What's the difference between P0440, P0442, and P0455?

P0440 is a general EVAP system malfunction — the system isn't working right but the ECM isn't specifying a leak size. P0442 = detected a small leak (more specific). P0455 = detected a large leak (most specific). P0440 is more common on older OBD2 vehicles; newer vehicles typically set P0442 or P0455 instead. All three start with the same first step: the gas cap.

Can I drive with P0440?

Yes — P0440 is an emissions-system fault only. It won't affect drivability or safety. You will likely fail an OBD2 emissions test. Fix it when convenient.

I replaced the gas cap and purge valve and P0440 still comes back. Now what?

Get a smoke test at a shop ($80–$120). This is the definitive diagnostic for EVAP leaks. Without it, you're replacing parts by guessing, which costs more than the diagnostic in the long run.

Can topping off my gas tank cause P0440?

Yes. When you add fuel after the pump clicks off, you're forcing liquid fuel into the vapor space above the tank — liquid that can reach and saturate the charcoal canister. A saturated canister sets EVAP codes including P0440. Stop topping off your tank — when the pump clicks, you're done.

P0440low severityGenerally driveable

P0440 Code: Evaporative Emission Control System Malfunction

What it means in plain English

P0440 is an older, more generic version of the EVAP leak codes. It means the ECM ran a self-test on the sealed fuel vapor system and got a result that doesn't fit the 'small leak' (P0442) or 'large leak' (P0455) categories specifically — instead flagging the system as generally not working as expected. You'll often see this on older OBD2 vehicles (pre-2000 era) or alongside a more specific EVAP code. The first step is always the same: check the gas cap.

Most likely causes — ranked

#1 Loose, damaged, or missing gas cap

most likely

Fix: Remove and reinstall the gas cap firmly. On threaded caps, turn until you hear audible clicks. On capless systems (Ford's Easy Fuel), make sure nothing is obstructing the sealing flap. Clear the code and drive 2–3 normal cycles. If the code doesn't return, the cap was the cause. A new gas cap is $10–$25.

DIY ~$20Shop ~$30

#2 Failed EVAP purge solenoid (canister purge valve)

likely

Fix: The purge solenoid opens during engine operation to draw stored fuel vapors from the charcoal canister into the intake manifold. If it's leaking, stuck open, or has failed electrically, it disrupts the system's ability to seal and self-test properly. Test resistance with a multimeter; replace if out of spec.

DIY ~$20Shop ~$160

evap purge valve repair guide

#3 Leaking or cracked vapor hose or line

possible

Fix: Inspect the rubber and plastic vapor lines connecting the fuel tank, charcoal canister, and purge valve. Heat and age cause cracking, especially where hoses connect to rigid fittings. A smoke test at a shop locates leaks precisely.

DIY ~$15Shop ~$200

#4 Saturated or damaged charcoal canister

possible

Fix: If the car has been frequently run with an overfilled gas tank (topping off after the pump clicks off), liquid fuel can saturate the charcoal canister and damage it. A saturated canister can't properly adsorb vapors and may fail the system self-test. Visual inspection and a smoke test identify canister issues.

DIY ~$60Shop ~$300

Get a FREE OBD2 scan first — no purchase required

AutoZone, O'Reilly Auto Parts, and Advance Auto Parts all scan your car's computer for free. Walk in, they plug in a scanner, you get a code in under 2 minutes. Then come back here and look up that code at eli5cars.com/obd2 for the plain-English explanation.

Pro tip: Take a photo of the code before they clear it.

P0440 Code: Evaporative Emission Control System Malfunction

Most likely causes — ranked

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Frequently asked questions