P1131 means the upstream oxygen sensor on bank 1 is reporting a lean air/fuel mixture and is not switching between rich and lean as expected. The code usually points to either a lazy or failed oxygen sensor, or a real lean condition that keeps the sensor pinned lean.
The oxygen sensor can become contaminated or worn with age, slowing or stopping its switching response and causing it to indicate lean. This is the single most common root cause for P1131.
Unmetered air entering the intake after the mass airflow sensor, or a vacuum leak, creates a persistent lean condition that the oxygen sensor correctly reports.
A leak in the exhaust manifold, flex pipe, or nearby gasket allows outside air to be drawn into the exhaust stream, causing the oxygen sensor to read falsely lean.
Insufficient fuel pressure from a weak pump, restricted filter, or faulty regulator can produce a genuine lean mixture that the sensor detects.
A dirty or failing mass airflow sensor can under-report incoming air, leading the engine controller to deliver too little fuel and resulting in a lean mixture.
PCM/ECM failure causing P1131 is extremely rare and should only be considered after the oxygen sensor, wiring, leaks, and fuel system have been thoroughly tested and ruled out.
Not without testing first. While a failed upstream oxygen sensor is the most common cause, vacuum leaks, exhaust leaks, and fuel delivery problems can set the same code and must be ruled out.
No. P0171 means the engine controller has determined the whole system is running lean. P1131 specifically means the upstream oxygen sensor is indicating lean and not switching properly, which may or may not be caused by a true lean condition.
PCM failure is very rarely the cause of P1131 despite being a common replacement recommendation. Verify the sensor, wiring, exhaust and intake leaks, and fuel system before considering the control module.
It can, especially if the underlying condition is a real lean mixture or a faulty sensor. Common effects include rough idle, hesitation, and reduced fuel economy, though some vehicles may run nearly normally.