P0091

Fuel Pressure Regulator 1 Control Circuit Low

P0091 is a generic OBD-II powertrain diagnostic trouble code: Fuel Pressure Regulator 1 Control Circuit Low. It is logged by the engine control unit when the fuel/inj monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
P0091
Group
Powertrain
System
Fuel/Inj
Severity
Warning (MIL on, possible limp mode)
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What P0091 means

P0091 is stored when the PCM detects that the voltage in the Fuel Pressure Regulator 1 control circuit is lower than the minimum acceptable threshold. The fuel pressure regulator (FPR) on modern common-rail diesel and GDI engines is an electrically operated solenoid valve — the PCM commands rail pressure by varying the duty cycle applied to the solenoid coil. A low-circuit condition means the driver circuit is seeing near-zero or zero voltage where it should see a pulsed signal, typically caused by an internal solenoid winding short to ground, a control wire shorted to chassis ground, or a failed PCM output driver circuit.

Without a functional pressure regulator command signal, the PCM loses the ability to reduce rail pressure below the maximum pump delivery. Depending on the engine architecture, this can result in runaway rail pressure (similar to P0088), or conversely, if the solenoid defaults to a fully-open bleed position, the rail may lose pressure entirely and cause a no-start or stall. The PCM recognises the invalid circuit state and typically commands a failsafe reduced injection quantity, resulting in limp mode or engine shutdown.

The solenoid may be integrated into the high-pressure pump body (as a metering/suction control valve) or mounted on the fuel rail as a separate proportional valve. Either location means the fault has direct authority over fuelling — this is a severe, high-priority code that warrants prompt repair.

Common causes

Most-frequently reported root causes when P0091 is logged.

  • 1
    Fuel pressure regulator solenoid coil internal short circuit, causing the winding to draw the signal line to near ground potential.
  • 2
    Control wire between the PCM and the regulator solenoid shorted to vehicle chassis ground.
  • 3
    Corroded or water-contaminated solenoid connector with low-resistance bridging between the control pin and a grounded surface.
  • 4
    Failed PCM output driver transistor for the FPR circuit, presenting the circuit low condition on the PCM side.
  • 5
    Broken or detached ground return wire causing the PCM to detect an abnormal circuit completion path.
  • 6
    Fuel pressure regulator solenoid mechanically seized in the closed or partially-open position, combined with an electrical fault.

Symptoms drivers notice

Check Engine Light (MIL) illuminated.
Engine enters limp mode with noticeably reduced power and throttle response.
Hard start or no-start condition — particularly on cold starts where precise rail pressure control is required.
Engine stalling at idle as rail pressure drops below the minimum needed for injector operation.
Rough running, misfires, or hesitation during acceleration due to unregulated fuel pressure.
Possible black smoke (diesel) or rich running condition if rail pressure defaults high.

How to diagnose P0091

A typical diagnostic flow when this code is present.

  1. 1
    Connect an OBD-II scanner and record freeze-frame data; note whether any companion fuel pressure codes (P0087, P0088, P0090) are also stored, as these indicate the pressure consequence of the regulator circuit failure.
  2. 2
    Visually inspect the FPR solenoid connector and harness for chafing, moisture intrusion, or corrosion; pay particular attention to routing near exhaust components and heat shields.
  3. 3
    With the key on and engine off, measure the voltage at the FPR solenoid control terminal relative to chassis ground — a reading near 0 V when the PCM is not commanding the solenoid confirms a persistent ground short.
  4. 4
    Disconnect the solenoid connector and measure solenoid coil resistance with a digital multimeter; compare to manufacturer specification (typically 2–10 Ω) — a reading near 0 Ω confirms an internal winding short.
  5. 5
    With the solenoid disconnected, re-check the harness-side control wire resistance to chassis ground; if it reads near 0 Ω, trace and repair the ground fault in the wiring harness.
  6. 6
    If wiring and solenoid test good, verify PCM output driver function using an oscilloscope on the control wire during cranking — absent or flat signal with no circuit faults indicates a failed PCM driver.
  7. 7
    After any repair, clear codes, start the engine, and monitor live rail pressure data to confirm the regulator is responding correctly to PCM commands before returning to service.

Vehicles where we've handled P0091

Platforms in our catalogue with confirmed P0091 coverage.

AUDI A4 20D
AUDI A6
2015
VW TRANSPORTER
AUDI A7 30D
AUDI A6 30D
2015
AUDI A7

Related powertrain codes

Frequently asked questions

What is the difference between P0091 (circuit low) and P0090 (control circuit)?

P0090 indicates a general control circuit fault without directional information. P0091 specifically means the circuit voltage is measurably low — pointing toward a ground short or internal solenoid winding short. P0092 (circuit high) would suggest an open circuit or short to supply voltage. The directional code narrows diagnostics significantly.

Can P0091 cause a no-start?

Yes. If the regulator solenoid is shorted internally and defaults the high-pressure pump to minimum or maximum delivery, the rail may not reach the minimum pressure required for injector operation, preventing the engine from starting. This is more common on cold starts when pressure demand is highest.

Is the fuel pressure regulator the same as the fuel pressure sensor?

No. The sensor (FRP sensor) measures rail pressure and sends a feedback voltage to the PCM. The regulator is an actuator — a solenoid-controlled valve that the PCM commands to vary pump output or bleed rail pressure. P0091 is strictly a regulator actuator circuit fault; the sensor is a separate component covered by different codes (P0192/P0193 etc.).

Should I clear P0091 and re-test to see if it comes back?

Only after a thorough inspection. Clearing the code without identifying the root cause risks a repeat fault mid-drive. If the solenoid is internally shorted, re-clearing will not resolve the loss of pressure control, and repeated limp-mode events can damage other fuel system components. Diagnose first, then clear and verify.

Disabling P0091 in software

RaceTune can permanently disable P0091 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.

Permanent
The monitor is disabled in the ECU itself — not just cleared. It cannot return.
Tailored to your file
Each patch is matched to your specific software version — never a one-size-fits-all file.
Reversible
The original file is always preserved. Reflash the stock to return the ECU to factory state.

ECUs with a P0091 disable in our catalogue

Confirmed coverage from our recipe database — we support many more families. Upload your file and our identifier will match it automatically.

  • Bosch EDC17C74 verified 2 software versions
  • Bosch EDC17CP44 verified 1 software version

Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.

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