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OBD-II fault code definitions, symptoms, causes, and step-by-step diagnostics.
Standardized generic DTCAuxiliary emissions controls

P0455 — Evaporative Emission Control System Leak Detected — Gross/Large Leak

The EVAP monitor detected a leak or inability to seal roughly equivalent to a large/gross leak.

Standardized generic definition
Evaporative Emission Control System Leak Detected — Gross/Large Leak
DTC category
Auxiliary emissions controls
Diagnostic priority
Usually low for immediate drivability, but the fault can increase evaporative emissions and prevent an emissions inspection from completing.
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What P0455 means

The EVAP monitor detected a leak or inability to seal roughly equivalent to a large/gross leak.

First priority: check cap/sealing components and disconnected hoses, then use the manufacturer-approved EVAP seal/smoke test.

Possible symptoms

  • Check Engine Light
  • Usually normal drivability
  • Possible fuel odor
  • Possible hard start after refueling if purge flow is abnormal

Causes to test

These are diagnostic possibilities, not a statistical parts-replacement ranking. Vehicle-specific service information takes priority.

  • Loose, damaged, or incorrect fuel cap where the system uses a sealed cap
  • Cracked/disconnected EVAP hose or canister connection
  • Purge valve leaking or electrically faulty
  • Vent valve stuck or electrically faulty
  • Fuel-tank pressure sensor fault
  • Damaged canister or leak-detection hardware

Step-by-step diagnostic workflow

  1. Check the fuel cap and filler sealing surface if applicable, but do not stop diagnosis there.
  2. Inspect EVAP hoses and canister connections for damage or disconnection.
  3. Check purge and vent valve electrical operation and whether either valve leaks when commanded closed.
  4. Use an approved low-pressure smoke/leak test following vehicle service procedures.
  5. Review fuel-tank pressure sensor response and monitor enable conditions.
  6. After repair, allow the EVAP monitor to run under the manufacturer-required fuel level and temperature conditions.
Preserve evidence: EPA OBD guidance emphasizes that stored DTCs and freeze-frame data are important to proper diagnosis. Save them before clearing codes.

Scan data worth checking

  • Fuel-tank pressure sensor
  • Purge command/duty cycle
  • Vent command
  • EVAP monitor status and Mode $06 where available
  • Fuel level at time of failure

Severity and drivability

Usually low for immediate drivability, but the fault can increase evaporative emissions and prevent an emissions inspection from completing.

Stop driving and investigate immediately if there is a flashing MIL, oil-pressure warning, overheating, fuel leakage, severe misfire, major loss of power, abnormal mechanical noise, or a transmission that is slipping badly.

Common diagnostic mistakes

  • Automatically replacing the gas cap for every EVAP code
  • Applying excessive smoke-test pressure
  • Clearing the code repeatedly and preventing the monitor from completing

Related generic codes

P0454 — Evaporative Emission Control System Pressure Sensor IntermittentP0456 — Evaporative Emission Control System Leak Detected — Very Small LeakP0453 — Evaporative Emission Control System Pressure Sensor High InputP0457 — Evaporative Emission Control System Leak Detected — Fuel Cap Loose/OffP0452 — Evaporative Emission Control System Pressure Sensor Low InputP0458 — Evaporative Emission Control System Purge Control Valve Circuit Low

Source and accuracy notes

Generic DTC structure and standardized definitions are governed by SAE J2012/J2012DA. Actual enable criteria, thresholds, circuit pinouts, test values, software conditions, and repair procedures vary by vehicle. Use current OEM service information for the exact year/engine/calibration and check applicable manufacturer technical service bulletins. See the site methodology and primary-reference policy.