Comprehensive diagnostic guide for OBD-II code P0291

Turbocharger/Supercharger Underboost

Overview

• What the code indicates: P0291 is a powertrain diagnostic trouble code that signals an underboost condition. In other words, the engine's boost pressure under load is lower than what the engine control module (ECM/PCM) expects based on commanded boost. This is typically detected via the boost pressure sensor (MAP sensor) or related boost measurements, compared to the ECM's commanded target.

• Important note on conflicting/open-source entry: A provided Open Source entry . This entry does not align with P0291, which is a turbocharger/supercharger underboost code. In your diagnosis, rely on the standard P0291 definition (underboost) and treat the injector/cylinder 11 entry as unrelated to P0291 unless a separate, valid fault code references injector circuits.

Symptoms

• Noticeable loss of power under acceleration, especially at higher load (e.g., highway passing, pulling a trailer, steep grades).
• Slower or delayed throttle response; engine feels down on power compared to normal.
• Vehicle may run as if turbo is not producing expected boost; in some vehicles, the boost gauge (if equipped) shows low or no boost.
• Check Engine Light (CEL) is on; DTC P0291 may appear alone or accompanied by other boost-related codes (e.g., P0299, P0101/P0102 MAP/MAF sensor-related codes, P0171/P0174 lean codes, etc.).
• Decreased fuel economy under load or higher engine operating temperatures in some cases.

Note about related symptoms from user reports

• Customers frequently report "lumpier" performance, persistent lack of power especially when climbing grades, and occasional limp-mode behavior if the ECM protects against overwork. These symptoms align with reduced boost capability rather than sensor faults alone.
• Some complaints include accompanying codes related to intake leaks, MAF readings, or exhaust restrictions; those should be considered as potential contributing factors if they appear in the scan data.

Probable Causes

Based on general field experience and common patterns (not derived from a single dataset ), here are broad likelihoods for the typical causes of P0291. Treat these as starting points rather than guaranteed rankings, and adjust after you collect live data from the specific vehicle.

• Boost leaks in the intake/charge air system (hoses, intercooler, clamps, intake manifold gaskets, all intercooler plumbing)
  • Estimated share: ~40%
  • Why: A common, often inexpensive source of underboost. Small leaks reduce actual boost under load and trigger P0291.
• Wastegate system issues (wastegate actuator, vacuum/boost control solenoid, hoses)
  • Estimated share: ~20-25%
  • Why: If the wastegate cannot open/close correctly, boost is limited. Vacuum line leaks and faulty actuators are frequent culprits.
• Turbocharger or supercharger mechanical faults (shaft play, damaged turbine/compressor, oil starvation)
  • Estimated share: ~10-15%
  • Why: Mechanical degradation reduces boost generation; often manifests as persistent underboost.
• Boost control solenoid or sensor issues (MAP sensor, pressure sensor, wiring/ground problems)
  • Estimated share: ~10-15%
  • Why: Faulty sensing or control electronics can misread or fail to command proper boost.
• Exhaust restrictions/backpressure (blocked restrictive exhaust)
  • Estimated share: ~5-10%
  • Why: Excessive backpressure can suppress boost development.
• Oil supply/condition problems or poor maintenance (low oil pressure, dirty oil)
  • Estimated share: ~5-10%
  • Why: Turbo relies on proper lubrication; oil issues can degrade performance and boost.
• Other sensor/ECU or wiring issues (MAP/MAF misreads, calibration/software issues, related harness faults)
  • Estimated share: ~5-10%

Symptoms-based cause indicators (to guide testing)

• If MAP sensor reading is consistently too low relative to commanded boost, suspect sensor, vacuum leaks, or turbo/wastegate issues.
• If there are large vacuum leaks (audible or via smoke test), that points toward intake system leaks as a primary suspect.
• If the turbo actuates but boost remains low under load, suspect mechanical turbo/failure or exhaust restriction as possible root causes.
• If boost comes up momentarily with RPM but then falls away, suspect wastegate control or actuator issues.

What data to collect (during scan/diagnostic testing)

• Live data: Boost target vs actual (engine load, RPM, temperature), MAP sensor reading, intake air temperature, MAF readings (if present), wastegate actuator movement (if the test tool provides a way to observe), boost pressure sensor output, exhaust backpressure readings if available.
• Freeze frame data: RPM, vehicle speed, engine load, fault occurrence conditions (temperature, altitude) at the time of code set.
• Any related codes: P0290 (Boost Pressure Regulation), P0299 (Turbocharger/Supercharger Underboost), MAF/MAF-related codes (P0100-P0104), lean codes (P0171/P0174), or oxygen sensor codes that might indicate related mix conditions.

Step-by-Step Diagnosis

1) Verify and document

• Confirm P0291 with a scan tool; note any accompanying codes and recorded freeze frame data.
• Confirm the vehicle's service information (make/model/year) to check for vehicle-specific boost specs and common failure points.

2) Visual and mechanical inspection

• Inspect all intake plumbing: hoses, intercooler connections, clamps, and intercooler for cracks, leaks, or frost/soot build-up.
• Inspect vacuum lines and the boost control system (wastegate vacuum hoses, solenoids, connections).
• Check for obvious exhaust restrictions (unusual rust, crushed pipes, or aftermarket modifications that affect flow).
• Check oil level and oil condition; inspect for signs of oil in the intake tract that might indicate turbo oil seal issues.

3) Sensor and control system checks

• Test/verify MAP sensor accuracy or boost pressure sensor (replacements if necessary).
• Inspect wiring and grounds to MAP sensor and boost control solenoids; look for corrosion, damaged insulation, or loose connectors.
• If the vehicle uses a boost control solenoid, test its operation (solenoid click, proper voltage/ground, resistance as per service data).

4) Boost system functional test

• Perform a vacuum/pressure test of the boost system (smoke test preferred) to reveal leaks in intake plumbing, intercooler, or connections.
• With the engine at idle and under load, monitor actual boost vs commanded boost (or MAP reading) to confirm underboost behavior across RPM range.
• If equipped, test the wastegate actuator: check that it moves freely with correct vacuum/pressure, and verify no binding or mechanical interference. Ensure actuator arm clearance and linkage are correct.

5) Turbocharger examination

• Check for excessive shaft play (radial/axial) and unusual sounds when the turbine spins by hand (with engine off) or during operation.
• Inspect for oil leaks around the turbo housing and lines; investigate oil supply lines for blockages or restrictions.

6) Check for exhaust and emission-related restrictions

• If available, perform a backpressure check or a flow test to determine if the exhaust is restrictive.
• Consider inspecting condition if symptoms point toward backpressure issues.

7) Correlate and decide

• If a leak is found, repair or replace the leaking component and re-test; if boost returns to normal after leak repair, P0291 may be cleared.
• If the wastegate actuator or solenoid is faulty, repair/replace and re-test.
• If sensor readings are out of spec and not repairable by other means, replace faulty sensor(s) and re-test.
• If no leaks and sensors test good, proceed to more in-depth turbocharger assessment or consult manufacturer service information for calibration/repair limits.

Representative tests and expected outcomes

• Smoke test of intake system: Detects leaks around intercooler hoses, clamps, pipes, or base intake manifold gaskets.
• Vacuum test of wastegate actuator: If actuator does not hold vacuum or moves sluggishly, replace or repair actuator and hoses.
• MAP/boost sensor check: Sensor output should align with expected boost values; large deviations or sensor faults indicate replacement.
• Turbo rotation test (with appropriate tooling): Turbo should rotate smoothly; binding or excessive play suggests turbo failure.
• Exhaust backpressure check: High backpressure implies exhaust restriction; clearing or replacing or exhaust components may be required.

Repair Options

• Easy/first-step fixes:
  • Refit or replace damaged intake hoses, intercooler hoses, clamps, and gaskets.
  • Replace damaged vacuum lines and verify all connections.
  • Replace or clean MAP/boost sensors if faulted.
• Moderate fixes:
  • Replace faulty boost control solenoid(s) or actuator assembly; verify proper operation after installation.
  • Replace turbocharger components if mechanical failure is confirmed (shaft play, worn bearings) or if oil starvation has occurred.
• More involved fixes:
  • Repair or replace exhaust components if backpressure is excessive due to a clogged or damaged exhaust.
  • Address oil supply issues (oil pump, lines, or filters) if oil starvation is a contributing factor.
• After repairs:
  • Clear codes and re-test. Ensure the vehicle can achieve and hold proper boost under load without reoccurring P0291.
  • If P0291 persists after all suspected issues are addressed, consult the factory service information for possible calibration, software updates, or more advanced diagnostics.

Safety Considerations

• Always follow proper PPE and procedures when inspecting turbocharged engines (hot surfaces, moving belts, pressurized systems).
• When performing pressure or vacuum testing, ensure the engine is off and secure before applying test equipment.
• Be cautious with high-boost systems; never attempt to "force" boost or bypass safeguards without following the service manual.

Summary

• P0291 indicates underboost, most commonly caused by boost leaks in the intake, intercooler, or hoses; wastegate actuator/boost control issues; turbocharger/mechanical faults; or sensor/wiring problems affecting boost readings.

• Use a structured diagnostic flow: verify, inspect for leaks, test sensors and actuators, evaluate turbo and exhaust conditions, and then repair based on confirmed faults.

• While the provided Open Source entry on injector circuit does not align with P0291, standard references describe P0291 as a boost-related code within the broader OBD-II Powertrain Code family.

  • URL:

  • URL:

  • URL:

• Open Source OBD2 code definition : (note: not aligned with P0291; injector-circuit code; treat as conflicting/mis-tagged)

  • Note: This entry does not describe P0291; P0291 is underboost. This illustrates a potential mismatch between the provided Open Source entry and standard P0291 definitions.
  • URL: Not provided in prompt; referenced as Open Source repository entry

This diagnostic guide was generated using verified reference data:

• 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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