P0442 Code: Evaporative Emission System Leak Detected (Small Leak)

What happens if you don't fix it

Symptoms you can actually notice

• Check engine light after refueling
• No drivability change
• Failed emissions test

What the computer is actually complaining about

The fuel tank does not vent to open air. Vapor runs into a charcoal canister and gets burned later. The computer closes a valve, pulls a slight vacuum on the whole tank, and watches how fast that vacuum bleeds off. A small leak bleeds slowly. Two failed self-tests in a row set P0442.

Most likely causes, cheapest check first

What you need in your hands

• OBD-II scanner

  A $25 Bluetooth reader plus a free phone app shows whether the EVAP monitor has run and stores the freeze frame.

• Replacement fuel cap Optional

  A cap with the right seal runs $10 to $30 at any parts store. Match the year, make and model.

• Propane or smoke test kit Optional

  A shop smoke test costs about $40. A $15 propane bottle and a length of hose let you do a rough version in the driveway.

Test it yourself, step by step

Do these in order. Each step tells you what to do, what to look at, what counts as fine, what counts as broken, and what you will actually see.

1. 1 Open the fuel door on the rear quarter panel, driver side or passenger side depending on the car, and unscrew the cap. Look at the rubber seal on the cap and the metal ring on the filler neck.

   Look at

   The seal for cracks, flat spots, grit, or a cap that rocks when you press it sideways.

   Fine if

   The rubber ring is soft, round, and sits flat, and the cap clicks three times when you screw it back on.

   Broken if

   The seal is hard, split, or missing, or the cap turns past the click. A cap that does not seal sets this code on its own.

   What you'll see A black rubber ring about the size of a half dollar. If it looks shiny and cracked like old plastic, that is your leak.

2. 2 Plug the scanner into the trapezoid port under the dash on the driver side, above your left knee. Key to ON, engine off, and open the monitor status screen.

   Look at

   Whether the EVAP monitor shows complete or not complete, and what the freeze frame recorded at the failure.

   Fine if

   The EVAP monitor reads complete and the code is stored, not pending. That means the leak repeats.

   Broken if

   The monitor reads not complete, or the code only shows pending. Clear it and drive a full tank before you buy anything.

   What you'll see A row of monitors with green, yellow or grey marks. Grey means the test has not run yet since the last clear.

3. 3 Open the hood and find the purge valve, a small black cylinder about the size of a film can, bolted on the engine bay side of the intake with one hose running to the canister line.

   Look at

   Every rubber hose from that valve back along the firewall for splits, loose clamps, and rubbed spots.

   Fine if

   Hoses are soft, clamps are tight, and no hose shows a shiny crack line or a wet spot.

   Broken if

   A split end, a clamp you can turn by hand, or a hose that crumbles when you squeeze it. A $10 hose fixes that.

   What you'll see Hard black rubber lines the width of your little finger, running from the valve toward the rear of the car along the inner fender.

4. 4 Jack the rear of the car and look above the spare tire well on the passenger side or driver side, where a black plastic box the size of a shoebox hangs under the floor.

   Look at

   The canister body, its two hose fittings, and the vent valve on top for cracks, road rash, and fuel stains.

   Fine if

   The box is dry, the fittings are seated, and no hose hangs loose or rubs on the fuel tank strap.

   Broken if

   A crack line, a fitting that wiggles, or a strong fuel smell at the box. Stop and let a shop look before you drive far.

   What you'll see A dull black plastic box with a light dusting of road dirt. Fresh wet spots or a fuel smell mean a liquid leak, not vapor.

5. 5 Buy a cap for your exact year, make and model, screw it on until it clicks three times, then clear the code from the driver seat with the scanner still plugged in under the dash.

   Look at

   Whether the code stays off through a full tank of gas and several cold starts.

   Fine if

   P0442 stays off after 100 miles and one refueling. The cap was the leak.

   Broken if

   The code returns within 50 miles. Keep the old cap in the box and move to a smoke test.

   What you'll see Three clear clicks from the cap, then a blank code screen after the clear. A pending code back in a day means the cap was not it.

6. 6 Have a shop hook a smoke machine to the test port on the EVAP line near the rear wheel well, or run a propane hose along the same joints with the engine idling.

   Look at

   Where the white smoke leaves the system, and whether it drifts from a hose joint, a clamp, or the canister seam.

   Fine if

   Smoke only exits at the open test port where the machine is connected. No smoke from any joint.

   Broken if

   Smoke curling from a hose end, a clamp, or a seam in the canister. That joint is the leak, and the fix is the part it belongs to.

   What you'll see A thin white trail that moves when you wiggle a hose. The spot where the trail grows is the leak, usually within a foot of the tank.

What the repair costs

Reference range compiled from 2 source type(s), as of 2026-11. This is not a quote — actual cost depends on your vehicle, region, and which part turns out to be the real cause.

Basis: editorial reference range, not a live parts quote · Data as of 2026-11

How to confirm it's really fixed

✓ Clear the code, fill the tank, and drive a full tank of mixed city and highway miles. The EVAP monitor needs a cold start, a warm-up and a steady cruise before it runs. If P0442 stays off after that, the repair held. If it returns, the leak is still open and the smoke test is the next step.

Common questions