Fault Tree: P0171 / P0172 Fuel Trim

Fuel trim codes are usually a measurement problem (air leak / MAF) or a delivery problem (fuel pressure / injectors). Use this flow to confirm which.

Petrol Lean (P0171) Rich (P0172)

Quick triage & tools

  • Look at trims: STFT + LTFT at idle and at ~2500 rpm (light throttle). Write them down.
  • Have ready: scan tool (live data), multimeter, and ideally a smoke tester or propane/brake-cleaner method (carefully).
  • Don’t skip basics: intake hose splits, PCV hoses, loose clamps, and recent work.

Deep-dive: Smoke testing done right → · Fuel trim patterns (advanced) →

Decision flow

  1. Step 1 — Confirm the pattern (idle vs off‑idle)
    • Lean mainly at idle (e.g. +15 to +30% at idle, improves at 2500 rpm) → suspect vacuum / unmetered air leak.
    • Lean everywhere (idle and 2500 rpm both high positive) → suspect low fuel pressure / delivery or MAF under‑reporting.
    • Rich everywhere (negative trims) → suspect leaking injector, high fuel pressure, EVAP purge stuck open, or a sensor plausibility issue.
  2. Step 2 — Check MAF/MAP plausibility
    • At hot idle: MAF (g/s) should be stable and plausible for engine size; MAP should reflect a healthy vacuum.
    • If MAF reads suspiciously low or noisy, clean/inspect sensor and check for air leaks after the MAF.
    If uncertain: compare MAF and MAP behaviour using Vacuum testing with a gauge →
  3. Step 3 — Air leaks (most common for P0171)
    • Smoke test intake, PCV, brake booster line, manifold gasket areas.
    • Check PCV valve / diaphragm (many modern engines tear and create a big unmetered leak).
    Use: Smoke testing →
  4. Step 4 — Fuel pressure & delivery
    • Low pressure → filter restriction, weak pump, low voltage at pump, failing regulator (if fitted).
    • High pressure → regulator fault (return-style) or control issue (returnless).
    Use: Fuel pressure testing basics →
  5. Step 5 — Injector / EVAP purge checks (common for rich P0172)
    • Look for fuel smell, hard hot restarts, or trims that go rich after idling.
    • Pinch off / block EVAP purge hose briefly at idle (if safe) and watch trims; a stuck purge can mimic injector leak.
    Safety: avoid fuel vapour ignition sources. If you’re not comfortable, stop and get help.
  6. Step 6 — O2 / lambda sanity
    • If trims and AFR readings disagree, verify O2 sensor wiring/heater and exhaust leaks upstream of sensor.
    • On wideband systems, compare commanded lambda vs measured.
    Related: O2 / lambda sensor testing →
  7. Step 7 — Confirm the fix
    • Clear codes, reset trims if appropriate, then repeat the same idle/2500 rpm trim snapshot.
    • Road test and recheck for pending codes.

Print / save checklist

Tick these off as you work. If you need to hand this to a mechanic, print it as a short job card.

  • Freeze-frame captured / conditions noted
  • Battery voltage checked (resting + cranking)
  • Basic visual checks (hoses, connectors, grounds, fuses)
  • One test at a time (don’t change multiple variables)
  • Confirm fix by reproducing the original condition
Print / Save as PDF Back to symptom selector

What to do next

Use the links below to deepen the test you’re about to perform, cross-check related codes, or jump to a faster symptom-led flow.

Find another symptom flow Jump to the symptom selector to locate the closest decision tree. Workshop Guides Deep-dive how-to tests: voltage drop, wiring diagrams, smoke testing, fuel pressure and more. Diagnostic Codes Look up DTC meanings, common causes, and related checks. AI Tools Use AI assistance to summarise symptoms, plan tests, or sanity-check a diagnosis.

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