P0442 Code on a GMC: EVAP System Small Leak Detected (GMC Guide)

P0442 on a GMC is GM's "EVAP System Small Leak Detected" — the engine computer sealed the fuel vapor system, watched it, and decided it lost pressure or vacuum through an opening of roughly 0.020 inch. That is a pinhole, which is why a P0442 can be so hard to find. On a GMC the surprise is where GM keeps pointing: not at the gas cap, but at the frame-mounted canister vent valve on Sierra and Yukon trucks, which GM has addressed in three separate bulletins for dust and water getting in. GMC also never uses the "very small leak" code P0456 on its gas engines — on a GMC, P0442 is the 0.020-inch leak code.

What Does P0442 Mean on a GMC?

Gasoline gives off vapor. Instead of venting it to the air, your GMC stores it in a charcoal canister and later draws it into the engine to be burned. Two valves run the show: the purge valve near the engine, which lets vapor into the intake, and the canister vent solenoid (CVS) at the canister, which lets fresh air in. A fuel tank pressure sensor watches what happens in between.

To check the system for leaks, the computer closes the vent valve so the tank, lines and canister are sealed. If pressure or vacuum leaks away faster than a calibrated limit, it sets P0442. GM's 2004 diagnostic table for the truck V8s and 4.3L V6 (the Sierra, Yukon, Savana and Envoy XL engines of that era) describes it as a small leak "(>= 0.020") in the EVAP system between the fuel fill cap and the purge solenoid." GM's 2019 table for the current Sierra and Yukon engines says the same 0.020 inch, adding that on some applications GM defines a small leak differently. It is a Type A code in those tables, but it is filtered through an averaging routine, so one bad test does not normally light the lamp on its own.

P0442 does not change how the truck drives. It is an emissions fault, and it will fail an emissions inspection.

Which GMCs Set P0442

Every gas-engine GMC with OBD-II can set P0442: Sierra 1500 and HD gas trucks, Yukon and Yukon XL, Savana, Canyon, Acadia, Terrain, Envoy and the older Sonoma, Jimmy and Safari. Duramax diesels have no gasoline EVAP system and don't use this code.

If a generic scanner shows "P0456" on your Sierra, re-read it with a tool that shows GM descriptors. Our P0456 guide explains why the two codes get confused.

How a GMC Runs the Leak Test

Most owners clear a P0442, see the light stay off for two days, and assume it's fixed. Knowing when the test runs explains why that proves nothing.

From the mid-2000s, GMC trucks use an engine-off natural vacuum (EONV) test. After you park a warm truck, the ECM closes the vent valve and watches tank pressure rise during the hot soak, then the vacuum that forms as the fuel cools. GM's 2004 truck table only runs it if the fuel level is between 15% and 85%, the drive lasted at least 10 minutes and 3 miles, coolant reached 70 °C, and the estimated outside temperature at the end of the drive was above 0 °C and below 33 °C. GM's 2019 Sierra/Yukon table widens the fuel window to 10–90% but asks for at least 600 seconds and 5 miles of driving and an estimated ambient between 0 and 35 °C, and won't repeat the test until at least 8 hours have passed.

So a Sierra that only makes two-mile trips, or one that is always kept full, may go a long time without a test. A Las Vegas summer afternoon above 35 °C won't run it either.

Sierra and Yukon Vent Valve Bulletins

GMC trucks mount the canister and vent valve under the truck, where they live with road dust, mud and spray. GM has three documents, all verified, that name P0442 alongside P0446, P0449 and P0455:

• 15-06-04-002 (April 2015) — 2011–2013 Sierra, 2014 Sierra 1500 and 2500/3500, and 2015 Sierra. Customers report the check engine light and a tank that's hard to fill. GM says dirt, dust or water entering the vent's fresh-air intake can restrict flow or corrode the valve. The fix is a revised, sealed CVS valve vented through a remote pre-filter mounted on top of the fuel filler housing; GM lists filter kit 23103351. GM is explicit: do not replace the canister unless it fails the leak test or has ingested excessive dirt, dust or water.
• 09-06-04-028D (April 2014) — 2007–2010 Yukon models and Yukon XL 1500, hybrids included. The same symptoms with moisture, water and corrosion at the CVS valve. GM supplies a revised vent assembly 20995472 and jumper harness 19257603.
• 23-NA-073 (version 5, March 2025) — 2019–2025 Sierra 1500 (including the 2022 Limited) and 2020–2025 Sierra 2500 HD/3500 HD, excluding cab-and-chassis trucks. For trucks driven in extremely dusty conditions, GM offers an in-line filter retrofit (filter assembly 86792068, foam filter 86792069) and calls the filter a maintenance item to check periodically. GM labels this a customer-pay alteration, not a warranty repair.

NHTSA's summaries also list GM bulletin 02-06-04-037 for 1999–2001 and 2003–2010 Sierra and Sierra Classic trucks used in dusty, farming or off-road conditions, with a replacement vent valve and relocated filter box — we could only read the summary, so treat it as corroborated.

Other GMC-Specific Causes GM Has Documented

• Filler vent hose connector (2013–2015). GM bulletin 15-NA-019 covers fuel odor while fueling or idling, with possible P0442 or P0455, on 2013 Sierra light duty and 2014–2015 Sierra 1500 (except regular cab) and 2013–2014 Yukon 1500 models. The cause is a fractured filler neck recirculation line connector.
• Savana cutaway fuel cap tether (2012–2013). GM's PI0873 says the cap tether on Savana 3500–4500 cutaways with an upfitter body and side-mounted tank can bind or get trapped under the cap seal, setting P0442 or P0455. The fix is moving the tether attachment.
• Acadia purge valve (2016–2017). Special Coverage N192210240 covers a purge valve that may not fully close, listing P0442 among its codes along with rough idle or stalling.
• 2019 Acadia and Terrain canister leaks. GM's PIE0528 engineering study asked for canisters leaking at the canister-to-vent-solenoid O-ring or through the fresh-air filter box.

Symptoms of P0442

• Check engine light, usually the only symptom
• A faint fuel smell near the rear of the truck or at the filler
• The pump nozzle clicking off early — hard to fill — on Sierras and Yukons with a contaminated vent path
• Fuel smell while filling on 2013–2015 Sierras and Yukons (15-NA-019)
• No change in how the engine runs, unless the cause is a purge valve that won't close (then rough idle is possible)
• Failed emissions inspection or "not ready" EVAP monitor

Causes and How to Confirm Each

How Is P0442 Diagnosed?

1. Confirm the code and check the freeze frame. Note fuel level and temperatures, and look for related codes such as P0446, P0449 or P0455.
2. Don't disturb anything first. GM's PIT4943D begins by saying not to disturb any fuel system components once P0442 is verified — reseating a cap or line first can hide the leak you're looking for.
3. Check the purge valve with the system sealed. Command the EVAP system closed, start the engine and watch tank pressure for about two minutes. Vacuum means the purge valve is passing.
4. Inspect GM's seven connections from 21-NA-124 — purge tube at the intake, purge tube to chassis line near the transmission, both lines at the canister, the vent valve line, the tank jumper and the fuel cap.
5. On a Sierra or Yukon, look at the vent valve and pre-filter for dirt, mud or water, and check your VIN against the three truck bulletins.
6. Smoke test under pressure and vacuum. GM warns test pressure must not exceed 13 inches of water (0.5 psi).
7. Repair, clear and let the monitor run. Keep fuel between about a quarter and three-quarters and allow several overnight soaks.

Parts and Cost

GM Documents Relevant to P0442 on a GMC

Have a dealer check your VIN against current bulletins and campaigns rather than relying on any article, including this one.

What Other P0442 Pages Get Wrong

• "It's the gas cap." GM's checklist has seven items and the cap is the last. On trucks the vent valve has three bulletins of its own.
• "Replace the charcoal canister." GM's Sierra and Yukon bulletins say not to unless it fails the leak test or is contaminated.
• Calling it a 0.040-inch leak. GM's tables define P0442 on GMC trucks as 0.020 inch.
• Treating P0456 and P0442 as different problems on a GMC. GMC gas engines report the 0.020-inch leak as P0442 only.
• Using a shop air hose or high pressure. GM's limit is 13 inches of water — about half a psi.
• Declaring it fixed after a day. The test needs the right fuel level, drive length and temperatures.

Frequently Asked Questions

Can I drive my GMC with P0442?

Yes. It doesn't affect how the vehicle drives. But it will fail an emissions inspection, and a light that's already on can hide a new problem.

My Sierra is hard to fill and has P0442. Are they related?

Very likely. That pairing is exactly what GM describes in 15-06-04-002 (2011–2015 Sierra) and 09-06-04-028D (2007–2010 Yukon): dust or water in the vent valve restricts the path air takes when the tank fills.

Is the 23-NA-073 filter covered by warranty?

No. GM calls it a customer-pay alteration for trucks used in extremely dusty conditions, and says the filter is a maintenance item to check periodically.

Will a new gas cap fix P0442?

Only if the cap was the leak. Reseat it, check the seal, and give the monitor several days. If the code returns, keep going through GM's connection checklist and smoke test.

How long until P0442 clears after a repair?

The leak test runs after a drive of at least several miles with fuel between roughly a quarter and three-quarters, followed by a long enough soak. Expect several days of normal driving.

Does my Acadia's purge valve coverage apply to P0442?

N192210240 lists P0442 among the codes for 2016–2017 Acadias whose purge valve may not fully close. The term is 10 years or 150,000 miles from the in-service date, so many have reached the end of it — a dealer VIN check is free.

Why did my scanner say P0456?

Generic tools sometimes show generic definitions. GMC gas engines use P0442 for a 0.020-inch leak; re-read with a GM-capable tool. For the whole EVAP family, see the GM EVAP leak codes guide.