CodelampCame back after a repair
P0300 came back after new spark plugs
P0300
- In plain words
- More than one cylinder, or no single one, is misfiring.
- Standard name
- Random/Multiple Cylinder Misfire Detected
- System
- Ignition and misfire
General information, not a mechanic's diagnosis: have a mechanic confirm before any repair.
What the computer saw
New plugs fix misfires when worn plugs were the cause. If P0300 is back, either something else is misfiring the engine, or something went wrong with the plug job itself.
Likely causes, most common first
- Weak ignition coils or plug wires. Old plugs can strain coils; a coil that was failing still fails.
- The wrong plugs, or the wrong gap. Plugs not made for the engine, or gapped wrong, can misfire.
- A coil boot damaged or not seated. A torn boot or a coil not pushed fully onto the plug.
- A vacuum leak or fuel problem. Lean mixtures misfire whatever the plugs.
- Low compression. Less common; found with a compression test.
What you can note yourself
- Note whether the light is steady or flashing, and tell the shop.
- Note how soon after the plug change it came back, and keep the plug part number.
What to tell the shop
- When the plugs were changed, which plugs, and by whom.
- Any cylinder codes now stored.
This page gives no verdict on whether a car is safe to drive; a mechanic can.