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Diagnostic code · Fuel & air

Honda P0171 Code: System Too Lean (Bank 1)

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What it meansToo much air or too little fuel — the computer maxed out adding fuel to compensate.
Most likely fixA vacuum / intake leak, or a dirty MAF sensor.
Safe to drive?Briefly — but running lean can overheat & misfire. Fix soon.
Typical cost$100–$400 (hose / MAF / fuel part).
A gas cap does not cause P0171 — that’s an EVAP code (P0455/P0456). P0171 is purely an air-vs-fuel balance problem.

What P0171 means

The engine computer trims fuel up and down to hold the ideal air/fuel ratio. When it has to keep adding fuel and still reads “lean,” it hits its correction limit and sets P0171. Something is letting in unmetered air or starving the engine of fuel.

Common causes most common first

  • Vacuum / intake leak
  • Dirty or failing MAF sensor
  • Weak fuel pump / clogged filter
  • Leaking or clogged injectors
  • Faulty upstream O2 sensor

Do these checks first

  1. Hunt vacuum leaks: spray a little carb cleaner around the intake boot and PCV hoses — an idle change means you found it.
  2. Clean the MAF sensor with MAF-specific cleaner (never touch the wire).
  3. Check fuel pressure / fuel trims — a weak pump shows lean trims that worsen under load.
See OBD2 scanners on Amazon A scanner with live data pays for itself on the first fix.

Repair cost

A cracked intake hose or PCV line is $20–$120. A MAF sensor is $120–$300 installed. A failing fuel pump is the pricier end, $400+.

FAQ

Is P0171 serious?
Not immediately, but sustained lean running raises combustion temps and can cause misfires or long-term engine damage — fix it within a week or two.
Can a dirty air filter cause P0171?
Rarely — a clogged filter usually pushes rich, not lean. Look at unmetered air (leaks) and the MAF first.
Will P0171 clear itself?
Only if the cause fixes itself (e.g. a hose reseats). Otherwise it returns after the next drive cycle.