P1126

Code

VOLKSWAGEN P — Powertrain

Long Term Fuel Trim Add Air Bank 2 System Too Lean

Brand: VOLKSWAGEN

Type: P — Powertrain

Views: UK: 26 EN: 40 RU: 25

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Page language: EN

Causes

Intake manifold or vacuum leak on Bank 2 side (unmetered air)
Faulty or contaminated mass airflow (MAF) sensor or MAP sensor
Low fuel pressure or weak fuel pump, clogged fuel filter
Restricted or partially clogged fuel injector(s) on Bank 2
Faulty upstream O2 (lambda) sensor or wiring/signal issues
Exhaust leak upstream of the O2 sensor for Bank 2

Symptoms

Malfunction Indicator Lamp (MIL) / Check Engine Light illuminated
Poor idle quality or rough running, especially at idle
Hesitation, stumble or reduced power under acceleration
Reduced fuel economy
Possible lean misfire codes or bank-specific misfiring
Smell of unburned air/fuel or backfiring in intake/exhaust in some cases

What to check

Read freeze-frame and pending codes; note fuel trim values, MAP/MAF, fuel pressure and O2 sensor voltages
Compare Short Term Fuel Trim (STFT) and LTFT for Bank 1 vs Bank 2
Visual inspection of intake manifold, vacuum hoses, PCV and charge-air plumbing on Bank 2
Smoke test the intake and vacuum system to find unmetered air leaks
Check fuel rail pressure under load and key-on; compare to manufacturer spec
Scan-stream O2 sensor (Bank 2 Sensor 1) voltage and response time; look for slow switching or low amplitude

Signal parameters

LTFT Bank 2: sustained positive offset (commonly > +10–20% depending on vehicle) indicating added fuel
STFT Bank 2: immediate compensations and oscillations around LTFT
O2 Sensor Bank 2 Sensor 1: low voltage or slow switching (should switch rapidly around 0.1–0.9 V on gasoline engines)
MAF: measured air flow (g/s) vs expected at idle and under load; abnormal low/high or erratic values
Fuel rail pressure: key-on and operating pressures compared to VW specs (example range 40–60 psi on many systems)
Intake manifold vacuum: lower-than-expected vacuum or fluctuations indicating leak

Diagnostic algorithm

Connect a reliable OBD-II scanner. Record LTFT/STFT, O2 sensor voltages, MAF readings, fuel pressure and any freeze-frame data.
Confirm code is current (not historic). Clear codes and perform a test drive while monitoring live data to reproduce the condition.
Visually inspect intake manifold, intake boots, vacuum lines, PCV and turbo charge pipes on Bank 2. Pay attention to split hoses and loose clamps.
Perform a smoke test of intake and vacuum systems to locate unmetered air leaks on Bank 2.
Check MAF sensor: inspect, clean with approved MAF cleaner if dirty, and verify readings vs expected. Replace if out of spec.
Measure fuel rail pressure (static and dynamic) and compare to manufacturer specification. Repair/replace fuel pump, pressure regulator, or filter if low.
Check injector operation on Bank 2: resistance, wiring, and flow. Perform balance or cylinder contribution test if available.
Monitor upstream O2 sensor (Bank 2 Sensor 1) behavior: voltage swings, response to induced rich/lean conditions. Test wiring and ground; replace sensor if faulty.
Inspect for exhaust leaks near the Bank 2 upstream O2 sensor that could skew sensor readings and mimic a lean condition.
If hardware checks good, review recent software/tune changes; reflashing to latest ECU calibration may be required.
After repairs, erase codes and verify LTFT for Bank 2 returns to near zero and that symptoms are resolved during a proper drive cycle.

Likely causes

Vacuum/intake leak specific to Bank 2 (most common)
Faulty MAF or MAP causing incorrect air measurement
Low fuel pressure or delivery issue
Upstream O2 sensor (Bank 2 Sensor 1) providing incorrect lean readings

Fault status

⚠️ Status

P1126 indicates the engine control is applying long-term positive fuel trim on Bank 2 to correct a detected lean condition. Inspect for unmetered air, MAF/MAP faults, fuel delivery problems, O2 sensor issues, and exhaust leaks. Perform targeted diagnostics before repair.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1-3 hours

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P1126 — Long Term Fuel Trim Add Air Bank 2 System Too Lean