P0442 - OBD-II Trouble Code

Evaporative System Malfunction, Small Leak

Our emissions expert has put together the following information about the P0442 fault code. We have also included diagnostic procedures you can take to your repair shop if the mechanic is having difficulty analyzing the code.

OBD II Fault Code

  • OBD II P0442

Fault Code Definition

  • Evaporative System Malfunction, Small Leak


  • Check Engine Light will illuminate
  • In most cases, there are no adverse conditions noticed by the driver
  • In some cases, there may be a noticeable fuel odor caused by the release of fuel vapors

Common Problems That Trigger the P0442 Code

  • Defective or damaged fuel cap
  • Distorted or damaged Fuel Tank Filler Neck
  • Small tear or puncture in the Evaporative system hose(s) and/or Carbon Canister
  • Defective Fuel Tank Sending Unit gasket or seal
  • Small split in a seam of the Carbon Canister
  • Defective Evaporative Vent Valve and/or Evaporative Purge Valve
  • Defective or damaged Fuel Tank
  • Defective Fuel Tank Pressure Sensor
  • Defective Leak Detection Pump
  • Slightly loose and/or worn clamps or hardened O-rings anywhere in the EVAP system

Common Misdiagnoses

  • Fuel cap
  • Evaporative Purge Valve
  • Evaporative Vent Valve

Polluting Gases Expelled
HCs (Hydrocarbons): Unburned droplets of raw fuel that smell, affect breathing, and contribute to smog

The Basics
The evaporative control (EVAP) system captures any raw fuel evaporating from the fuel storage system (e.g. the fuel tank, filler neck, and fuel cap). Under precise operating conditions—dictated by engine temperature, speed, and load—the EVAP system stores and purges these captured fuel vapors back into the combustion process.

Want to Learn More?
The EVAP system is designed not only to capture, store, and purge any raw fuel vapors that leak from areas in the Fuel Storage system, but also to run a series of self-tests that confirm or deny the operational and vapor holding ability of the system. This is an important task because at least 20 percent of vehicle-produced air pollution originates from malfunctioning Vehicle Fuel Storage systems.

There are many ways to "leak test" the EVAP system, but most perform the leak test when the vehicle is sitting (like over night) or during the initial start-up after the vehicle has been sitting over night. The EVAP system's operational performance is also tracked by the Powertrain Computer by reading the change in the oxygen sensor voltages and short term fuel trim whenever the stored vapors are released or "purged" back into the combustion process. These values should indicate that fuel is being added to the system and that the overall mixture is getting richer. The purging process occurs when the vehicle is under acceleration, which is when most vehicles require additional fuel.

P0442 Diagnostic Theory for Shops and Technicians
The P0442 code indicates that there is a small leak in the EVAP system, but this is somewhat misleading. What the code really indicates is that the EVAP system will not hold a specified level of vacuum for a specified amount of time when it performs its leak test.

Here is how the evaporative leak test is performed by the Powertrain Computer:

  1. When the leak test is performed, the vehicle must have been sitting for at least four to eight hours so that the engine temperature and outside air temperature are identical. There must also be between 15 and 85 percent fuel in the tank—this is to provide a baseline for the test since gasoline and diesel are volatile fluids that expand and vaporize easily with warm temperatures.
  2. When the leak test initiates, the Vapor Canister Vent Valve is closed to prevent any fresh air from entering the EVAP system. The Purge Valve is also sealed off.
  3. The Leak Detection Pump operates to build a vacuum in the entire Evaporative System (see the Leak Detection Pump information below). After a specified time interval—usually about ten seconds—the Purge Valve is shut off and the vacuum level in the system is measured by the Fuel Tank Pressure Sensor.
  4. Finally, a countdown initiates, which measures the rate at which the vacuum decays in the system. If the vacuum decays faster than the specified rate on two successive tests, then the Powertrain Computer will fail the EVAP system and trigger the P0442 code. Most modern EVAP systems will fail the leak test with a pin-sized hole anywhere in the EVAP system, which amounts to 0.020 of an inch or a ½ millimeter.

Two Other Commonly Used Variations of Evaporative System Leak Test
The Leak Detection Pump (LDP) uses Engine Vacuum to operate a bellows-like pump to build vacuum in the EVAP system once the Evaporative Vent Valve has sealed the system. The LDP has a diaphragm that changes shape as vacuum builds. When the desired amount of vacuum has been reached, the diaphragm opens a set of electrical contacts that disables the LDP and stops the vacuum from building any further. Then the system starts a timer and monitors how long it takes for the diaphragm to relax and the electrical contacts to close. If the diaphragm relaxes in less than the required amount of time and the points touch too early on two consecutive tests, a code P0442 is set by the Powertrain Computer. The LDP system is common on Chrysler/Jeep and many European vehicles.

The Stationary or Sitting Evaporative Leak Test is performed when the vehicle is sitting. The test uses changes in the Fuel Tank Pressure Sensor data to determine whether or not there is an EVAP system Leak. Once the vehicle is shut off for a pre-determined amount of time, the Powertrain Computer closes the Evaporative Vent Valve. Any pressure changes in the Fuel Tank Pressure are closely monitored. Under normal circumstances, the pressure inside the Fuel Tank should decrease slowly as the fuel cools off. If the pressure falls off a little faster than the maximum allowable rate for two consecutive tests, the system will set a code P0442. This type of system is used on many GM and Ford vehicles.

In very hot climates, the EVAP system might never run its monitor because when air temperatures stay over or around 100 degrees, the fuel becomes too volatile, making it impossible for any EVAP system to perform an accurate leak test.

Common Tests for the Evaporative System

  • Retrieve the code and write down the freeze frame information to be used as a baseline to test and verify any repair. If there are any other EVAP system codes, make special note of those as well, as they may be triggering the P0442. It is important to address these codes before you attempt to repair the P0442. Purge Valve or Vent Valve codes (P0446, P0443) can trigger a P0442. If the Vent Valve or Purge Valve is not functioning correctly, it may lead to a small leak that will set the P0442. The same applies to any Fuel Tank Pressure Sensor codes (P0451, P0452) or Leak Detection Pump codes (P1495).
  • Perform a careful and close visual inspection of all accessible hoses and components in the EVAP system for any signs of damage or degradation.
  • Using a scan tool, pay very close attention to the Fuel Tank Pressure readings. Does the Fuel Tank Pressure Sensor work properly? If it doesn't, the system will think that no pressure or vacuum is being created when the EVAP monitor is performed when, in fact, there is a pressure/vacuum being created that Fuel Tank Pressure Sensor is unable to read. The Fuel Tank Pressure Sensor is the primary feedback sensor that the Powertrain Computer relies on for the leak test data each time the EVAP monitor is run.
  • Inspect and test the fuel cap to determine how well it fits onto the Fuel Tank Filler Neck. Make sure the Fuel Cap Seal isn’t dry or cracked. If the cap will not seal or hold vacuum/pressure, then it can trigger the P0442 code.
  • Verify that the Purge Valve and the Vent Valve work properly and hold vacuum for a sustained amount of time—at least thirty to sixty seconds with no decay. If either one of these valves function improperly, the system will not develop and/or hold the proper amount of vacuum. Energize both the Purge Valve and Vent Valve several times to test their vacuum-holding ability during and after each time they are energized. It is not uncommon for these devices to fail on an intermittent basis
  • Perform an "in bay" Leak Test with a bi-directional scan tool and track how well the Leak Detection Pump performs. You may have to isolate the Leak Detection Pump by pinching off the Purge and Vent Valve. In some cases, a manual Leak Detection Pump performance test is needed—this can be done by manually energizing the contact points on the Leak Detection Pump.
  • If all the components seem to function properly, then perform another smoke test of the entire EVAP system. This will usually root out any leaks that are hidden behind and/or under components of the vehicle. Pay close attention to the Fuel Tank Filler Neck, the Carbon Canister, and the Fuel Tank itself, especially where the Fuel Pump and Fuel Level Sending unit are located and sealed. Occasionally when a Fuel Pump is replaced, the seal isn't replaced or properly installed. This can cause small leaks in the system. You might have to remove the rear seats to further inspect and pinpoint the source of a Fuel Tank leak.