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P1171

System Too Lean Banks 1 And 2 (Lean Fuel Fault).

  • Jaguar
  • Powertrain
  • Manufacturer code
  • Medium severity

How this code is built

P
SystemPowertrain
1
Defined byThe manufacturer
1
AreaFuel and air metering
71
FaultThe specific fault in that area

What it means

On a Jaguar, P1171 means the engine control module has seen the fuel system running too lean on both banks (1 and 2) at the same time. The long-term and short-term fuel trims have been pushed to their limit adding fuel, yet the oxygen sensors still report excess air. Because both banks are affected, the cause is usually something common to the whole engine rather than a single cylinder bank fault.

Symptoms

  • Malfunction indicator lamp on
  • Rough or unstable idle
  • Hesitation or flat spots on acceleration
  • Engine surging or occasional stalling
  • Poor fuel economy or reduced power
  • Hissing noise from the engine bay

Possible causes

  • Unmetered air entering after the mass airflow sensor, such as a split intake hose, loose clip or leaking intake manifold gasketlikely
  • Vacuum leak from a perished vacuum hose, brake servo hose or crankcase breather hoselikely
  • Contaminated or faulty mass airflow sensor under-reading the air entering the enginelikely
  • Low fuel pressure from a weak fuel pump, blocked fuel filter or faulty pressure regulator, depending on modelpossible
  • Exhaust leak ahead of the oxygen sensors letting in air and giving false lean readingspossible
  • Restricted or leaking fuel injectors affecting both bankspossible
  • Faulty engine control module or outdated calibration, only where a manufacturer update is documented for the modelrare

How to diagnose

  1. Read all stored and pending codes and record the freeze frame data, noting engine load, speed and temperature when the fault set
  2. Check for other related codes such as mass airflow, oxygen sensor or misfire faults, and deal with those first
  3. Visually inspect the intake pipework, clips, breather hoses and vacuum hoses for splits, disconnections and wear
  4. Look at live long-term and short-term fuel trims at idle and at about 2,500 rpm; a trim that corrects at higher revs points towards a mass airflow or fuel delivery fault, while one that is worst at idle suggests a vacuum leak
  5. Check the mass airflow sensor reading against expected values and inspect the sensing element for contamination
  6. Smoke test the intake system to locate leaks, and check the exhaust for leaks upstream of the oxygen sensors
  7. Measure fuel pressure and compare with the specification for the specific model and engine
  8. Check injector operation and balance if fuel pressure and air leaks are ruled out

How to fix

  • Replace split or loose intake hoses and tighten or replace clips and seals
  • Replace leaking vacuum or breather hoses and any failed intake manifold gaskets
  • Clean or replace the mass airflow sensor as the testing indicates
  • Replace the fuel filter, fuel pump or pressure regulator if pressure is below specification, as applicable to the model
  • Repair exhaust leaks upstream of the oxygen sensors
  • Clean or replace faulty injectors, and apply a manufacturer calibration update only if one is documented for the fault

Can I keep driving?

Medium severity

The car can usually be driven short distances, but a persistent lean condition can cause hesitation, stalling and possible engine or catalyst damage over time. Book a diagnosis soon and avoid heavy loads or hard acceleration until it is fixed.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.

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