What Does GM Code P0456 Mean?
- GM P0456 definition: Evaporative emission control system leak (small).
- Issue Severity: LOW – Continued driving for a short period of time is okay
- Repair Urgency: Get this fixed within the next month to prevent drivability problems and excessive fuel consumption.
- Diagnosis: It is unlikely the driver will notice any symptoms with check engine light code P0456 other than a slight odor of fuel, a slight decrease in fuel economy, and the check engine light. However, as with all check engine lights, it is recommended that you get it fixed as soon as possible so the engine can be running at the proper specifications to prevent further damage.
The EVAPorative emission control (EVAP) system prevents fuel vapors from escaping into the atmosphere. The fuel vapors from the fuel tank are absorbed and stored by charcoal pellets in the charcoal canister. The vent control valve is controlled by the engine control module (ECM) and allows air to flow into the charcoal canister to purge the gas vapors into the engine air intake to be burned. The flow of the gas vapors from the charcoal canister to the engine air intake is controlled by a purge volume control valve. The vent control valve is usually open when the engine is at normal operating temperature, and the purge volume control valve is commanded on when the ECM is ready to burn the built-up fuel vapors. However, when the GM is turned off the ECM performs a leak test to ensure the EVAPorative emission control system is working properly. During the leak test, the ECM closes the vent control valve and purge valve to seal the EVAPorative system. If the EVAP system does not maintain the pressure, the ECM recognizes an EVAPorative emission control leak. In the case of P0456, it is a small leak, less than .020” in diameter.
GM P0456 Causes
There are many potential causes of code P0456.
- Loose or damaged gas cap
- Leaking or disconnected EVAP hose
- Faulty purge volume control valve
- Faulty canister vent control valve
- Charcoal canister leak
- Leaking fuel tank
GM P0456 Symptoms
- Check Engine Light is on
- Decreased fuel economy
- Increased vehicle emissions
- Fuel smell
How Do I Fix GM Code P0456?
With an EVAP fault, the first step is to get it diagnosed to figure out what is causing the problem.
If your GM has this fault and you’re not comfortable diagnosing this issue at home, we recommend finding a RepairPal certified shop nearby to pinpoint the problem and give an accurate estimate for repairs.
These shops can not only help you figure out what’s going wrong before you waste time and money on the wrong parts, but they also offer a minimum 12-month, 12,000-mile warranty and stand behind all their estimates with guaranteed fair pricing.
How Much Does It Cost to Fix GM Code P0456?
P0456 can be caused by anything from a loose gas cap to a faulty valve to a leaking fuel tank. It’s impossible to give an accurate estimate without properly diagnosing the issue first.
If you take your car to a shop for diagnosis, most shops will start with an hour of “diag time” (the time spent in labor diagnosing your specific issue). Depending on the shop’s labor rate, this typically costs somewhere between $75-$150. Many, if not most, shops will apply this diagnosis fee to any required repairs if you have them perform the repairs for you. From there, a shop will be able to give you an accurate estimate for repairs to fix your P0456 code.
Possible Repair Costs for GM P0456
For error code P0456, one or more of the below repairs may be needed to solve the underlying issue. For each possible repair, the estimated cost of repair includes the cost of the relevant parts and the cost of labor required to make the repair.
- Gas Cap $20-$60
- Evap Purge Volume Control Valve $150-$200
- Charcoal Canister Vent Control Valve $150-$200
- Replacement Evap Line $50-$100
- Charcoal Canister $200-$600
DIY Steps to Diagnose GM Code P0456
GM engine code P0456 could be caused by a number of things, including a loose gas cap, a faulty valve, or a leaking fuel tank. If you’d like to try to fix code P0456 at home without throwing money at parts, you’ll want to follow the steps below for proper diagnosis. Keep in mind this is an intermediate-level diagnosis and repair and not recommended for beginners. Diagnosis requires more specialized equipment beyond what the FIXD Sensor can provide and it can be a time and labor-intensive process for inexperienced DIYers.
DIY difficulty level: Intermediate
This repair requires mechanical knowledge and is not recommended for beginners.
Tools/parts needed (our top picks from Amazon):
- Fused power wire
- Ground wire
STEP 1: USE FIXD TO ENSURE NO OTHER ENGINE CODES ARE PRESENT.
Scan your GM to verify P0456 is the only code present. If other codes are present regarding fuel pressure or the fuel system, repair and diagnose those first. If it is paired with P0441, P0440, and/or P0446, there is most likely a solenoid failure, leaking charcoal canister, or a more complex EVAP leak.
STEP 2: CHECK YOUR GAS CAP.
Inspect your GM’s gas cap to see if it is loose or damaged. If your gas cap is loose, tighten it and clear the code. Inspect your gas cap for physical damage or deterioration. However, it should be noted that damage to the gas cap or deterioration of its components is not always noticeable. If your gas cap was not loose and you do not see any indications of failure, try replacing the gas cap anyway and clearing the codes. Gas caps are relatively inexpensive and are often the fix for code P0456.
STEP 3: CHECK HOSES.
Check for cracked or disconnected EVAP hoses near or connected to the engine air box. Replace cracked/disconnected hoses. Clear the code.
STEP 4: CHECK FUEL TANK AND CHARCOAL CANISTER.
Inspect the fuel tank and charcoal canister for damage and leaks. Replace if necessary.
STEP 5: CHECK PURGE VOLUME CONTROL VALVE.
Check the purge volume control valve for proper operation. This valve is normally not powered on and when at rest, with no power source applied, does not allow air to pass through. It can get sticky, causing leaks.
The purge volume control valve is usually under the hood near the airbox or intake manifold. To test, remove the hoses from either side of the purge volume control valve with the key and engine off. Blow through openings with no power supplied. If you can’t blow through them, they are sealing properly and are most likely not the cause of this medium EVAP leak.
STEP 6: CHECK CHARCOAL CANISTER VENT CONTROL VALVE.
Check the charcoal canister vent control valve for proper operation. This valve is normally not powered on and when at rest, with no power source applied, allows air to pass through. It can get sticky, causing leaks, or the internal solenoid can fail and not operate properly.
The charcoal canister vent control valve is usually connected to the charcoal canister underneath the vehicle. To test, remove the hoses from either side of the charcoal canister vent control valve with the key and engine off, unplug it from the GM, and remove it from the vehicle. Blow through openings with no power supplied. Air should pass through. Now, supply a fused power source to one side and ground to the other of the electrical connector. Blow through the openings again. If you can’t blow through them, they are sealing properly and are most likely not the cause of this medium EVAP leak.
STEP 7: RUN A SMOKE TEST.
The leak causing code P0456 is often too small to see. If you have completed all of the previous diagnostic steps, a smoke test may be necessary. You can purchase a smoke tester from Amazon to do it yourself, or you may want to take it to a shop to find the leak.
Common GM P0456 diagnosis mistakes
Many assume that a loose fuel cap is the only problem and do not perform all the tests necessary to diagnose the full EVAP system. The P0456 leak is a bit trickier to diagnose. Also, GM may have technical service bulletins regarding EVAP codes. Be sure to check if your particular year and model has any technical service bulletins available to save time diagnosing and/or misdiagnosing your GM.
Still Need Help Fixing GM Code P0456?
If you’ve followed the steps above and are still experiencing check engine code P0456, please contact the FIXD Mechanic Hotline if you’re a FIXD Premium subscriber or find a RepairPal certified shop near you to get the right repairs at a fair price.
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