CodelampTwo codes together
P0171 and P0174 together: both sides of the engine are lean
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
P0174
- In plain words
- The engine is getting too much air or too little fuel on bank 2.
- Standard name
- System Too Lean (Bank 2)
- System
- Fuel and air mixture
General information, not a mechanic's diagnosis: have a mechanic confirm before any repair.
What the computer saw
P0171 says bank 1 is lean and P0174 says bank 2 is lean. Each side has its own fuel injectors and front oxygen sensor, so when both go lean at once, the cause is usually something they share, not a part on one side.
Likely causes, most common first
- A vacuum leak in the shared intake. A cracked PCV hose, a brake booster hose or an intake gasket lets air reach both banks.
- A dirty or failing mass airflow sensor. One sensor measures the air for the whole engine. If it reads low, both sides run lean.
- Low fuel pressure. One pump and one filter feed both banks.
- A cracked intake boot after the airflow sensor. Air that enters after the sensor is never measured.
What you can note yourself
- Note when it runs worst: idle, cold start or highway.
- Note any recent work that opened the intake, such as an air filter change.
What to tell the shop
- Both codes, and that they came together.
- Ask for a smoke test and a fuel pressure check before sensors are replaced.
This page gives no verdict on whether a car is safe to drive; a mechanic can.