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.
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.
3-7-12
2002 Ford E-250. Vacuum forms in gas tank after driving a few miles. The bottom
of the fuel tank gets sucked upwards. Turn off engine the gas tank pops back into
place. No check engine codes. Replaced fuel pump, fuel vapor separators, and
fuel tank pressure sensor. After two days time problem started again.
3-7-12
Any idea of what the cost may be to have the EVAP Leak addressed for a 2000
GMC Jimmy?
That depends on what the problem is. If it is just a vent
valve, then expect to pay about $150 parts and labor. If it
is a fuel tank seal, than maybe $300 pats and labor.
Could just be a bad gas cap. If the leak is hard to find,
then labor hours will be most of the cost. And this
depends on what the shop charges. Dealerships range
from $90- $130 per hour. Local shops are $60 - $100.
You do not have a sensor problem, you have a vent
valve problem. It is not venting when it should be, thus
causing an extreme vacuum.
3-27-12
I have a 99 Hyundai accent and recently found out the car I recently purchased is
missing the evap canister. Of course check engine light is on. I have been having
problems after driving for a while I blow a noticable amount of white smoke out the
tailpipe. I am hoping buying and installing one will remove that issue. Also is not a
blown head gasket. Any ideas.
A missing canister should not cause the engine to blow
smoke. You have other problems.
4-2-12
Since you are very smart let me throw this at you.  I have a 2009 Dodge Ram Hemi
came with 26 Gallon tank no codes. I decided I wanted a 32 gallon tank so I bought
one from salvage yard and removed 26 and moved over the fuel pump assembly
which is same part number for both. Hooked all back up and everything looks fine
but now I am getting a gas cap light and check engine where engine is lit solid.
Check with the scanner I have it says 0440 evap emissions code. What do you
think I did wrong to cause this error. I have rechecked the connections which is
about 6 things if you include both ends of quick connects and hoses. I did mess up
wire harness with one purple wire and one black wire that connects to rear of tank
by the vent hose and had to make a wire harness with spade terminals. I doubt it is
really gas cap since the gas cap worked before on 26. What do you think it could
be? Maybe some junk that got in the lines?
You may have blown a fuse, have a leak in an EVAP line
or the pump unit seal, or maybe the pressure sensor and
computer is not calibrated for a larger tank.
4-27-12
This was very helpful I'm testing the voltage that was not correct on a 2007 Malibu
so I replaced the PCM now it's ok. Thank you.
P2421 Evaporative Emission System Vent Valve Stuck Open on a 2007 Volvo
XC90. How do I fix?
Replace the valve.
I have a 1997 Camaro. P1441 came up in i changed the purge solenoid valve in it
came on again i really dont no what to do i need help.
I have a 1999 Grand AM 2.4l that stalls on hot days after driving for 15 minutes. It
also stalls when running in park. I don't have a ses light on. I do have data sensor
information.  Output driver 2 open I'm not sure what that means. The mechanic
also noted the oxygen sensor was not working as well as it could be. Thank you for
your help.
Mine said p1441 if i replace the purge solenoid will that fix it?
I think that code is for the EGR valve.
I have a 2002 Avalanche with the warning light showing. Code P0446 comes up
which is associated with EVAP Vent System Performance. Any idea what would be
causing this and how to fix?
Probably the EVAP vent valve is bad. Very common
problem. It is located above the spare tire. There is a
redesigned kit at the Dealer to repair this problem.
I hope you can help me i have a 2004 dodge stratus and i have been through so
much with car my engine light is on and has been for ever i need a smog and the
code i am getting says that my car is running lean i have changed everything
possible what are my options please help.
Check for vacuum leaks,check fuel pressure and check
all the
oxygen sensors for proper operation.
More Comments And Add Your Own >>
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.
2002 Honda Passport (same as Isuzu Rodeo). Codes P0446 and P1441.  Using a
Mity-pump, vacuum holds at the purge valve and in the line from the valve to the
canister (line pulled off of canister and vacuum pulled from that end).  Purge valve
opens when a voltage is applied from a 9V battery.  Vent valve at canister has
open flow and valve closes when a voltage is applied from a 9V battery.  I did not
test vacuum between gas cap (seal looks fine) and canister (lines are too large for
pump).  Canister itself will hold pressure, but not vacuum.  Can this be the
problem?
Yes, the canister not holding vacuum could mean that it
is cracked. Vacuum pulls it one way, pressure the other.
But, just because the vent and purge valve seem to be
working when voltage applied, does not mean the
computer is applying voltage properly. You need to use a
scanner that can turn these items on and off. That will
rule out the computer, valves and wiring.
I have a 2001 Hyundai Accent 1.6L and I have had the EVAP CONTROL
CANISTER replaced twice now. Is there anything I can pick up at like Auto Zone to
help keep it clean?
If the canister is getting full of dirt, there may be a filter
for it. Check with your dealer for a redesigned unit. If it is
filling up with gas, then you have a problem in the system.
I have a 2001 Rodeo that keeps haveing the same error code saying bad gas cap.
I replaced it with one but still got the error. I was told it has to have a factory cap. I
am also getting horrible mileage 8.6. I was told the bad gas cap would cause this
also. any ideas?
If the gas cap is not fixing it, then you have an EVAP
system leak. Possibly a bad purge valve or vent valve.
The check gas cap light comes on because the computer
senses a leak,but cannot determine what it is. A large
leak acts like a gas cap loose or missing, and is a
common thing to happen, so thats why the system has
this warning.

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Tom Said:
hi ya i have a mk4 golf 1.4 2003 with engine light staying on and when its running engine jadderys and there a ticking sound come from evaporates
emission evaporative purge valve any ideas?

Bill Said:
Reply to Tom: If you mean the engine is ticking and the check engine light is on, check the oil level and get the computer codes scanned.
Evaporative Emissions System (EVAP) Description