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DTC codes / Volkswagen codes / P1129

P1129 · VolkswagenLong Term Fuel Trim Bank 2 System Too Rich

P1129 on Volkswagen vehicles means the engine control module has detected that the long-term fuel trim on bank 2 has reached its rich limit, so the computer is subtracting as much fuel as it can and still cannot bring the air/fuel mixture back to stoichiometric. Bank 2 is running too rich and the ECU has run out of adjustment authority, which usually points to excess fuel entering that bank or a false rich reading from the oxygen sensor.

SystemPowertrain
Code typeVolkswagen specific
SeverityMedium
Fix difficultyProfessional
AreaFuel and air metering

Symptoms you may notice

  • Often no symptoms besides the warning light, especially if the rich condition is mild and the ECU can still keep driveability acceptable.
  • A noticeable drop in fuel economy because bank 2 is receiving more fuel than it can burn efficiently.
  • Black sooty exhaust smoke or a strong fuel smell from the tailpipe, particularly at idle or during cold starts.
  • Rough idle, misfires, or engine stumbling on the affected bank because excess fuel fouls the spark plugs.
  • A sluggish or surging feeling during acceleration as the ECU struggles to correct a mixture it can no longer control.
  • Illumination of the check engine light, sometimes accompanied by a traction control or EPC light on Volkswagen models.

Common causes, most likely first

  1. A leaking or stuck-open fuel injector on bank 2, which is the most common cause of a genuine rich condition on one bank.
  2. A faulty or contaminated upstream oxygen sensor on bank 2 sending a false rich signal to the ECU.
  3. High fuel pressure from a failing fuel pressure regulator or a restricted fuel return line, affecting bank 2 more than bank 1.
  4. A leaking intake manifold gasket or vacuum leak that the mass airflow sensor does not see, causing the ECU to over-fuel bank 2.
  5. A stuck-open purge valve or EVAP system fault that allows fuel vapor to enter the intake and enrich bank 2.
  6. Worn spark plugs or ignition coils on bank 2 causing incomplete combustion that the oxygen sensor reads as rich.
  7. A restricted air filter or dirty mass airflow sensor that skews the base fuel calculation, though this usually affects both banks.

How to diagnose and fix it

  1. Connect a scan tool and record live data for long-term fuel trim on both banks, plus upstream and downstream oxygen sensor voltages, to confirm bank 2 is genuinely rich and not a sensor artifact.
  2. Inspect the bank 2 spark plugs for black soot or fuel fouling, which is a quick and inexpensive confirmation of a rich mixture on that bank.
  3. Check fuel pressure with a gauge and compare it to the Volkswagen specification for your engine, looking for pressure that stays high after the pump shuts off.
  4. Perform a injector balance test or swap bank 2 injectors to bank 1 and see if the rich trim follows the injector, which isolates a leaking injector without guessing.
  5. Smoke-test the intake system for vacuum leaks and check the EVAP purge valve for sticking open, since both can add unmetered fuel or air that skews bank 2.
  6. Scope or swap the bank 2 upstream oxygen sensor with a known-good unit to rule out a false rich signal before replacing expensive parts.
  7. After repairing the root cause, clear the fault codes and drive the vehicle through a complete readiness cycle to confirm long-term fuel trim on bank 2 returns to a normal range.

What a rich bank 2 really tells you

  • A P1129 does not always mean the engine is actually running rich; a lazy or contaminated bank 2 oxygen sensor can report a false rich condition and cause the ECU to pull fuel it should not, so always verify with a scope or a known-good sensor before condemning injectors.
  • Volkswagen long-term fuel trim on bank 2 can be skewed by a leaking injector that only drips when the engine is hot, so a cold-start injector balance test may look normal while a hot-restart test reveals the fault.
  • A stuck-open EVAP purge valve is a commonly missed cause because it feeds fuel vapor into the intake that the mass airflow sensor never measures, and the resulting rich trim often affects bank 2 more due to intake runner geometry.
  • Exhaust leaks upstream of the bank 2 oxygen sensor can pull in outside air and make the sensor read lean, causing the ECU to add fuel and then overcorrect into a rich trim on that bank, so inspect the exhaust before replacing sensors.
  • On some Volkswagen engines, a failing high-pressure fuel pump or a worn cam follower can cause excess fuel to enter bank 2 without setting a fuel pressure code, so check fuel pressure and cam follower wear when trim data points rich.

Can I keep driving with P1129?

You can usually drive the vehicle short distances to a repair shop, but avoid hard acceleration and long trips because a persistent rich condition can foul spark plugs, damage the catalytic converter, and dilute engine oil. If the check engine light is flashing or the engine is misfiring, stop driving and have it towed.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for Volkswagen vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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