P0300 Code on a Buick: Random/Multiple Cylinder Misfire Detected (Buick Guide)

P0300 on a Buick is "Engine Misfire Detected" — the engine computer counted misfires it could not pin on one cylinder, or on several at once. Most guides treat it as a spark plug and coil job, and sometimes it is. But on Buicks GM has documented some very different causes behind this exact code: a coolant leak into the cylinder through porous block casting on the 3.6L V6 in the Enclave, LaCrosse and Regal, cracked pistons on the 1.4L turbo Encore, broken spark plug insulators on later Encores, a loose ground stud on the Encore GX, and mis-installed rocker arms on the 2025 Enclave. Knowing which Buick you have changes what gets checked first.

What Does P0300 Mean on a Buick?

Every GM engine computer watches the crankshaft speed sensor for tiny slowdowns between firing events. When a cylinder fails to fire properly, the crankshaft decelerates for an instant, and the ECM counts that as a misfire against the cylinder it believes was firing.

When the misfires are clearly on one cylinder, you get a cylinder-specific code — P0301 through P0306 on a V6 (P0301–P0304 on a four-cylinder). P0300 sets when the misfire is random, moves around, or hits more than one cylinder. That is why P0300 frequently arrives with one or more cylinder codes beside it, and why GM's own bulletins list P0301–P0306 as related keywords.

A flashing check engine light means the misfire rate is high enough to damage the catalytic converter. And "random" points away from a single coil or plug, toward things every cylinder shares — fuel, air leaks, grounds, oil or coolant problems, or several worn parts at once.

For the full GM-wide explanation of the misfire monitor, see our P0300 GM diagnosis and repair guide. This page focuses on what GM has documented specifically for Buick models.

Which Buicks Set P0300 — and Have GM Paperwork Behind It

Any Buick can set P0300. The ones where GM has published a bulletin naming the code are:

We did not find a GM bulletin we could read that names P0300 on the 3800 V6 (LeSabre, Park Avenue, Regal, early LaCrosse and Lucerne) or the 2.4L Ecotec LaCrosse, Regal and Verano. The code certainly sets on those engines; GM simply has not published a Buick-specific pattern we could verify.

3.6L V6: The Cold-Start Coolant Leak

This is the Buick P0300 story most owners never hear about. GM bulletin 18-NA-115 — which replaced a long-running preliminary information document, PIP4959 — has grown to cover the 2009–2025 Enclave, 2009–2019 LaCrosse, 2009–2011 Allure and 2018–2019 Regal, plus a long list of Cadillac, Chevrolet and GMC V6 models.

GM's condition is specific:

• The engine misfires and/or idles rough on a cold start.
• The misfire count is medium to high and always on the same one or two cylinders.
• The condition goes away once the engine warms up.

The cause GM gives is "possible engine cylinder block porosity causing a pinhole at the liner to deck face casting area, allowing engine coolant to leak into the affected cylinder." Coolant seeps in while the engine sits, and the wet cylinder misfires until it burns off.

GM's procedure is to add dye to the cooling system, warm the engine fully (GM notes the thermostat should be fully open at 107 °C / 225 °F), pressurize the cooling system once the engine has cold-soaked, and inspect the suspect cylinders with a blacklight borescope for dye streaming down the liner. If the borescope is inconclusive, the cylinder head may have to come off. GM warns technicians not to confuse normal deck pitting, or residual fuel on the piston crown, with the real thing.

The correction is blunt: "If this condition is verified, the engine must be replaced." There is no plug, coil or injector that fixes it.

2017 LaCrosse and the 3.6L LGX

GM's PIP5493B told technicians that on a 2017 LaCrosse in for the first time with P0300 or P0304 on cylinder 4, they should clear the codes, try to duplicate the conditions and not replace parts while engineering worked on a fix. For P0302 or P0305 on these engines, it directed replacing the rocker arm oil control valve first.

Encore and Encore GX Turbo Fours

The small Buick crossovers have the densest stack of GM misfire documents.

1.4L LUJ/LUV (2013–2019 Encore). Bulletin 19-NA-090 ties P0300 with a misfire and low compression to a broken piston ring land, often alongside oil consumption or leaks. GM's instruction: if the cylinder wall is not damaged, do not replace the engine — replace all four pistons and rings. Separately, PIP5353A (2014–2017 Encore) warns that fuel injector control circuits can be connected crossed, causing misfires only at certain rpm that are hard to duplicate. The check is to compare injector circuit wire colors against the schematic.

1.4L LE2 (2016–2019 Encore). Bulletin 18-NA-171 covers a rough-running engine with P0300, a misfire and low compression in one or more cylinders, caused by a damaged piston. GM's procedure is a compression test, then a cylinder leakage test. If leakage to the crankcase is found and the cylinder walls are undamaged, all four pistons are replaced. GM also notes that after the mechanical repair the throttle body should be cleaned and its idle airflow learn reset, because a rough-running engine can teach the ECM an incorrect value.

1.4L LE2 (2020–2021 Encore). PIP5657C describes misfires from a broken spark plug center insulator. Turn each plug upside down — the broken insulator can slide down over the center electrode. If found, GM replaces all four plugs.

Encore GX. Bulletin 21-NA-215 covers early 2021–2022 Encore GX models (1.2L LIH and 1.3L L3T) with a pop noise and power loss under hard acceleration, and codes including P0300, P0351 and P0352 — caused by a loose G106 ground. GM inspects the stud and tightens it; no parts. On the 2025 Encore GX, PIP6082 links surging or random misfire with cam timing codes to debris in the cam actuators.

2025 Enclave 2.5L turbo (LK0). Bulletin 25-NA-102 blames P0300 and P050D on rocker arms mis-assembled at the plant, confirmed with a borescope before the camshaft module comes off.

Older Buicks: Rainier, Lucerne, Terraza

On the 2005–2007 Rainier with the 4.2L inline-six (LL8), PIP4013D describes P0300 from single-cylinder misfires that occur only at idle and vanish above idle. GM's suspects are a damaged valve, valve seat or piston ring, found with a cylinder leakage test — and GM advises regulating shop air to 50–60 psi, because full pressure can seat a sticky valve and hide the leak. A leaking intake valve leads to cylinder head replacement per bulletin 07-06-01-018.

On the 2009–2011 Lucerne, 2005–2006 Terraza and 2006 Rendezvous with the 3.5L or 3.9L V6, PIP5064C covers P0300 after a remanufactured engine is installed, starting with oil pressure (30–45 psi at 1,850 rpm) and the cam phaser filter.

Symptoms of P0300

• Check engine light on or flashing
• Rough idle, shake or stumble — on 3.6L Buicks, often worst on a cold start and gone when warm
• Hesitation or loss of power under acceleration
• Poor fuel economy, fuel smell from the exhaust
• Cylinder codes (P0301–P0306) set alongside
• On Encore 1.4L: oil consumption, blue smoke or oil leaks at the same time
• On Encore GX: a pop noise under hard acceleration, with communication codes in other modules

Causes and How to Confirm Each

How Is P0300 Diagnosed on a Buick?

1. Record every code and the freeze-frame data. Engine temperature at the time of the misfire matters — a cold-start misfire on a 3.6L points one way, a hot-highway misfire another.
2. Watch misfire counts per cylinder. A scan tool that shows the current and history misfire counters tells you whether the "random" misfire is really one or two cylinders.
3. Check for open bulletins and recalls by VIN before buying parts.
4. Inspect plugs and coils and swap a coil to see if the misfire follows it.
5. Check fuel trims and look for vacuum leaks. Lean trims on all banks point to air or fuel, not ignition.
6. Run a compression test and cylinder leakage test if a cylinder is consistently weak — this is GM's first step on the Encore and Rainier bulletins.
7. On a 3.6L with a cold-start-only misfire, follow 18-NA-115: coolant dye, cold-soak pressure test and borescope.
8. Clear the codes and confirm the repair under the same conditions that set it.

Parts and Cost

Order plugs and coils by VIN. GM specifies different plugs for different Buick engines, and the wrong heat range or gap can create the misfire you are trying to fix.

GM Documents That Name P0300 on Buick Models

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

What Other P0300 Pages Get Wrong

• "P0300 means plugs and coils." On a 3.6L Enclave or LaCrosse with a cold-start-only misfire, GM's documented cause is coolant entering the cylinder — and no ignition part fixes it.
• "A misfire that goes away when warm isn't serious." On these V6 engines that is precisely GM's warning sign for block porosity.
• "Low compression on an Encore means a new engine." GM's bulletins say to replace pistons and rings when the cylinder walls are undamaged.
• Ignoring the other codes. On an Encore GX, P0300 alongside communication codes points to a ground, not the ignition system.
• Skipping the throttle body relearn. GM's Encore bulletin calls for cleaning and an idle airflow reset after a mechanical repair.

Frequently Asked Questions

Is it safe to drive a Buick with P0300?

For a short distance with a steady light and mild symptoms, usually. If the check engine light is flashing, reduce load and get it checked — a heavy misfire can overheat and damage the catalytic converter.

My Enclave misfires only on cold mornings. What should I ask for?

Ask the shop to check for GM bulletin 18-NA-115, which describes cold-start misfire on the same one or two cylinders caused by coolant entering the cylinder. It requires a dye and borescope inspection, not just a plug change.

Will new spark plugs fix P0300 on my Encore?

Sometimes. On 2020–2021 Encores GM documented broken spark plug insulators. But on 2013–2019 Encores GM also documented damaged pistons and ring lands, so a compression test is worth doing if the code returns.

What is the difference between P0300 and P0301?

P0301 means the ECM pinned the misfire on cylinder 1. P0300 means the misfire is random or on several cylinders. See our P0301 Buick guide for the cylinder-specific version.

Can bad gas cause P0300?

Contaminated or low-quality fuel can affect all cylinders and set random misfire. Rule out the documented causes for your engine before blaming fuel.

Does GM cover the 3.6L block porosity repair?

18-NA-115 is a technical bulletin, not a special coverage, so coverage depends on your powertrain warranty status. A dealer can check your VIN for free.