P0171 – What You Need to Know


The Check Engine Light code P0171 code means that Bank 1 (typically the side containing Cylinder #1 with V6 and V8 engines, inline engines only have the one bank) has too much air or not enough fuel. Extended driving with this code can cause engine damage and/or shorten the life of your catalytic converter(s), so it’s a good idea to get this sort of code looked at ASAP.

P0171 Potential Causes

  • Failing or contaminated (dirty) mass airflow sensor
  • Vacuum leaks – PCV hoses, vacuum hoses, intake manifold gasket, etc.
  • Failing fuel pump
  • Clogged or dirty fuel injectors
  • Clogged fuel filter
  • Exhaust leaks
  • Faulty O2 sensor
  • Faulty AFR sensor

How Do I Fix It?

If your engine is running lean and you’re not comfortable diagnosing this issue at home, we recommend finding an experienced, local auto repair shop to pinpoint the cause of the problem and give you an accurate estimate.

Should you decide to take your to a shop for diagnosis, most shops will start with an hour of “diag” (the time spent in labor diagnosing your specific issue). Depending on the shop’s labor rate, this typically costs somewhere between $75-$150. Many, if not most, shops will apply this diagnosis fee to any required repairs if you have them perform the repairs for you.

If you decide to handle the diagnosis yourself, you may want to start with checking for vacuum leaks or air intake duct leaks, especially on MAF-based vehicles. Common leakage areas include injector O-rings and intake manifold gaskets, seals and O-rings, all of which are more likely to act up when cold, since seals soften and expand as they warm up. Keep in mind this is an intermediate-level diagnosis and repair and not recommended for beginners.

Next Steps

Once you think you’ve located and repaired the cause of the problem, it’s helpful to have a code scanner with live data capability for verification. Scroll through the PID list and highlight the Short Term and Long Term Fuel Trims for Bank 1. These should both be single digit numbers between -9 and 9. Ideally, hovering right around 0.

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