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Home / DTC / P21F7 — Fuel Control System B Too Rich Off Idle Bank 2

P21F7 — Fuel Control System B Too Rich Off Idle Bank 2

Detailed page for trouble code P21F7.

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P21F7

Generic P — Powertrain

Fuel Control System B Too Rich Off Idle Bank 2

Brand: Generic
Type: P — Powertrain
Views: UK: 10 EN: 14 RU: 6
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Page language: EN

Causes

  • Leaking or stuck‑open fuel injector(s) on bank 2
  • Fuel pressure too high (faulty regulator, blocked return, faulty fuel pressure sensor)
  • Mass air flow (MAF) sensor or intake air measurement errors (overreporting airflow)
  • Faulty or contaminated upstream oxygen sensor (Bank 2) or heater circuit issues giving biased rich signal
  • Shorted/wiring fault in injector, O2, MAF, or fuel pressure sensor circuits
  • Excessive fuel vapor from a stuck open EVAP purge valve during off‑idle

Symptoms

  • Check Engine Light (MIL) illuminated with P21F7 stored
  • Rich exhaust smell (fuel odor) from tailpipe
  • Poor fuel economy
  • Black/sooty exhaust or carbon‑fouled spark plugs (on affected bank)
  • Possible hesitation, stumble or roughness off idle or during light throttle
  • Excessive black smoke (in severe cases)

What to check

  • Read freeze‑frame and live data with a scan tool; confirm the code is bank‑2 off‑idle and note conditions (rpm, load, throttle position, fuel pressure).
  • Monitor short‑term and long‑term fuel trims (STFT/LTFT) for Bank 1 and Bank 2 during idle and off‑idle/running: look for large negative trims on Bank 2.
  • Compare upstream (sensor 1) and downstream (sensor 2) O2 sensor voltages for Bank 2; check heater status and response.
  • Check MAF sensor output (grams/sec or voltage/frequency) against expected values for engine speed/load; inspect and clean MAF if contaminated.
  • Measure fuel rail pressure and compare to spec under key‑on and running conditions; observe for overpressure or rail pressure spikes.
  • Perform a visual wiring and connector inspection for injectors, O2 sensor(s), MAF, fuel pressure sensor and EVAP purge; check for damage, corrosion or open/short.

Signal parameters

  • Short‑Term Fuel Trim (STFT) Bank 2: normally around ±10%; significant negative values (e.g., < -10%) suggest a rich condition
  • Long‑Term Fuel Trim (LTFT) Bank 2: persistent negative LTFT indicates the PCM is compensating for an overly rich condition
  • Upstream O2 Sensor (Bank 2 Sensor 1) voltage: should oscillate ~0.1–0.9 V in closed loop; stuck high or biased high can indicate rich reading
  • Downstream O2 Sensor (Bank 2 Sensor 2) voltage: should be steadier; unusually high or matching upstream constantly may indicate actual rich or sensor fault
  • MAF sensor output (g/s or frequency/voltage): compare actual vs expected for RPM/load — over‑reporting causes extra fuel
  • Fuel rail pressure (psi/bar or kPa): verify within manufacturer spec and stable under load

Diagnostic algorithm

  1. Verify code and conditions: record freeze‑frame data and live parameters (STFT/LTFT, O2 voltages, MAF, fuel pressure, throttle) and note when code sets (rpm, load, temp).
  2. Clear codes and reproduce: drive to replicate off‑idle condition; confirm P21F7 returns and capture live data during the event.
  3. Examine fuel trims: if Bank 2 shows significant negative trims while Bank 1 is normal, focus on hardware for Bank 2 (injectors, rail pressure, O2 sensor).
  4. Inspect for fuel leaks: visually check injectors, rail, and fuel lines on Bank 2 for external leaks or seepage.
  5. Check fuel pressure: measure static and dynamic fuel rail pressure and compare to spec; investigate regulator or return restrictions if pressure is high.
  6. Test injectors: perform an injector flow/leak test or swap suspect injector(s) with Bank 1 equivalent to see if the fault follows injector(s).
  7. Verify O2 sensor operation: inspect heater circuit, compare upstream Bank 2 O2 waveform to Bank 1; if O2 reads rich but other indicators show normal fuel pressure/MAF, consider replacing the O2 sensor.
  8. Inspect MAF/IAT: clean or test the MAF sensor and verify MAF readings correspond to expected airflow at given RPM/load; replace if over‑reporting.
  9. Check EVAP purge valve operation: confirm it is not stuck open during conditions where code sets; repair or replace if faulty.
  10. Verify wiring/grounds: backprobe and wiggle‑test connectors and harness for injectors, sensors and fuel pressure sensor for intermittent shorts or opens.
  11. After repairs, clear codes and perform a road test under the same off‑idle conditions to confirm the issue is resolved and trims are within normal range.

Likely causes

  • Leaking or stuck injector(s) on Bank 2
  • High fuel rail pressure (regulator or return restriction)
  • Faulty/upstream O2 sensor on Bank 2 reporting rich
  • MAF sensor over‑reporting airflow or contaminated MAF
  • EVAP purge valve stuck open

Fault status

⚠️ Status
Fuel Control System B Too Rich Off Idle Bank 2 — PCM detected excessive fuel (rich condition) on Bank 2 when off idle. Check fuel delivery, injectors, sensors and related wiring.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.5-3.0 hours
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