P0304 Code on a GMC: Cylinder 4 Misfire Detected (GMC Guide)

P0304 on a GMC is GM's "Cylinder 4 Misfire Detected." On a Sierra, Yukon, Savana or Envoy V8 with Active Fuel Management, that is a loaded code: cylinder 4 is one of the four cylinders GM switches off in V4 mode, and GM has published a bulletin that names cylinder 4 by number on 2014–2015 Sierras and Yukons. On a four-cylinder Terrain, Acadia or Canyon, cylinder 4 is an end cylinder, which brings its own GM documents into play. The surprise: on an AFM V8, GM's first diagnostic step isn't a coil swap with the engine running. It's a running compression test that reads whether cylinder 4's valves are opening at all, plus an oil pressure check, because low oil pressure feeding the lifter oil manifold can kill AFM lifters. This guide covers where cylinder 4 is on each GMC engine, the GM documents that name it, and the order to test in.

What Does P0304 Mean on a GMC?

The ECM counts a misfire when cylinder 4's firing stroke fails to speed up the crankshaft the way the others do. Enough of those and it stores P0304, normally with P0300. If several cylinders are counting, our P0300 GMC hub sorts GM's documents by pattern. A P0304 that travels with P0301, P0306 or P0307 on a GMC V8 is the AFM group, and that's covered below. For general background on GM's misfire monitor, see our P0300 GM guide.

Every gasoline GMC can set P0304, including every four-cylinder model. Duramax diesels follow separate diagnostics, and the electric Hummer and Sierra EV can't set it.

Where Cylinder 4 Is on Your GMC

On a Savana, cylinder 4 is reached from inside the cab under the engine cover, so a swap test there takes longer than on a pickup. On the Acadia V6 it's on the front bank, one of the easier coils to reach.

Sierra and Yukon: Cylinder 4 Is an AFM Cylinder

GM's 20-NA-038 states that on its AFM V8s the ECM deactivates cylinders 1, 7, 6 and 4 in V4 mode. Cylinder 4 gets a two-stage AFM lifter that can unlock when it shouldn't or collapse. Three GM documents name it.

PIP5259D covers the 2014–2015 Sierra, Yukon and Yukon XL with the 5.3L L83 or 6.2L L86. GM's wording is direct: "engine misfire on cylinder 4, 6 and/or tick noise," from an AFM lifter mechanically collapsed or stuck, possibly from internal locking pin damage due to oil aeration. If the valves on the cylinder aren't moving, GM replaces the AFM lifters and guide on the affected bank — for cylinder 4, the passenger side — and also replaces the oil pump pickup seal. GM's warranty labor allowance in that document runs from 11.7 to 14.3 hours depending on model and drive, which tells you how big the job is. The later 15-06-01-002O carries the same lifter story for 2014–2018 Sierras and 2015–2020 Yukons.

PIP4568S covers the earlier trucks: 2007–2013 Sierra, 2007–2014 Yukon and 2006–2009 Envoy with AFM V8s (including the 5.3L LH6 in the Envoy and the 6.0L LFA hybrid). It lists misfire on cylinder 1, 4, 6 or 7, with or without a tick.

One more applies after repairs. PIP5083 covers 2007–2013 Savana, Sierra and Yukon trucks with the 4.8L L20, 5.3L LC9 or 6.0L L96 that misfire on cylinders 1, 4, 6 and 7 after a camshaft replacement. GM's tip is to check the last four digits stamped on the end of the camshaft against the right part for the engine.

GM's AFM Tests for Cylinder 4

PIP4568S gives the sequence GM wants before heads come off:

1. AFM running compression test. If cylinder 4's running compression stays below 25 psi whether the AFM solenoid is commanded on or off, the lifter is collapsed, stuck or unlocking at start-up.
2. Valve lifter oil manifold (VLOM) test. GM's service procedure compares air leakage across all four solenoids. A VLOM fault is replaced first and the engine re-evaluated.
3. Watch the valves. With the valve covers off, a technician watches the cylinder 1, 4, 6 and 7 rockers while cranking. If an AFM cylinder's valves stop moving, the lifter is the cause.
4. Check oil pressure. GM says most good engines show about 25 psi or more at hot idle with new oil on the scan tool. At 19 psi or less, GM wants the AFM oil pressure relief valve tested, because low pressure to the VLOM can damage AFM lifters.

When a lifter is confirmed on these earlier trucks, GM replaces the VLOM, all AFM lifters and all plastic lifter guides, and inspects the cam lobes through the lifter bores. For a repeat failure, GM adds an oil pump to deal with possible aeration. GM's broader pushrod tip, 19-NA-219, adds a no-parts trick for locating a tick: press on the valve side of each rocker with a wooden hammer handle. A change in the noise points to that cylinder's lifter or cam lobe.

On DFM trucks, every cylinder can deactivate. 23-NA-043 and PIP5861 cover lifter failures with P0300–P0308 on 2020–2022 Sierra 1500 and Yukon trucks inside specific build windows.

Cylinder 4 on Four-Cylinder and Inline GMCs

Older Canyon inline four. GM's PIP3472C says contaminated fuel trapped in the returnless rail misfires cylinder 1 on deceleration, "or on the opposite end cylinder during or following hard acceleration," and that the end cylinder depends on the engine. On the 2.8L LK5 and 2.9L LLV four in the 2006–2012 Canyon, that opposite end cylinder is cylinder 4. GM's fix is cleaning the tank, lines, rail and injectors, and it warns a sticking valve can stay behind. GM also says fuel-contamination repairs aren't treated as warranty repairs.

Terrain 1.5T. Three GM documents describe broken spark plug insulators: PIE0505 (2018–2019, P0300–P0304), PIP5657C (2018–2021) and 21-NA-255 (2018–2022). GM's check is to pull the plugs and turn each upside down; a broken center insulator will slide down the electrode. GM replaces all four plugs when it finds one.

Terrain 2.0T. PI1178F covers the 2018 Terrain 2.0L LTG with a misfire and low compression on one cylinder from a crack in the piston between the compression rings. GM confirms with a leak-down into the crankcase, and if the cylinder wall is usable, replaces all four piston assemblies and rod bearings.

2.7T and 2.5T. On the 2019–2025 Sierra 1500 and 2023–2025 Canyon 2.7L, 25-NA-037 lists P0304 with a tapping noise from worn exhaust roller followers; if one has failed, GM replaces the set. On the 2024 Acadia 2.5T, 25-NA-102 lists P0304 for rocker arms installed wrong at the plant, confirmed with a borescope while turning the engine by hand.

Cylinder 4 on V6 GMCs

On the 4.3L and every 3.6L, cylinder 4 always fires, so AFM lifter documents don't point to it. On the Gen 2 3.6L in the 2017–2019 Acadia and Canyon, GM's PIP5493G lists P0301–P0306 from broken or dislodged rockers (SRFFs) or bent camshaft actuator pins, so a non-AFM cylinder can still have a valvetrain fault. On older V6 Acadias and Terrains, the injector-harness and coil-ground documents in our P0303 GMC guide apply to cylinder 4 too.

Symptoms of P0304

• Check engine light, flashing during a heavy misfire
• A tick from the passenger-side valve cover on a Sierra or Yukon V8
• A misfire that starts when the truck drops into or comes out of V4 mode
• Low oil pressure reading at hot idle alongside the misfire (AFM trucks)
• A stumble under hard acceleration on an older four-cylinder Canyon
• A sudden misfire on a Terrain 1.5T with no warning (broken plug insulator)

Causes and How to Confirm Each

How Is P0304 Diagnosed on a GMC?

1. Identify the engine RPO and confirm whether it has AFM, DFM or neither.
2. Record freeze frame and per-cylinder counts. Note whether P0301, P0306 or P0307 are also counting.
3. Swap the coil and plug to another cylinder. If the misfire follows, you've found it.
4. Inspect the plug for cracks, oil fouling or a broken insulator.
5. Test the injector with GDS2 balance or AFIT.
6. On an AFM V8, run GM's AFM compression test, the VLOM test and an oil pressure check at hot idle.
7. Watch cylinder 4's valves with the cover off before committing to a lifter job.
8. Run leak-down if nothing else isolates it, then check your VIN for open bulletins and recalls.

Parts and Cost

GM Documents That Name P0304 or Cylinder 4 on GMC Models

Have a dealer check your VIN for open recalls, bulletins and special coverages. The check is free.

What Other Pages Get Wrong

• "A P0304 is a coil until proven otherwise." On an AFM Sierra or Yukon, GM names cylinder 4 in its lifter documents. If the swap test doesn't move the misfire, go to GM's compression test.
• "Just replace the one bad lifter." GM's documents call for the lifters and guides on the affected bank (and on earlier trucks, all AFM lifters and the VLOM), plus attention to oil pressure and aeration.
• "Cylinder 4 is at the rear of the passenger side." On a GMC V8 it's second from the front on the passenger side. On a lengthwise inline four, it is the rear cylinder.
• Ignoring oil pressure. GM ties low oil pressure to the VLOM directly to AFM lifter damage.

Frequently Asked Questions

Where is cylinder 4 on a GMC Yukon?

Second from the front on the passenger-side bank. GM numbers that bank 2-4-6-8 and the driver bank 1-3-5-7.

Is P0304 on my 2015 Sierra a lifter?

It can be. GM's PIP5259D names cylinder 4 and 6 misfires and tick on 2014–2015 Sierras with the L83 or L86. Rule out the coil, plug and injector first, then have the valves checked.

Will disabling AFM fix a cylinder 4 misfire?

Not if the lifter is already collapsed or stuck. GM's tests are designed to find a lifter that isn't opening the valve whether AFM is on or off.

Why does GM care about oil pressure for a misfire?

The AFM lifters are switched by oil pressure through the VLOM. GM's PIP4568S says low oil pressure to the VLOM can damage AFM lifters, so pressure is checked as part of the diagnosis.

My Terrain 1.5T has P0304. Do I replace just one plug?

GM's documents for the 1.5T say that if any plug has a broken or cracked insulator, all four plugs are replaced.

Can I drive with P0304?

With a flashing light, reduce load and get it checked soon to protect the catalytic converter. On an AFM truck with a tick, don't put it off. GM's lifter documents have technicians inspect the cam lobes during the repair, which tells you the damage doesn't always stay in the lifter.