P0131: O2 Sensor Circuit Low Voltage — Bank 1 Sensor 1
What to do
Expect poorer economy and possibly hesitation. Ask for an exhaust leak check ahead of the sensor — a leaking manifold gasket produces this code with a healthy sensor fitted.
What this code means
The upstream bank 1 sensor is reporting persistently low voltage, which for a conventional zirconia sensor means it is seeing a lean mixture — or has lost its signal to a ground short. The critical question is whether the sensor is lying or telling the truth: a genuine exhaust leak upstream of it draws in fresh air and produces an honest lean reading from a perfectly good sensor. Check for leaks before replacing anything.
Likely causes
Start here
- Exhaust leak before the sensor · moderate
- Faulty oxygen sensor · easy
- Vacuum leak in intake manifold or hoses · moderate
Less common
- Wiring shorted to ground · moderate
- Low fuel pressure · moderate
- Contaminated or failing mass air flow sensor · easy
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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