Vapor Canister Purge Valve Function

Unless you’re a mechanic or an engineer, you’ve pr

Unless you’re a mechanic or an engineer, you’ve probably never heard of a canister purge valve before. To begin with, it’s an emissions component that redirects excess fuel vapors back into the engine.

The entire system that it’s a part of is your vehicle’s EVAP (Evaporative Emission) system. It works in conjunction with the charcoal canister to capture and redirect excess fuel vapors so the engine can burn them again.

This reduces your emissions by burning the fuel vapors instead of letting them out of the air, essentially maximizing the amount of energy produced and minimizing the number of harmful chemicals released as a byproduct.

Because it’s effective at reducing the overall amount of harmful emissions, many auto manufacturers have started to implement them into new vehicles to meet rising emission standards.

But when the vapor canister purge valve gets stuck open, you get too much air in the system that wrecks performance. On the other hand, if it gets stuck closed, your emission levels skyrocket.


What Are The Symptoms Of a Failing Purge Valve?

When your purge valve fails or starts to malfunction, you’ll likely see a check engine light. If the vehicle’s computer senses an unusually high or low purge from the valve, it will kick an error code and illuminate the check engine light. Other common symptoms of a failing purge valve include:

Poor Fuel Economy

If the purge valve is malfunctioning, it can allow fuel vapors that your vehicle actually needs to burn to escape. The vehicle will have to use more fuel to produce the same level of power, causing a reduction in fuel economy.

Engine And Performance Issues

If the purge valve is stuck open, it can allow a vacuum leak to form. That means that air can flow into the combustion chamber at unexpected levels, which will mess with the engine’s air-fuel ratio and cause issues.

What repairs can fix the P0443 code?

  • Testing and replacing the purge control valve
  • Repairing the damaged wiring to the purge control valve and secure from getting damaged again
  • Replacing the purge vent valve

How to Check if the Canister Purge Valve Works Properly?

First, you’ll need to remove the canister purge valve. Usually, you can find this part inside the engine bay, near the fuel-delivery system. Most purge valves look the same: a small black plastic box with two or three small rubber hoses on either side and an electrical connector on the top. If you aren’t sure where the purge valve is located in your car, take a look into the EVAP system diagram inside your car’s repair manual.

Remove the electrical connector that provides electricity to the solenoid and remove the vapor lines. Next, remove the whole component. Most purge valves are held in place with one bolt or slid on a rubber bushing.

Testing the Canister Purge Valve Solenoid

After you’ve removed the part, you can check if it functions properly. To do that, you’ll need a car battery and jumper cables the solenoid click, and a vacuum pump for testing the sealing properties.

Connect the car battery to the control side of the solenoid to see if it closes/opens appropriately. If your purge valve has more than two pins inside the connector, check the purge valve electrical diagram to find out which pins control the solenoid. You should hear an audible click every time you apply voltage to the system. If not, the solenoid might not work.

On most purge valves, you’ll find only two p
On most purge valves, you’ll find only two pins controlling the solenoid. One is the positive wire coming from the fuse and the other is the ground controlled by the PCM.

Testing the Seal

To verify the proper sealing of the purge valve, use a vacuum pump to create a vacuum inside the canister purge valve and check the gauge for at least 5 minutes. If the purge valve doesn’t seal properly, the vacuum will drop. When this happens, the best bet is to replace it right away.

If the purge valve clicks just fine and you’re still having an EVAP system DTC code, it probably means that the solenoid is not getting power or ground. The next thing to do is then to test the control circuit.

Testing the Canister Purge Valve Control Circuit

The easiest and first thing to test is the positive side. Once your car starts, the canister purge valve fuse supplies power to the control side of the solenoid. Use a test light or a multimeter to test the fuse and make sure it isn’t burned. If it’s not, replace the fuse and test it again. If everything seems good, it’s time to test the ground.

Since the ground is supplied by the PCM, you’ll need to activate it in order to test it. To do that, you’ll need an OBD2 scanner with special test features. Select the “special test” or “activation test” tab and search for the canister purge valve test mode. Click on it and activate it. Once activated, the PCM will start sending ground to the purge valve control output pin. Use your multimeter to make surge ground is supplied to that specific pin. If it’s not, multiple causes may come to mind. A sensor might not work properly, a communication circuit might be open, or the PCM might be faulty. In all cases, exhaustive troubleshooting might be required and you should probably leave that to professionals.

On the other hand, if the ground actually comes out of the PCM, the electrical circuit between the PCM and the purge valve might be open. Find the stripped or damaged wire and repair it.

What causes the P0443 code?

  • The ECM has commanded the purge control valve to open and detected either an incomplete open circuit or a short in the circuit.

  • The cause of code P0443 can be a purge control valve internal open circuit or the connector is corroded causing loss of contact with the valve.

  • The code can also be set if the wiring to the valve is damaged between the ECM and the purge valve causing an open circuit if the wire is cut or a short if the wire is shorted to ground or a power source.

Code For Evap Leak

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P0443 Diagnosis: Mercury Sable

A good code scanner can really help aid you in diagnosing P0443.  You’ll be able to command the purge valve to open.  When it opens, you should be able to hear it.  If you don’t hear anything at all, you’ll need to check to see if the purge valve solenoid is getting any voltage.  If it is, and the ground is good, it’s almost certainly the solenoid that is at fault.

If there isn’t voltage at the solenoid, you’ll need to go back and see if there is a wiring issue.  You’re going to be looking for frayed and shorted wiring going to the solenoid.

Here’s a good YouTube video on how to diagnose P0443:

What are the common causes of code P0443 ?

Typical causes of code P0443 could include any or all of the following-

  • Damaged, burnt, shorted, or corroded wiring and/or connectors.
  • Open circuits.
  • Purge valve stuck in the open position.
  • Failed, or failing PCM. Note however that this is rare event, and the fault must be sought elsewhere before any controller is replaced.

NOTE: Although code P0443 is rarely caused by issues other than wiring problems and defects in the valve itself, there is one notable exception to this. On early 2000’s Dodge Ram trucks, this code can be set by a short circuit in the seat belt warning chime circuit, since this circuit shares a fuse with both the CD changer and the EVAP system. This is a common problem, so consult the relevant manual on the correct procedure to prevent a continual recurrence of the problem.

Connecting the lab scope

Correct functioning of the canister purge valve can be checked by measuring the following signal voltages, see figure 1:

Channel Probe Voltage Range
1 Signal voltage at negative side of canister purge Signal voltage at negative side of canister purge valve 80 V
Ground at battery Ground at battery
2 Power supply at canister purge valve Power supply at canister purge valve 20 V
Ground at battery Ground at battery
Figure 1: Measuring diagram
Figure 1: Measuring diagram

Figure 2: Measuring a working canister purge valve

The lab scope is connected to the canister purge valve via a Measure lead TP-C1812B and Back Probe TP-BP85 and is set to normal scope mode.

P0443 Symptoms: Mercury Sable

There typically won’t be much in the way of noticeable symptoms associated with P0443 and your Sable.  Here are the most common ones:

  • Check Engine Light– This is often the only symptom that you’ll see with this code.
  • Gas Smell– If the purge valve is stuck shut, when the fuel vapor builds up it will escape into the air.  You may smell gas when you aren’t moving very fast.
  • Lean Condition– A lean condition indicates that the ignition process is occurring with too much air and not enough fuel in the mixture.  If it’s bad enough, it can cause the engine to run rough.  This is not very common though.  You may see P0171 or P0174 with this code if that’s the case.

How do you troubleshoot code P0443 ?

NOTE #1: Code P0443 specifically refers to issues in the purge valve control circuit, and leaks in the system, or issues with other EVAP circuits/components will typically not set this code. However, on some Hyundai (mostly Elantra, Santa Fe, Tucson, and Tiburon models) and some VAG models from the early 2000’s, a stuck open purge valve will typically produce code P0441, while on some Mazda products from around the same era, a stuck open purge valve will typically produce code P0446 along with a variety of other EVAP related codes.

NOTE #2: The EVAP purge valve must not be confused with the EVAP vent valve. The primary function of the vent valve is to allow fresh air to enter the system to help displace fuel vapors along sometimes-long vacuum lines towards the engine. Always refer to the manual for the application being worked on to correctly locate the components that are being diagnosed or worked on.

NOTE #3: A repair manual or wiring diagram for the application being worked on, as well as a good quality digital multimeter and a hand-held vacuum pump fitted with a gauge are required items to diagnose code P0443.

Record all codes present, as well as all available freeze frame data. This information can be very useful should an intermittent fault be diagnosed later on. Refer to the notes above if any other codes are present along with code P0443. Bear in mind that if multiple codes are present, they must be diagnosed and resolved in the order in which they were stored. Failure to do this will almost certainly result in a misdiagnosis.

If the code persists after clearing all codes, consult the manual on the location, routing, color-coding, and function of all wiring associated with the purge valve. Perform a thorough visual inspection of all associated wiring; look for damaged, burnt, disconnected, shorted, or corroded wiring and connectors. Make repairs as required, clear all codes, and retest the system to see which codes (if any) return.

NOTE:  Most applications typically require that several drive cycles be completed before EVAP codes can be fully cleared.

If no visible damage is found on the wiring, perform input voltage, ground, continuity, and resistance checks on all associated wiring, but be sure to disconnect the purge valve from the PCM to prevent damage to the controller.

Pay particular attention to the resistance of the purge valve input voltage wire, as well as the signal wire going to the PCM. Resistance values on these wires must match the values stated in the manual exactly. Make repairs, or replace wiring as required to ensure that all obtained readings fall within the manufacturer’s specifications. Clear all codes after repairs are complete, operate the vehicle, and retest the system to see if the code returns.

If the code persists despite having made repairs to wiring, suspect a defective purge valve. There are several ways to test the purge valve’s operation, but removing the valve from the system makes testing the valve considerably easier. Note that while most EVAP purge valves are rated for full battery voltage, there are exceptions to this, so always consult the manual for the correct input voltage before applying direct current to any component.

As a first step in testing the purge valve, test its internal resistance and compare this value with the value stated in the manual, and replace the valve if it does not test within stated specifications. If the resistance checks out, determine the correct input voltage and apply direct current to the valve, but note that the valve must be properly grounded.

On some valves, an audible “click” when current is applied will indicate that the control solenoid in the valve is working, but be aware that the absence of an audible “click” does not necessarily mean that the valve is defective, since some purge valves operate silently.

WARNING: When applying direct current to the purge valve from the vehicles’ battery, make absolutely sure that short circuits cannot happen. Short circuits can destroy the battery as well as the purge valve, in addition to causing serious burns when the test wires overheat. The better option is to use a battery charger to provide current for testing purposes, but whatever the source of the current, consult the manual on the correct procedure to apply direct current to the purge valve solenoid.

Even if the purge valve is known to open and close, how well it works must be tested as well. To do this, attach the vacuum pump securely to one opening of the valve, and draw a vacuum that registers on the gauge.

Purge valves are normally closed, so provided the test equipment is not defective in any way, the vacuum must hold if the valve is in good condition. Keep a close watch on the vacuum gauge- if the vacuum decays the valve is defective, and it must be replaced. If on the other hand, the vacuum does not decay over the space of about 60 seconds, apply direct current to the valve. If the valve works as intended, the vacuum will decay almost immediately: if it does not, the valve is also defective, and it must be replaced as well.

NOTE:  Testing of the purge valve is required because it forms part of the control circuit on the one hand, and for the fact that a defective purge valve can set code P0443 on some applications. Note however that where the code is set by a defective valve, other EVAP related codes are almost certain to be present as well.

Reassemble the EVAP system after all repairs are complete, and operate the vehicle for multiple drive cycles before scanning the system again to see if the code returns.

If the code returns but it is certain that all electrical values fall within specifications, the purge valve works as intended, and that all electrical repairs had been performed to industry standards, it is likely that an intermittent fault is present. Be aware that intermittent faults can be extremely challenging and time consuming to find and repair, and in some cases, it may be necessary to allow the fault to worsen before an accurate and definitive repair can be made.

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Innova CarScan OBD2 Scanner

NewYall Vapor Canister

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