Code
P1178
JAGUAR
P — Powertrain
Long Term Fuel Trim Too Lean Banks 1 And 2 (AMFR)
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UK: 25
EN: 54
RU: 25
AI status
Completed
Completed
100%
Causes
- Unmetered air entering the intake (vacuum or intake manifold leaks downstream of the MAF)
- Contaminated or faulty mass air flow (MAF) sensor or improper MAF signal
- Low fuel pressure or weak fuel pump/fuel filter restriction
- Clogged or leaking fuel injectors or injector flow imbalance
- Faulty oxygen sensors (O2) or wideband sensor providing incorrect lean signal
- Faulty MAP/TP sensors, or intake air temperature (IAT) sensor errors
Symptoms
- Illuminated MIL/check engine lamp
- Rough idle, hesitation, or lack of power under load
- Poor fuel economy
- Hard starting or stumble on acceleration
- Persistently high positive long‑term fuel trim values on both banks in live data
What to check
- Read freeze frame and pending/related codes; note engine load, RPM, coolant temp and MAF voltage at fault occurrence
- Check live data: LTFT and STFT for both banks, MAF g/s or V, O2 sensor voltages or wideband AFR, fuel pressure, MAP and IAT
- Visually inspect intake boots, hoses, vacuum lines, PCV and EVAP plumbing for leaks or disconnections
- Perform a smoke test of intake and vacuum system to locate unmetered air
- Measure fuel rail pressure (static and under load) against manufacturer spec
- Check MAF sensor for contamination and wiring integrity; inspect air filter and intake tract
Signal parameters
- Long Term Fuel Trim (LTFT) Bank 1 & Bank 2: typical target ~0% (repair threshold commonly >+10% to +25%)
- Short Term Fuel Trim (STFT) Bank 1 & Bank 2: fluctuating around 0% normally
- MAF: reported g/s or voltage — compare to expected for RPM/load (dirty MAF often reads low)
- Fuel rail pressure: typical returnless systems ~40–60 psi (manufacturer specific) — check spec
- O2 sensor voltage (narrowband): switching between ~0.1–0.9 V in closed loop; stuck low (
- Wideband AFR: >14.7:1 under conditions where stoich is expected indicates lean
Diagnostic algorithm
- Retrieve and record all DTCs and freeze frame data. Attempt to reproduce condition while monitoring live data (LTFT, STFT, MAF, O2, fuel pressure, MAP).
- Verify both banks actually show sustained positive LTFT. If only one bank is affected intermittently, investigate that bank first.
- Inspect intake tract for obvious leaks, displaced hoses, loose clamps, or damaged boots. Perform a smoke test of intake and vacuum system to find hidden leaks.
- Evaluate MAF sensor: check wiring and connector, inspect and clean MAF with proper MAF cleaner if contaminated. Re-test to see if trims normalize.
- Measure fuel pressure (static and during cranking/running). Compare to manufacturer specs. If low, test fuel pump, filter, and pressure regulator.
- Check injector operation: use injector balance or relative fuel trim tests to detect clogged or leaking injectors. Inspect injector wiring.
- Monitor O2/wideband sensors for proper response. Check wiring and grounds. Consider replacing sensor only after verifying upstream fueling/air issues are addressed.
- Check MAP/IAT sensors and throttle body for faults or contamination that could mislead fuel calculations.
- Inspect exhaust for leaks upstream of O2 sensors that could cause false lean readings.
- If all mechanical and sensor checks are normal, check for software/PCM updates or consult manufacturer technical bulletins before replacing major components.
- After any repair, clear codes and perform a road test to verify LTFT values return to acceptable range and code does not reset.
Likely causes
- Large intake boot/manifold leak after MAF (affects both banks)
- Dirty or failed MAF reading low air mass
- Low fuel pressure (failing pump/regulator or clogged filter) causing lean condition on both banks
- Massive vacuum leak at throttle body or PCV/EVAP plumbing
- Faulty wideband O2 sensor or shared wiring harness fault giving false lean reading
Fault status
Status
ECM detected sustained lean long‑term fuel trims on both banks and set P1178. The controller is adding fuel to compensate; investigate for unmetered air, low fuel pressure, faulty MAF/O2 sensors, or related faults.
Repair difficulty: Medium
Diagnostic time: 1-3 hours
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