OBD II Code P0442
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.
Check Engine Light
Emission Gas Definitions
OBD II Fault Code
Fault Code Definition
- Evaporative System Malfunction, Small Leak
Common Problems That Trigger the P0442 Code
- 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
- 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
- Fuel cap
- Evaporative Purge Valve
- Evaporative Vent Valve
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.
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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 Power Train 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
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 Power Train Computer
- 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.
- 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.
- 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.
- 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 Power Train 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.