P0301 Check Engine Code

Symptoms

• Check engine light — may flash under load with active misfire
• Rough idle or stumble you can feel at low speed
• P0301 alone or with P0300
• Recent tune-up where cylinder 1 plug or coil was disturbed
• Fuel trim normal or lean on the bank that includes cylinder 1

Common causes

• Failed coil-on-plug or spark plug on cylinder 1
• Cracked plug insulator or wrong plug gap
• Injector restriction on cylinder 1
• Low compression on cylinder 1 (after ignition is ruled out)
• Vacuum leak affecting cylinder 1 runner on some intakes

Often sounds like

• P0300 random misfire — multiple cylinders or unstable counts
• P0302 through P0308 — other cylinders; a swap test isolates coil-related faults

How we check it

1. 1 Confirm misfire counts on cylinder 1 with a scan tool
2. 2 Inspect plug and coil; swap coil test when safe and applicable
3. 3 Injector and compression tests when ignition looks good

What happens next when you bring it in?

1. 1 Call with whether the light flashes and whether roughness is constant or only when cold.
2. 2 We isolate cylinder 1 on scan data before quoting plugs, coils, or deeper work.

Questions we get a lot

What does P0301 mean?

P0301 is Cylinder 1 Misfire Detected. Cylinder 1 is the reference cylinder for bank 1 on your engine. The computer counted enough misfire events on that cylinder to store a code.

Can I swap the coil to test P0301?

On some engines, moving the coil to another cylinder and watching misfire counts move with it speeds diagnosis. We do that when the design allows — instead of guessing at parts.

Is P0301 an emergency?

A steady light with mild roughness — get it scanned soon. A flashing light means the engine is misfiring while you drive; drive gently and come in quickly to protect the catalytic converter.