Summary
P0170 means the PCM has detected that Bank 1 fuel trims are outside their acceptable operating range — the engine can't maintain the correct air-fuel ratio within its normal correction limits. The most common causes are a vacuum leak (30%), a dirty or failing MAF sensor (25%), and a faulty O2 sensor (20%). This code often appears alongside P0171 (lean) or P0172 (rich) codes. Start by reading fuel trims to determine if the engine is running lean or rich, then work from there.
Severity: Moderate
Safe to drive: Yes, short distances — lean or rich running damages catalytic converters over time
Repair cost: $8–$600 depending on cause
DIY difficulty: Easy to Moderate
What does P0170 mean?
The PCM continuously adjusts fuel delivery to maintain a 14.7:1 air-fuel ratio using O2 sensor feedback. It tracks these adjustments as fuel trims: short-term fuel trim (STFT) for immediate corrections and long-term fuel trim (LTFT) for persistent patterns. Together, these trims typically stay within plus or minus 10% during normal operation.
P0170 sets when the total fuel correction on Bank 1 has drifted outside the PCM's acceptable range — typically beyond plus or minus 20–25%. Unlike P0171 (lean) or P0172 (rich), P0170 is a general "fuel trim out of range" code. Some manufacturers use it instead of or in addition to the directional lean/rich codes.
The code doesn't tell you which direction the trims have shifted. You need to read live fuel trim data to determine whether the engine is running lean (high positive LTFT) or rich (high negative LTFT), then diagnose accordingly.