoid and the N276 HP fuel pump solenoid. Replaced the 0.5mm dia red/blue wire that supplied ground to the J757 relay coil ground from to the ECU 94 pin main harness at terminal 32 (T94/32) due to poor continuity, see Audi Current flow diagram No. 78/3. Note, all fuses were checked and none were blown.
July-12 (1 HR): Replaced the turbocharger waste gate solenoid (N75) with a new Audi part (codes returned).
Recent Maintenance Activity (likely unrelated):
June-6: New thermostat manifold (06D-121-111-G)
May-16: New PCV valve and gasket installed: 06F-129-101-R
April-02: New serpentine belt and tensioner (06D-903-137-E and 06B-903-133-E)
April-02: Oil change 5w40 synthetic
Mar-22: 4 new Audi Coil Packs (06E-905-115-E) and 4 new OEM Spark plugs (06H-905-601-A)
Dec-29: New HPFP cam follower (worn) and replaced drivetrain lubricants
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