CodelampTwo codes together
P0171 and P0300 together: lean, and misfiring
P0171
- In plain words
- The engine is getting too much air or too little fuel on bank 1.
- Standard name
- System Too Lean (Bank 1)
- System
- Fuel and air mixture
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
A lean mixture is harder to light. When there is enough extra air, cylinders start to misfire, so P0171 and P0300 often share one cause. Fixing the lean condition often clears both.
Likely causes, most common first
- A vacuum leak. The most common shared cause: air that bypasses the airflow sensor leans the mixture until cylinders miss.
- A dirty or failing mass airflow sensor. Too little fuel for the real air.
- Low fuel pressure. A weak pump or clogged filter starves every cylinder.
- Worn spark plugs on top of a lean mixture. Weak plugs fail first when the mixture is lean.
What you can note yourself
- Note whether P0174 is also stored, and whether the light flashes.
- Note whether it is worst at idle, where vacuum leaks show most.
What to tell the shop
- All the codes, and that the lean and misfire codes came together.
- Ask them to look for the lean cause before replacing ignition parts.
This page gives no verdict on whether a car is safe to drive; a mechanic can.