What the evap emission system does
Your car's evaporative emission control system — usually called the evap system — captures fuel vapors that would otherwise escape from your fuel tank and fuel lines into the air. Instead of releasing those vapors, the system stores them in a charcoal canister and routes them back into the engine to be burned as part of normal combustion. This prevents volatile organic compounds from entering the atmosphere.
The evap system is required on every gasoline-powered vehicle sold in the United States. It sits between your fuel tank and your engine, working quietly in the background whether you are driving or parked. When something goes wrong with the system, your check engine light turns on and your vehicle may fail an emissions test.
Key Takeaways
- The evap system captures fuel vapors from your tank and routes them back to the engine instead of letting them escape into the air.
- A faulty evap system triggers a check engine light and will cause your vehicle to fail an emissions inspection.
- Common problems include a loose gas cap, a cracked hose, or a failed charcoal canister, ranging from a five-dollar fix to several hundred dollars.
- You can clear a check engine light yourself by tightening your gas cap, but the underlying problem may still exist.
- Evap system repairs must be done by a mechanic, but understanding how the system works helps you understand what the repair actually costs.
The main parts of an evap system
The evap system has four core components. The fuel tank stores gasoline and is sealed except for a vent line. The charcoal canister sits underneath or near the vehicle and absorbs fuel vapors when the engine is off. The purge valve opens when the engine is running to let those stored vapors flow into the engine. The vent valve allows fresh air into the canister so it can absorb more vapors.
Hoses and lines connect all four parts. A loose or cracked hose is one of the most common reasons an evap system fails. The gas cap itself is also part of the system — it seals the fuel tank opening and prevents vapors from escaping there. Many check engine lights are triggered by nothing more than a gas cap that is not tight enough or is damaged.
Why your check engine light comes on
Modern vehicles have an onboard diagnostic system that monitors the evap system for leaks. If the system detects a leak — whether it is a cracked hose, a faulty valve, or a loose gas cap — it stores a diagnostic trouble code and illuminates your check engine light. The code tells a mechanic which part of the system is failing.
A small leak might not cause any noticeable change in how your car drives, but it will still trigger the light. A large leak can cause your vehicle to fail an emissions test. In some states, you cannot renew your vehicle registration if your check engine light is on and the vehicle fails emissions testing.
Common evap system problems and their causes
A loose or missing gas cap is the single most common cause of an evap system fault code. The cap seals the fuel tank, and if it is not tight or if the seal inside the cap has degraded, vapors escape and the system detects a leak. Replacing a gas cap costs between five and twenty dollars.
A cracked or disconnected hose is the second most common problem. The rubber hoses that carry vapors from the tank to the canister and from the canister to the engine can crack, split, or come loose from their fittings. A mechanic can usually replace a single hose for fifty to one hundred fifty dollars, depending on which hose and how accessible it is.
A failed purge valve or vent valve prevents the system from moving vapors correctly. These valves can stick open or closed, or the solenoid that controls them can fail. Replacing a valve typically costs one hundred to three hundred dollars including labor.
A saturated or failed charcoal canister means the canister can no longer absorb vapors. This usually happens after many years of use or if the system has had a large leak that flooded the canister with liquid fuel. Canister replacement is the most expensive evap repair, often ranging from three hundred to eight hundred dollars depending on the vehicle.
How to check your gas cap first
If your check engine light is on, the first step is to pull over safely and check your gas cap. Open your fuel door, look at the cap, and make sure it is tight. If it is loose, tighten it firmly until you hear or feel a click. If the cap looks cracked or the rubber seal looks dried out or damaged, you should replace it.
After tightening or replacing the cap, drive your vehicle normally for a few days. Many vehicles will clear the check engine light on their own after you have fixed the problem and the system has run through its diagnostic cycle. If the light stays on after a week of normal driving, the problem is something other than the gas cap and you will need a mechanic to read the diagnostic code.
When to see a mechanic about evap problems
If your check engine light remains on after you have checked the gas cap, or if you notice fuel smell around your vehicle, you should have a mechanic read the diagnostic trouble code. The code will tell you exactly which part of the system is failing. Many repair shops will read the code for free or for a small fee, usually between twenty and fifty dollars.
Do not ignore an evap system fault code. Depending on your state and your vehicle's age, a failed emissions test can prevent you from registering your vehicle. The longer you wait, the more damage a leak can cause — a small leak in a hose is cheaper to fix than a canister that has been flooded with fuel for months.
Evap system repairs and costs
The cost of an evap repair depends entirely on which part has failed. A gas cap replacement is five to twenty dollars. A hose replacement is fifty to one hundred fifty dollars. A valve replacement is one hundred to three hundred dollars. A charcoal canister replacement is three hundred to eight hundred dollars. These ranges vary by vehicle make and model, by your location, and by whether you go to a dealership or an independent shop.
Before you authorize any repair, ask the mechanic to show you the diagnostic code and explain which part failed and why. Ask whether they recommend replacing just the failed part or replacing multiple components at once. Some mechanics will recommend replacing the canister and valves together if they suspect the canister has been exposed to liquid fuel, because a new canister can fail quickly if the purge valve is still faulty.
Frequently Asked Questions
Can I drive my car if the evap system is broken?
Yes, a broken evap system will not prevent your car from running. You can drive it to a mechanic. However, you will not be able to pass an emissions test, and in many states you cannot renew your vehicle registration without passing that test. The longer you wait to fix it, the more expensive the repair may become.
Will tightening my gas cap clear the check engine light?
Tightening the gas cap may clear the light if the cap was the problem, but only after the vehicle has gone through its diagnostic cycle, which usually takes a few days of normal driving. If the light comes back on, the problem is something else in the evap system and you will need a mechanic to diagnose it.
What does it mean if I smell fuel around my car?
A fuel smell usually indicates a leak somewhere in the fuel system or evap system. This could be a cracked hose, a loose connection, or a failed valve. Have a mechanic inspect the vehicle as soon as you can, because a fuel leak is a safety concern in addition to being an emissions problem.
Is an evap system repair covered by my car's warranty?
Most manufacturer warranties cover evap system repairs for three years or 36,000 miles, whichever comes first. Some extended warranties cover longer. Check your warranty paperwork or contact your dealership to see what is covered. If your vehicle is out of warranty, you will pay for the repair out of pocket.
Can I replace the charcoal canister myself?
The charcoal canister is usually located underneath the vehicle and requires removing parts of the undercarriage to access. Most people do not have the tools or experience to do this safely. A mechanic can do it in one to two hours. If you are experienced with vehicle repair, consult your vehicle's service manual before attempting it yourself.