The evaporative emission (EVAP) control system limits the fuel vapors from escaping into the
atmosphere. The EVAP transfers the fuel vapors from the sealed fuel tank to an activated carbon
storage device, the EVAP canister. The EVAP canister stores the vapors until the engine is able to use
the extra fuel vapor. When the engine is able to use the extra fuel vapor, the intake air flow purges the
fuel vapor from the carbon element and then the normal combustion process consumes the fuel vapor.
The system is required in order to detect the evaporative fuel system leaks as small as 0.040 inch
between the fuel filler cap and the EVAP canister purge valve. The system can test the evaporative
system integrity by applying a vacuum signal, ported or manifold, to the fuel tank in order to create a
small vacuum.
The powertrain control module (PCM) then monitors the ability of the system to maintain the vacuum. If
the vacuum remains for a specified period of time, then there are no evaporative leaks, and a PASS is
reported by the PCM. If there is a leak, the system either will not achieve a vacuum, or a vacuum cannot
be maintained. Usually a fault can only be detected after a cold start with a trip of sufficient length and
driving conditions to run the needed tests. The enhanced evaporative system diagnostic conducts
sub-tests in order to detect the fault conditions. If the diagnostic fails a sub-test, the PCM stores a
diagnostic trouble code (DTC) in order to indicate the type of fault detected.
The EVAP diagnostic tests include the following tests:
Weak Vacuum Test: This tests for large leaks and blockages in the EVAP system. The fuel tank
pressure (FTP) sensor detects a low fuel tank pressure resulting from a large vacuum leak or a
restriction in the vacuum supply to the fuel tank. The FTP sensor value should increase over a period
of time. If the PCM does not detect an increase, a malfunction is indicated by setting DTC P0440.
Weak Vacuum Follow-up Test: This tests the EVAP system after the Weak Vacuum Test has failed.
The FTP sensor looks for high tank vacuum over a period of time. If this occurs the Weak Vacuum Test
will pass. If Weak Vacuum does not pass, the test continues through this ignition cycle.
Vacuum Decay Test: This tests for small leaks in the EVAP system. The decay rate is determined by
measuring the change in the fuel tank pressure over a period of time. If the decay rate is more than a
calibrated value, another Vacuum Decay Test will be initiated. If vacuum decay is still more than a
calibrated value, a malfunction is indicated by setting DTC P0442.
Fuel Vapor Build Pressure Test: This test is used to determine the vacuum leak size. When the
vacuum has decayed to near zero, the PCM subtracts the measured slope from the vacuum decay to
calculate the leak size. If the calculation indicates that a leak is present, the Small Leak Test will fail.
Canister Vent Restriction Test: This tests for a restriction in the EVAP vent system. The FTP sensor
looks for sufficient vacuum from the engine intake manifold. After a period of time and conditions have
been met, this test will pass. If the vacuum value is too high, a malfunction is indicated by setting DTC
P0446.
Purge Valve Leak Test: This tests for small leaks to the intake manifold. This is accomplished by
sealing the EVAP system purge valve and the EVAP vent valve and allowing the PCM to monitor the
FTP sensor. The FTP sensor value should not increase. If the PCM detects an increase, a malfunction
is indicated by setting DTC P1441.
Diagnostic Test DTC Possible Causes
Weak Vacuum Test Large Leak Test P0440 A large leak, above .060 in. or blockage somewhere in the
system
Vacuum Decay Test Small Leak Test P0442 A small leak somewhere in the system, .020-.060 in.
Canister Vent Restriction Test P0446 A restriction in the EVAP vent system
Purge Valve Leak Test P1441 A stuck open or partially open purge valve
Fuel Tank Pressure Sensor.
The fuel tank pressure (FTP) sensor measures the difference between the air pressure, or vacuum, in
the fuel tank and the outside air pressure.
The sensor mounts at the top of the fuel tank sending unit. The PCM supplies a 5 volt reference
voltage and ground to the sensor. The sensor provides a signal voltage between 0.1-4.9 volts to the
PCM. When the air pressure in the fuel tank is equal to the outside air pressure, such as when the fuel
fill cap is removed, the output voltage of the sensor will measure 1.3-1.7 volts.
When the air pressure in the tank is 1.25 kPa (4.5 inches Hg), the sensor output voltage should
measure 1.25 kPa (0.5 volts).
The sensor voltage increases to approximately 4.5 volts at -3.75 kPa (14 inches Hg).
EVAP Canister Purge Valve.
When energized, the evaporative canister purge valve allows the fuel vapor to flow from the EVAP
canister to the engine. The normally closed valve is pulse width modulated by the PCM in order to
precisely control the vapor flow. The valve opens during the Enhanced Evaporative Diagnostic Test in
order to create a vacuum in the fuel tank and then closed in order to seal the system.
EVAP Vent Valve.
The evaporative canister vent valve not only allows the fresh outside air to the EVAP canister during
the purge modes, but also allows the diagnostic to pull a vacuum on the fuel tank by closing the vent
valve. This valve is normally open.
EVAP Canister.
The EVAP canister, filled with charcoal pellets, stores the fuel vapors from the fuel tank. Vacuum is
pulled from the fuel tank through the vapor pipe. The system goes open loop. (2). Fresh air (3) is mixed
with the vapors and pulled to the intake manifold from the canister through the purge line (1). The
engine vacuum purges the EVAP canister during normal driving.
atmosphere. The EVAP transfers the fuel vapors from the sealed fuel tank to an activated carbon
storage device, the EVAP canister. The EVAP canister stores the vapors until the engine is able to use
the extra fuel vapor. When the engine is able to use the extra fuel vapor, the intake air flow purges the
fuel vapor from the carbon element and then the normal combustion process consumes the fuel vapor.
The system is required in order to detect the evaporative fuel system leaks as small as 0.040 inch
between the fuel filler cap and the EVAP canister purge valve. The system can test the evaporative
system integrity by applying a vacuum signal, ported or manifold, to the fuel tank in order to create a
small vacuum.
The powertrain control module (PCM) then monitors the ability of the system to maintain the vacuum. If
the vacuum remains for a specified period of time, then there are no evaporative leaks, and a PASS is
reported by the PCM. If there is a leak, the system either will not achieve a vacuum, or a vacuum cannot
be maintained. Usually a fault can only be detected after a cold start with a trip of sufficient length and
driving conditions to run the needed tests. The enhanced evaporative system diagnostic conducts
sub-tests in order to detect the fault conditions. If the diagnostic fails a sub-test, the PCM stores a
diagnostic trouble code (DTC) in order to indicate the type of fault detected.
The EVAP diagnostic tests include the following tests:
Weak Vacuum Test: This tests for large leaks and blockages in the EVAP system. The fuel tank
pressure (FTP) sensor detects a low fuel tank pressure resulting from a large vacuum leak or a
restriction in the vacuum supply to the fuel tank. The FTP sensor value should increase over a period
of time. If the PCM does not detect an increase, a malfunction is indicated by setting DTC P0440.
Weak Vacuum Follow-up Test: This tests the EVAP system after the Weak Vacuum Test has failed.
The FTP sensor looks for high tank vacuum over a period of time. If this occurs the Weak Vacuum Test
will pass. If Weak Vacuum does not pass, the test continues through this ignition cycle.
Vacuum Decay Test: This tests for small leaks in the EVAP system. The decay rate is determined by
measuring the change in the fuel tank pressure over a period of time. If the decay rate is more than a
calibrated value, another Vacuum Decay Test will be initiated. If vacuum decay is still more than a
calibrated value, a malfunction is indicated by setting DTC P0442.
Fuel Vapor Build Pressure Test: This test is used to determine the vacuum leak size. When the
vacuum has decayed to near zero, the PCM subtracts the measured slope from the vacuum decay to
calculate the leak size. If the calculation indicates that a leak is present, the Small Leak Test will fail.
Canister Vent Restriction Test: This tests for a restriction in the EVAP vent system. The FTP sensor
looks for sufficient vacuum from the engine intake manifold. After a period of time and conditions have
been met, this test will pass. If the vacuum value is too high, a malfunction is indicated by setting DTC
P0446.
Purge Valve Leak Test: This tests for small leaks to the intake manifold. This is accomplished by
sealing the EVAP system purge valve and the EVAP vent valve and allowing the PCM to monitor the
FTP sensor. The FTP sensor value should not increase. If the PCM detects an increase, a malfunction
is indicated by setting DTC P1441.
Diagnostic Test DTC Possible Causes
Weak Vacuum Test Large Leak Test P0440 A large leak, above .060 in. or blockage somewhere in the
system
Vacuum Decay Test Small Leak Test P0442 A small leak somewhere in the system, .020-.060 in.
Canister Vent Restriction Test P0446 A restriction in the EVAP vent system
Purge Valve Leak Test P1441 A stuck open or partially open purge valve
Fuel Tank Pressure Sensor.
The fuel tank pressure (FTP) sensor measures the difference between the air pressure, or vacuum, in
the fuel tank and the outside air pressure.
The sensor mounts at the top of the fuel tank sending unit. The PCM supplies a 5 volt reference
voltage and ground to the sensor. The sensor provides a signal voltage between 0.1-4.9 volts to the
PCM. When the air pressure in the fuel tank is equal to the outside air pressure, such as when the fuel
fill cap is removed, the output voltage of the sensor will measure 1.3-1.7 volts.
When the air pressure in the tank is 1.25 kPa (4.5 inches Hg), the sensor output voltage should
measure 1.25 kPa (0.5 volts).
The sensor voltage increases to approximately 4.5 volts at -3.75 kPa (14 inches Hg).
EVAP Canister Purge Valve.
When energized, the evaporative canister purge valve allows the fuel vapor to flow from the EVAP
canister to the engine. The normally closed valve is pulse width modulated by the PCM in order to
precisely control the vapor flow. The valve opens during the Enhanced Evaporative Diagnostic Test in
order to create a vacuum in the fuel tank and then closed in order to seal the system.
EVAP Vent Valve.
The evaporative canister vent valve not only allows the fresh outside air to the EVAP canister during
the purge modes, but also allows the diagnostic to pull a vacuum on the fuel tank by closing the vent
valve. This valve is normally open.
EVAP Canister.
The EVAP canister, filled with charcoal pellets, stores the fuel vapors from the fuel tank. Vacuum is
pulled from the fuel tank through the vapor pipe. The system goes open loop. (2). Fresh air (3) is mixed
with the vapors and pulled to the intake manifold from the canister through the purge line (1). The
engine vacuum purges the EVAP canister during normal driving.
Evaporative Emissions System Description and Operation (EVAP)
| 6-7-11 what is code to turn off light? |
Comments
| 6-15-11 You will have to fix the EVAP system problem, for the MIL to turn off. |
| 8-3-11 What can happen if you don't fix it? |
| That depends on what the stored computer code number is. Most EVAP system codes will have no ill effect of the vehicles operation, other than a constant check engine light and unable to pass emissions testing. |
| 9-14-11 I have the same problem in my car Mazda 3 2007, I want to change the sensor but I don't know the name of it and where is Locate it. Do you know ? |
| 10-11-11 I have P0446 and I changed the purged solenoid, vent solenoid, and gas cap and still same. |
| That depends on what the stored computer code number is. Most EVAP system codes will have no ill effect of the vehicles operation, other than a constant check engine light and unable to pass emissions testing. |
| 10-13-11 If you re using a smoke machine and smoke is coming out to the breather is that normal? |
| It depends if you've got everything closed off. You should only use the smoke machine after the vent valve and purge solenoids are commanded closed, and the gas cap is on tightly, or on an adapter tightly. If you've introduced smoke into the system without doing the steps listed above, you can have smoke coming out everywhere and it will not help you find your leak. |
| 11-13-11 When does the EVAP test occur -- At cold engine start, on highway driving? After resetting the DTC P1441, P1442 and P1446, it takes 3 to 4 days for the MIL light to turn ON and for the INC on status to disappear. DTC small leak, but found a open CCV (large vacuum leak), so the CCV got replaced. |
| 11-17-11 what is the evaporation control valve? That's what my code is for. Is that in the canister? 2006 GMC truck. |
| Several parameters must be met to run this test. Highway driving, then sitting at a stop, engine warm, A/C off. |
| If you mean the vent valve, it is in the rear under the spare tire, or on the rear of the gas tank. If you mean the purge valve solenoid, it is on the engine. |
| 1-20-12 2001 Pontiac Grand Am GT ABS/TRAC off warning light on but shows no codes. Is there a reset button for these warning lights to go off? |
| No rest button. You need to have the computer scanned for codes, and the problem repaired, then the computer scanner can clear the codes. |
