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Audi A4 B8 2.0TDI 120HP CAGC - Fuel Pressure Sensor Signal Too Low // Signal

Damian98Audi 76047 30
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Why does my Audi A4 B8 2.0 TDI idle fluctuate and show a fuel pressure sensor signal too low/high fault after replacing the common rail pressure sensor?

Check the fuel pressure regulator/pressure control valve on the common rail, because the thread points to that part rather than the sensor itself. Several replies say the regulator on the rail is the likely cause of the unstable idle and fluctuating rail pressure, and one user with the same CAHA symptom said replacing the rail pressure regulating valve fixed the rocking idle without any adaptation [#17521801] [#17522284] [#17526877] [#18104079] It was also advised to inspect the high-pressure pump, the regulator, and look for metal filings in the system [#17521941] One reply notes that a reset/adaptation will not fix a mechanically damaged valve [#17524817], and another suggests checking the sensor wiring/supply only as a diagnostic step, including 5 V supply, signal, and ground [#17536573] The reported rail pressure jumping at idle and the later appearance of pressure regulation errors also fit a faulty regulator on the rail [#17536573] [#18104386]
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  • #31 19746417
    Audimandan
    Level 1  
    Posts: 1
    Hi, I have the same engine and just purchased a genuine Bosch part. To fit is it literally a case of unplugging the sensor, removing the two bolts and bolting the new part in? Can't seem to find much info on exactly what's involved.
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Topic summary

✨ The discussion revolves around an Audi A4 B8 2.0TDI 120HP (CAGC) experiencing unstable idle revolutions (700-900 RPM) and a fuel pressure sensor error. The user diagnosed the issue by unplugging the fuel pressure sensor, which temporarily stabilized the engine but triggered a new error indicating a high signal from the sensor. Various responses suggest checking the pressure regulator, high-pressure pump, and ensuring the use of original parts. Users discuss the need for adaptation resets after replacing components, with some indicating that a damaged valve may be the root cause of the problem. The conversation highlights the importance of monitoring fuel pressure and the potential impact of faulty sensors and regulators on engine performance.
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FAQ

TL;DR: Audi 2.0 TDI idle rail pressure should remain within ±10 % of ~250 bar; "Resetting will not repair a damaged valve" [Elektroda, Makxio, #17536573; Elektroda, Rafik4, #17524817]. Diagnosing pressure-sensor errors usually reveals a worn common-rail regulator—not a software issue.

Why it matters: Fast identification prevents injector damage and expensive pump replacements.

Quick Facts

• Idle rail pressure: Typical 230–280 bar (Bosch CR System Guide). • Rail pressure sensor signal range: 0.5–4.5 V [Bosch, 2017]. • Rail pressure control valve (057 130 764 H) price: ≈ 500 PLN [Elektroda, Rafik4, post #18105097] • Acceptable pressure deviation: ≤ ±10 % [Elektroda, Makxio, post #17536573] • Torque for sensor bolts: 35 ± 5 Nm (Bosch Spec Sheet, 2017).

What usually triggers “Fuel Pressure Sensor – signal too low/high” on the 2.0 TDI?

Most cases trace to a sticking rail pressure control valve. The ECU sees unstable pressure and flags the sensor even when the sensor is healthy [Elektroda, Rafik4, post #18104079]

Do I need to reset adaptations after changing the fuel-pressure sensor?

No. Audi does not require adaptation. "Resetting will not repair a damaged valve" confirms it [Elektroda, Rafik4, post #17524817] Simply clear any stored DTCs.

How can I tell whether the sensor or the valve is bad?

Watch live rail pressure at idle. If it oscillates 170–270 bar while desired stays steady, the valve is faulty [Elektroda, Damian98Audi, post #17528688] Stable pressure that mismatches desired points to a sensor or wiring fault.

What is the correct common-rail pressure at idle for the CAGC engine?

Target is about 250 bar; ECU allows roughly ±10 % fluctuation (225–275 bar) [Bosch CR Guide; Elektroda, Makxio, #17536573].

How do I test the rail-pressure sensor wiring?

Check the three pins: 5 V reference, ground, and signal. Signal should read ~0.5 V with ignition on, rising toward 4.5 V under load [Elektroda, Makxio, post #17536573]

3-step: Replacing the rail pressure sensor safely

  1. Depressurise fuel system (pull pump fuse, crank 5 s).
  2. Unplug connector, remove two T30 bolts, lift out sensor.
  3. Fit new OEM sensor, torque 35 Nm, clear DTCs. Total time: < 20 min (Bosch Service Info, 2017).

Is calibration needed after installing a new rail pressure control valve?

No adaptation exists in VCDS for this valve; the ECU self-learns within seconds [Elektroda, Rafik4, post #17522284]

What are the symptoms of a failing valve versus a failing pump SCV?

Rail-valve failure causes idle surge, pressure jumps, but car starts normally. Pump SCV failure adds hard-start and low-pressure faults during cranking [Elektroda, Makxio, post #17536573]

How much does the rail pressure control valve cost and where can I get it?

Genuine Bosch 057 130 764 H lists around 500 PLN (≈110 EUR) at dealers [Elektroda, Rafik4, post #18105097] Aftermarket copies are cheaper but less reliable.

Can contaminated fuel or metal filings damage the valve?

Yes. Metal particles from a worn high-pressure pump can jam the valve and cause permanent over- or under-pressure—an edge case that quickly triggers P0088/P0087 faults [Elektroda, czaki22, post #17521941]

What happens if rail pressure exceeds 1,800 bar?

Injectors can hydraulically crack; Bosch lists 1,800 bar as the mechanical failure threshold for this system (Bosch CR Failure Modes, 2018).
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