P0171: System Too Lean (Bank 1)
Your engine is running lean on Bank 1 - meaning there's too much air and not enough fuel in the air/fuel mixture. This is one of the most common check engine codes and is usually inexpensive to fix.
Symptoms
- Check engine light
- Rough idle
- Hesitation on acceleration
- Popping or backfiring
- Possible stalling
Causes (Ranked by Likelihood)
50 Most Common OBD2 Codes - Free Printable
Plain-English explanations for the codes mechanics charge $100+ to diagnose.
No spam. Unsubscribe anytime.
DIY Fix Steps
⚠ Safety first: work on a cold engine, chock wheels, use jack stands - never rely on a floor jack alone.
Remove the MAF sensor (2 bolts, 1 connector). Spray 10-15 bursts of MAF sensor cleaner on the wire element. Let dry 20 min. Reinstall. This fixes it 40% of the time for free.
With engine at idle, spray carb cleaner around: intake manifold gaskets, throttle body base, PCV valve and hose, brake booster hose. RPM change or smoothing = found the leak.
A torn or improperly seated air filter allows unmetered air. Check the intake boot from air box to throttle body for cracks.
Fuel pressure at idle: typically 40-60 PSI. Low pressure = weak pump or clogged filter. Buy a fuel pressure gauge ($25) and check before replacing the pump.
If the lean code returns after cleaning, replace the MAF sensor ($30-120 for aftermarket). Clear codes and drive 2 cycles.