P0300 — Random/Multiple Cylinder Misfire

Flashing vs. Solid Check Engine Light

A solid check engine light is a stored fault — drive to a shop within a few days. A flashing or blinking check engine light means an active misfire is happening right now and raw fuel is entering the exhaust system, which can destroy the catalytic converter within minutes. Pull over and don't drive until the cause is found.

Common Causes

• Worn spark plugs (most common — replace all at once)
• Failed ignition coil(s) — random misfires often mean multiple coils are aging
• Vacuum leak — creates lean condition causing widespread misfire
• Low fuel pressure — failing fuel pump or clogged filter
• Failing fuel injectors
• Low compression — serious internal engine wear (head gasket, worn rings, bent valve)

Start with the Cheap Fixes

If the vehicle has high mileage and hasn't had spark plugs recently, start there — a full set of plugs is $80–$200 and fixes P0300 more often than anything else. If plugs are new, swap ignition coils between cylinders and see if the misfire follows the coil. A single bad coil causing random misfires across cylinders is common on vehicles with aged coil packs.

Repair Cost

• Spark plug replacement: $80–$440 depending on engine
• Ignition coil replacement (one): $150–$350
• Vacuum leak repair: $100–$400
• Fuel injector cleaning or replacement: $200–$600
• Low compression (serious): $1,500–$4,000+ for engine repair