P0442 Code: EVAP System Leak Detected (Small Leak)
The EVAP system leaks through an opening of about 0.040 inch — a cracked hose, loose joint or tired seal.
Safety first · Consult a professional
- Fuel vapor is flammable. No smoking or open flames; work outdoors and fix fuel leaks before driving.
At a glance
Severity: Minor
- Meaning
- EVAP System Leak Detected (Small Leak)
- System
- Emission Controls · Powertrain
- Safe to drive?
- Yes, short term
- Typical repair cost
- $15–$650
- DIY difficulty
- Moderate
The EVAP system traps gasoline vapor from the tank in a charcoal canister and later burns it in the engine. To make sure no vapor escapes, the ECU seals the system and measures how fast pressure or vacuum leaks away, or, on vehicles that test with the engine off, how hard a small pump has to work compared with a built-in reference orifice. P0442 means the leak rate matches an opening of roughly 0.040 inch (about 1 mm): too small to smell most of the time, but too big to pass. Because the test only runs under specific conditions, the code can come and go, and the leak rarely shows up on a quick look — a smoke test is usually the only reliable way to find it.
Can I keep driving? Safe to drive; the engine runs normally. The main downside is that the lit check engine light can hide a new, more serious fault, so don't leave it for months.
How to read P0442
- P Powertrain
- 0 Generic (SAE)
- 4 Auxiliary emission controls
- 42 Specific fault
Symptoms
- Check engine light, often returning a few days after being cleared
- No change in how the car drives
- Occasional faint fuel smell near the rear of the car on hot days
- EVAP monitor won't show 'ready' for an emissions test
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Cracked or hardened EVAP hose, often the filler-neck vent line or canister lines under the car
- Gas cap seal worn or hardened even though the cap clicks tight
- Purge valve not sealing fully (counts as a leak on engine-off tests)
- Canister vent valve not sealing completely when closed
- Leaking seal at the fuel pump module or sender lock ring on top of the tank
- Cracked charcoal canister or loose connection at the canister
- Pinhole rust in the fuel filler neck
Diagnosis step by step
Record conditions before clearing
Save the freeze frame and note fuel level and ambient temperature. Many EVAP tests only run with the tank roughly between 1/4 and 3/4 full and outside temperatures in a moderate range (about 40–95°F), which tells you when the fault can be reproduced.
Tools: OBD-II scanner
Check the EVAP Mode 6 result
If your scanner shows on-board test results, look at the EVAP small-leak test. A value just over the limit suggests a tired seal or cap; one far over it suggests a clearly cracked hose or loose connection.
Tools: OBD-II scanner
Replace a questionable cap first
If the cap seal is hardened, cracked or the cap is an aftermarket universal part, replace it with an OEM-quality cap before paying for deeper diagnosis. It's cheap and removes one variable.
Tools: Flashlight
Smoke test with the vent valve closed
Command the vent valve closed with a scan tool (or cap the vent side), then feed EVAP-rated smoke at low pressure into the service port or filler neck. Give it several minutes and use a bright light; small leaks produce only a thin wisp at a hose joint, the canister or the tank top.
Tools: Smoke machine, OBD-II scanner, Flashlight
Check that the purge valve holds vacuum
Disconnect the purge valve and apply vacuum with a hand pump while it's unpowered. It must hold vacuum; if it bleeds down, it's leaking internally and should be replaced.
Tools: Vacuum gauge
Inspect the top of the tank
On many vehicles the fuel pump module is reached through an access panel under the rear seat or in the cargo floor. Look for smoke, fuel dampness or a lock ring that isn't fully seated.
Tools: Flashlight, Screwdriver
Confirm the repair with the monitor
Clear the code and let the car complete the EVAP test, which may take several days and overnight cold starts. Only a completed monitor with no pending code proves the leak is fixed.
Tools: OBD-II scanner
Where the parts are
- 1Gas cap
- 2Charcoal canister
- 3Purge valve
- 4Vent valve
- 5Fuel pump
- 6Intake manifold
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Replacement gas cap | Yes | $15–$45 |
| EVAP smoke test diagnosis | No | $100–$200 |
| Replace EVAP hose or connector | Yes | $100–$300 |
| Replace purge or vent valve | Yes | $100–$300 |
| Replace charcoal canister or filler neck | No | $250–$650 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Notes by brand
Field data
| Chevrolet | 11 | Corvette Zr1, Cobalt, Corvette Z06 | |
|---|---|---|---|
| Hyundai | 2 | Sonata, Veloster | |
| Mazda | 2 | CX-7, Mazda3 |
Source: NHTSA Complaints API (US government open data), analyzed by DTC mention.
Emissions / smog test impact
Will fail
An active P0442 fails OBD-based emissions and smog tests. Even after a repair, the EVAP monitor often takes longest to become 'ready', and most states allow only one unready monitor on 2001-and-newer vehicles, so plan a few days of normal driving before the test.
Frequently asked questions
How big is a 'small' leak?
The test is calibrated around an opening of about 0.040 inch (1 mm) — roughly a pinhole or a hairline crack in a hose. That's why you usually can't see or smell it without a smoke machine.
Why does P0442 come and go?
The leak test runs only under certain fuel levels, temperatures and after the car has sat for hours. A leak at a hose joint can also open and close with temperature, so the result varies from day to day.
Should I just replace the gas cap?
It's a reasonable first step when the cap is old or aftermarket, because it costs little. If the code returns with a good cap, stop guessing and get a smoke test.
What's the difference between P0442 and P0456?
Both are leak codes; P0442 is the 0.040-inch test and P0456 the even smaller 0.020-inch test. A P0442 leak is usually easier to find with smoke than a P0456 leak.