Code Definition

P1128 — Long Term Fuel Trim B1 System Too Lean

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

About P1128

P1128 is a manufacturer-specific powertrain diagnostic trouble code recorded for Porsche 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

On Porsche vehicles, the P1128 fault code indicates the Engine Control Module (ECM) has learned a long term fuel trim correction for Bank 1 that is at the lean limit, meaning it is adding a large amount of fuel to keep Bank 1 at the correct air to fuel ratio. The code sets when the long term fuel trim for Bank 1 exceeds the allowed positive correction threshold. This points to either unmetered air entering Bank 1, an exhaust leak ahead of the Bank 1 oxygen sensor, or a fuel delivery shortfall affecting that bank. When this code is set the driver may notice rough idle, hesitation, reduced power...

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