Comprehensive diagnostic guide for OBD-II code P0191

What This Code Means

• P0191 = Fuel Rail Pressure Sensor A Circuit Range/Performance. This corresponds to the Fuel Rail Pressure Sensor Circuit Range/Performance on Wikipedia.

Context from the real world

• Symptoms tied to P0191 in actual complaints include:
  • Rough idle, engine light on, and stalling concerns.
  • Engine shut-down behavior with the check engine light on in some cases; lean-condition codes (P0171/P0172) also appear in the same complaints.
  • In several complaints, the reports involve engine performance issues during turns or at idle, with intermittent loss of drive/reliability until the issue is addressed.
• Takeaway from the NHTSA data: P0191 in these complaints is commonly associated with fuel delivery/pressure concerns and lean-condition codes, and it can coincide with idle instability and potential engine shutdown in some situations. See complaints: Complaints 1 and 3.

Technical Context

• P0191 indicates a fault in the fuel rail pressure sensor circuit or the sensor itself, i.e., the sensor's range or performance is outside expected parameters or the circuit (signal, reference, or ground) is not behaving properly. This aligns with the fuel rail pressure/sensor theme described in the Open Source code definition.
• OBD-II and Powertrain code concepts provide a framework for diagnosing this as a fuel-supply/fuel-pressure-related issue rather than a completely unrelated system fault. See the general OBD-II discussion of Diagnostic Trouble Codes and Powertrain Codes on Wikipedia.

Probable Causes

• Primary/Direct causes (most commonly implicated with P0191):
  • Fuel rail pressure sensor (FRPS) or its wiring/circuit: sensor failure, poor 5-volt reference, bad ground, or faulty signal wiring.
  • Sensor circuit issues causing out-of-range or erratic readings that trip P0191.
  • These direct sensor/circuit problems are reflected by the presence of P0191 in Complaints 1 and 3.
    Estimated share based on complaint patterns: about 50%.
• Secondary/associated causes (often tied to fuel delivery or rail pressure):
  • Fuel pump performance, failing/weak pump, or fuel delivery pressure not meeting commanded rail pressure.
  • Fuel rail pressure regulator or fuel filter/restrictions that prevent proper rail pressure, contributing to lean conditions (P0171/P0172) seen in Complaints 1 and 3.
    Estimated share based on complaint patterns: about 40%.
• Electrical/wiring/harness issues (outside of the FRPS sensor itself but affecting rail pressure readings):
  • Corroded/loose connectors, damaged wiring harnesses around FRPS or related sensors, causing intermittent circuit faults.
    Estimated share: about 10%.
    Note: These proportions reflect the frequency and type of issues described in the cited NHTSA complaints and the typical clustering of P0191 with lean codes in those reports. The data set is small, so treat percentages as approximate guidance rather than exact probabilities.

Diagnostic Approach

Safety note: If the vehicle stalls, loses power steering, or behaves unpredictably (especially during turns or near intersections), pull over safely and avoid continued operation until the fault is diagnosed and repaired. Fuel system diagnostics can involve pressurized lines; follow standard shop safety practices and use appropriate personal protective equipment.

1) Prepare and confirm

• Retrieve all active DTCs with a scan tool and record freeze-frame data (engine RPM, engine load, temperature, fuel trim values, etc.). Note if P0171/P0172 or other fuel-system-related codes accompany P0191.
• Confirm the symptom pattern: idle roughness, stalling, limp mode, engine light on, and any relevant drivability events from the customer description or complaints.

2) Visual and basic electrical inspection

• Inspect the FRPS connector and wiring harness for damage, corrosion, bent pins, or loose connections at the sensor and near the fuel rail.
• Check for obvious fuel leaks around the fuel rail, lines, and pressure regulator (external to the sensor).
• Inspect ground points and battery/engine grounds that could affect sensor signals.

3) Compare live data to expectations

• With the engine running (or during cranking if required by the vehicle), review:
  • FRPS sensor signal voltage vs. engine conditions (if the tool provides it). Look for abnormal or highly erratic readings.
  • 5-volt reference and ground integrity for the FRPS circuit (verify continuity and proper reference supply).
  • Real-time fuel rail pressure vs. commanded rail pressure (if the scanner can display both), and compare at idle and during throttle/load.
  • If available, monitor related fuel trims (short-term and long-term). Concurrent lean codes (P0171/P0172) with P0191 can point toward fuel delivery or air-fuel mixture issues.

4) Fuel system mechanical/pressure verification

• Perform a proper fuel pressure test using the vehicle's service spec:
  • Measure actual rail pressure with the engine running and at idle, and compare to the manufacturer's specifications for the engine.
  • If possible, perform a pressure hold test (key on, engine off; observe the rail pressure with the line capped) to detect leaks or a failing pump/regulator.
• If rail pressure is consistently low or fluctuating while the sensor readings are reasonable, suspect the fuel pump, regulator, or a restriction in the fuel delivery path.

5) Sensor and circuit verification

• If rail pressure is in spec but P0191 persists, focus on the FRPS circuit:
  • Check for proper 5V reference voltage to the FRPS and a solid ground.
  • Check continuity and resistance of the FRPS signal circuit from the sensor to the PCM, looking for shorts, opens, or damaged wiring.
  • Inspect the FRPS sensor itself; swapping with a known-good sensor can help confirm sensor integrity if you have that capability.
• If the sensor/wiring checks out, but readings are still inconsistent, consider PCM-related issues or software update/service bulletins as a last resort after confirming sensor/circuit integrity.

6) Evaluate the fuel delivery pathway

• If fuel pressure tests indicate abnormal performance (low pressure, or pressure not matching commanded values during various engine loads), inspect or service:
  • Fuel pump health and condition (may show signs of weakness under demand).
  • Fuel rail components or a faulty pressure regulator.
  • Fuel filter (if applicable) and any restrictions in fuel supply lines.
• Repair or replace faulty components as indicated by the test results.

7) Re-test and verify

• Clear codes after repairs and perform a road test or simulated drive to confirm:
  • No reoccurrence of P0191.
  • Stability of idle, absence of stalling, and consistent performance under load.
• If codes reappear, revisit the FRPS circuit and fuel-delivery path with more in-depth diagnostics, and consider additional data sources or service bulletins.

8) Documentation and escalation

• Document all measurements, readings, and component tests performed.
• If the codes persist after sensor/circuit and fuel-delivery path checks, consider PCM/software-related issues or vehicle-specific service bulletins. Check for recalls or TSBs related to fuel system or FRPS for the specific make/model/year.

Documentation

• P0191 points to a fuel rail pressure sensor circuit or sensor issue, often appearing with lean-condition codes (P0171/P0172) and drivability concerns such as rough idle or stalling.

• Root causes can be sensor/circuit faults, fuel delivery/rail pressure issues, or wiring/connectors. The most actionable path is to test and verify fuel pressure first, then inspect the FRPS circuit, followed by sensor replacement if needed.

• Given the safety implications (engine shutdown or limp mode in some cases), a careful, methodical diagnostic approach is essential, with proper measurement of actual rail pressure and sensor circuit integrity.

• P0191 definition: Fuel Rail Pressure Sensor Circuit Range/Performance (A circuit). This provides the standard code meaning used in the diagnostic guide.

• OBD-II general context: Diagnostic Trouble Codes and Powertrain Codes sections describe the framework in which P0191 resides and how codes are organized and interpreted.

  • OBD-II > Diagnostic Trouble Codes
  • OBD-II > Powertrain Codes

• Real-world symptom patterns: Complaints 1 and 3 mention P0191 in conjunction with rough idle, engine light, stalling, and lean-condition codes (P0171/P0172), illustrating how P0191 can present in the field and how it often correlates with fuel-system/pressure issues.

  • Complaint 1: 2013 Equinox/rough idle, stalling behavior, limp mode after driving; P0191 and P0172 reported.
  • Complaint 3: Lean-condition codes (P0171/P0172) with P0191; stalling concerns during turns and at idle.
    Notes: The other complaints (2 and 4) discuss engine shutdown and various electrical/electronic function concerns but do not explicitly tie to P0191; however, they illustrate the broader context of drivability and electronic control-related symptoms found in the NHTSA dataset.

Caution

• This diagnostic guide uses the available sources to shape symptom interpretation and a practical diagnostic flow. The data set is limited, and vehicle-specific service information (actual fuel rail pressure specs, sensor voltage ranges, and wiring diagrams) should be consulted from factory service information for the exact vehicle under repair. If in doubt, consult a technician with vehicle-specific data and consider vehicle recalls or TSBs related to fuel system components.

This diagnostic guide was generated using verified reference data:

• NHTSA Consumer Complaints: 4 real-world reports analyzed
• Wikipedia Technical Articles: OBD-II
• Open-Source OBD2 Data: N/A (MIT)

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

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