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Evaporative System Monitor

Non-Continuous

One of the slowest monitors to complete. Requires an 8-hour cold soak followed by 15-30 minutes of driving, and may need a second overnight soak for enhanced EVAP leak tests. Total elapsed time: 1-2 days.

What This Monitor Checks

The Evaporative System (EVAP) Monitor tests the fuel vapor containment system for leaks by performing either a vacuum decay test (sealing the system and monitoring pressure loss) or a positive pressure test (on newer enhanced EVAP systems). The PCM closes the vent valve and purge valve, then monitors the fuel tank pressure sensor (FTP) to detect leaks as small as 0.020 inches in diameter. It also verifies proper purge flow by monitoring fuel trim shifts when the purge solenoid is commanded open.

Why It Matters for Emissions

Fuel vapors (hydrocarbons) escaping from the fuel system are a significant source of evaporative emissions, contributing to ground-level ozone formation. The EVAP system captures and recycles these vapors; even a 0.020-inch leak can release several grams of HC per day, exceeding EPA evaporative emissions limits.

Drive Cycle Steps

1

Ensure the vehicle has cold-soaked for at least 8 hours (engine off, parked) so that fuel tank temperature has stabilized near ambient — temperature differentials cause vapor pressure changes that interfere with leak detection.

2

Verify the fuel level is between 15% and 85% — too full prevents vapor space for the test, too empty may cause the fuel pump to uncover and create false pressure readings.

3

Start the engine and drive at steady speeds between 30-55 mph for 10-15 minutes to allow the PCM to run the EVAP purge flow test and confirm vapor canister operation.

4

After the initial drive, the leak test often runs during a subsequent idle period or during the next engine-off soak period (enhanced EVAP systems run the leak test with the engine off using an onboard pump).

5

Do not open or loosen the fuel cap during the test cycle or for several hours before it — this resets the test.

Prerequisites

  • Fuel level must be between 15% and 85% of tank capacity as reported by the fuel level sender.
  • Ambient temperature must be between 40°F (4°C) and 95°F (35°C) — extreme temperatures cause excessive vapor pressure variation.
  • The vehicle must have cold-soaked for at least 8 hours with the fuel cap properly sealed and tightened.
  • No active DTCs related to the fuel tank pressure sensor, purge solenoid, or vent valve circuits.

Common Failure Reasons

  • Loose, damaged, or missing fuel cap — the single most common cause of EVAP DTCs and monitor failures.
  • Cracked or dry-rotted EVAP hoses and connections, especially the line from the fuel tank to the charcoal canister.
  • Saturated or damaged charcoal canister from fuel overfilling (topping off) or liquid fuel ingestion.
  • Failed purge solenoid (stuck open or closed) preventing proper vapor flow management.
  • Cracked fuel tank filler neck or deteriorated O-ring seals at the fuel pump module flange.

Pro Tips

  • This is the most difficult monitor to set — plan for it first when preparing a vehicle for emissions testing. Start the cold soak the night before and drive the EVAP cycle first thing in the morning.
  • Never top off the fuel tank (continue fueling after the first click) — liquid fuel saturates the charcoal canister and causes P0446/P0441 codes that prevent monitor completion.
  • For enhanced EVAP systems (2000+ model year), the leak test runs with the engine off using a small onboard pump — you may hear a brief whirring sound from the fuel tank area after shutdown.
  • Professional-grade EVAP smoke machines can detect leaks as small as 0.010 inches that the onboard system struggles to isolate, saving hours of diagnostic time.

Frequently Asked Questions

What does the Evaporative System Monitor check?

The Evaporative System (EVAP) Monitor tests the fuel vapor containment system for leaks by performing either a vacuum decay test (sealing the system and monitoring pressure loss) or a positive pressure test (on newer enhanced EVAP systems). The PCM closes the vent valve and purge valve, then monitors the fuel tank pressure sensor (FTP) to detect leaks as small as 0.020 inches in diameter. It also verifies proper purge flow by monitoring fuel trim shifts when the purge solenoid is commanded open.

How do I get the Evaporative System Monitor to set ready?

Follow the drive cycle: Ensure the vehicle has cold-soaked for at least 8 hours (engine off, parked) so that fuel tank temperature has stabilized near ambient — temperature differentials cause vapor pressure changes that interfere with leak detection. Verify the fuel level is between 15% and 85% — too full prevents vapor space for the test, too empty may cause the fuel pump to uncover and create false pressure readings. Start the engine and drive at steady speeds between 30-55 mph for 10-15 minutes to allow the PCM to run the EVAP purge flow test and confirm vapor canister operation. After the initial drive, the leak test often runs during a subsequent idle period or during the next engine-off soak period (enhanced EVAP systems run the leak test with the engine off using an onboard pump). Do not open or loosen the fuel cap during the test cycle or for several hours before it — this resets the test. Estimated completion: One of the slowest monitors to complete. Requires an 8-hour cold soak followed by 15-30 minutes of driving, and may need a second overnight soak for enhanced EVAP leak tests. Total elapsed time: 1-2 days.

Why does the Evaporative System Monitor keep failing?

Common failure reasons include: Loose, damaged, or missing fuel cap — the single most common cause of EVAP DTCs and monitor failures.; Cracked or dry-rotted EVAP hoses and connections, especially the line from the fuel tank to the charcoal canister.; Saturated or damaged charcoal canister from fuel overfilling (topping off) or liquid fuel ingestion..

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Quick Info

Type

Non-Continuous

Completion

One of the slowest monitors to complete. Requires an 8-hour cold soak followed by 15-30 minutes of driving, and may need a second overnight soak for enhanced EVAP leak tests. Total elapsed time: 1-2 days.

Resets on Clear

Yes