Vacuum hoses and intake leaks codes

How its codes differ

• Lean codes (P0171, P0174) are the usual sign of an intake leak: air that enters after the airflow sensor is not measured, so the computer under-fuels. A leak in the shared intake can lean both banks; a leak at one bank's gasket can lean one side.
• P0101 can set when an intake leak makes the airflow reading disagree with what the computer expects from throttle and engine speed.
• P0300 can follow a large leak, because a mixture lean enough can misfire, often worst at idle.
• EVAP leak codes (P0455, P0442, P0456) involve the hoses on the fuel-vapour side rather than the intake side: a split or loose EVAP hose stops the system sealing for its leak test.
• P0106 (MAP sensor range) can come from an air leak between the throttle and the intake valves, including the MAP sensor's own seal.
• P0507 (idle speed higher than expected) is often an air leak into the intake after the throttle.
• P0401 (too little EGR flow) can be a leaking or disconnected vacuum hose on engines with a vacuum-operated EGR valve.
• P0497 (low purge flow) can come from a blocked, kinked, disconnected or leaking purge line, and P0457 can be set by a leak elsewhere in the EVAP system rather than at the cap.

Codes

• P0171

  Engine computer has reached its limit adding fuel on bank 1: the mixture is lean

  Severity: Monitor

• P0174

  Engine computer has reached its limit adding fuel on bank 2: the mixture is lean

  Severity: Monitor

• P0101

  Airflow sensor reading doesn't match what the engine computer expects for the throttle opening

  Severity: Inspect soon

• P0300

  Random or multiple-cylinder misfire detected

  Severity: Inspect soon

• P0440

  EVAP system fault (general)

  Severity: Monitor

• P0441

  EVAP purge flow incorrect

  Severity: Monitor

• P0442

  EVAP system small leak detected

  Severity: Monitor

• P0446

  EVAP vent control performance problem

  Severity: Monitor

• P0455

  EVAP system large leak detected

  Severity: Monitor

• P0456

  EVAP system very small leak detected

  Severity: Monitor

• P0496

  EVAP purge flow when purge should be off

  Severity: Monitor

• P0106

  Manifold pressure (MAP) sensor reading doesn't match what the engine computer expects

  Severity: Inspect soon

• P0507

  Idle speed higher than the computer is aiming for

  Severity: Monitor

• P0401

  Exhaust gas recirculation (EGR) flow insufficient detected

  Severity: Monitor

• P0497

  EVAP system low purge flow

  Severity: Monitor

• P0457

  EVAP leak detected from the fuel cap being loose or off

  Severity: Monitor

Signs it's failing

• Check engine light with lean codes
• Rough or high idle
• Hissing noise from the engine bay
• EVAP leak codes when the leak is on the fuel-vapour side

How to test it

1. With the engine off, look along the vacuum hoses, PCV hose and the intake duct after the airflow sensor for cracks, splits, loose clamps or disconnected ends. (Low risk)

2. With a scan tool, compare fuel trims at idle and at a steady 2500 rpm. Trims that are high at idle and fall toward normal at higher speed suggest an air leak. (Use caution)

   Warning: Keep hands, hair, clothing and tools clear of the fan and belts while the engine is running, and do not run the engine in a closed garage.

3. A smoke test finds leaks that cannot be seen or heard, on both the intake and the EVAP side; this is usually a shop test. (Best left to a shop)

   Warning: Smoke testing pressurises the intake or fuel-vapour system and should be done by a technician with the right equipment.

After the repair

• Recheck fuel trims at idle and 2500 rpm: they should move back toward normal.
• Clear codes only after the repair, then drive normally over several days with both city and highway driving so the readiness monitors can complete; clearing codes or disconnecting the battery resets them to not ready, which fails an emissions test.