🌬️ Lean Fuel Mixture Codes
TL;DR
Lean codes (P0171 Bank 1, P0174 Bank 2, plus P2195/P2187) mean too much air or too little fuel. Severity: medium. Top causes: a vacuum/intake leak, a dirty MAF sensor, a weak fuel pump/filter, or a stuck-open PCV valve. Check for vacuum leaks and clean the MAF first.
Codes in this family
Shared causes
- Unmetered air (vacuum) leak — cracked hose or intake gasket
- Dirty or failing mass air flow (MAF) sensor
- Stuck-open PCV valve (acts as a vacuum leak)
- Weak fuel pump, clogged filter or dirty injectors (not enough fuel)
- Faulty oxygen sensor reporting wrongly
Parts commonly involved
Related symptoms
Frequently asked questions
What does a lean code (P0171) mean?
It means the air-fuel mixture has too much air or too little fuel, so the computer has been adding fuel to compensate and finally flagged it. P0171 is Bank 1; P0174 is the same fault on Bank 2.
What is the most common cause of a lean code?
An unmetered air (vacuum) leak and a dirty MAF sensor are the two most common causes. Check intake hoses and gaskets for leaks and clean the MAF before chasing fuel-supply causes.
Can a lean condition damage the engine?
Persistent lean running raises combustion temperatures and can cause misfires and, over time, damage — so fix a lean code rather than ignoring it, especially if you also see misfire codes.