Code Definition

P1128 — Long Term Fuel Trim B1 System Too Lean

System: Powertrain  |  Type: Manufacturer-Specific  |  Make: Volkswagen

About P1128

P1128 is a manufacturer-specific powertrain diagnostic trouble code recorded for Volkswagen vehicles. It belongs to the P-prefix family of codes, which covers engine, transmission, emissions, or fuel-system faults.

Scanning & clearing: A generic OBD-II scan tool will usually read and clear powertrain (P) codes. For live data, freeze-frame capture, and bi-directional tests you may need a more capable scan tool that supports the vehicle's ECM protocol.

P1128 is a manufacturer-specific code. The same code string on a different make may have a different meaning. Always confirm the code's definition against the vehicle's service information for the specific make and model under test.

Code Information

The Engine Control Module (ECM) uses oxygen sensor feedback on Bank 1 to fine tune injector delivery and hold the air fuel ratio close to the stoichiometric target of 14.7 to 1. Long Term Fuel Trim (LTFT) is the learned long range fuel correction stored by the ECM. DTC P1128 sets when the Bank 1 long term fuel trim reaches its maximum positive correction limit because the ECM has had to add a large amount of fuel to compensate for a persistently lean mixture. A lean condition is most often caused by unmetered air entering the engine or by insufficient fuel delivery. Symptoms can include rough ...

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