P0136
PowertrainOxygen O2 Sensor Circuit Low Voltage (Bank 1, Sensor 2)
The bank 1 sensor 2 (rear) oxygen sensor circuit has a fault, the signal isn't behaving as expected. This is the sensor downstream of the catalytic converter, used to monitor how well the cat is doing its job.
ⓘ Information only. This page provides general educational information about fault code P0136. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0136 mean?
P0136 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Oxygen O2 Sensor Circuit Low Voltage (Bank 1, Sensor 2).
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light, sometimes the only symptom
- • Slightly worse fuel economy
- • Generally normal drivability
- • On some cars, the cat efficiency monitor (P0420) eventually appears alongside if the rear sensor isn't working at all
Possible causes
Causes commonly associated with P0136, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Failed rear sensor, age-related, sensors typically last 80,000-120,000 miles
- 2. Open circuit in the sensor wiring, often where the loom passes a heat shield
- 3. Connector unplugged or corroded, the rear sensor sees road spray and salt
- 4. Less commonly, an exhaust leak ahead of the rear sensor pulling false air into the reading
- 5. Damaged cat substrate causing unburned fuel to reach the rear sensor
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Visual inspection of the sensor connector, water ingress is the silent cause of a meaningful share of P0136s
- 2. Read live data: a healthy rear sensor sits roughly 0.6-0.8V steady when the cat is functioning; stuck at one extreme suggests sensor or open circuit
- 3. Check sensor wiring for damage between the sensor and the ECU
- 4. Inspect the exhaust around the rear sensor and the cat for leaks (hold a piece of paper near suspect joints with the engine running)
- 5. Test sensor resistance and signal output if the wiring is good
Common questions about P0136
Does P0136 mean my cat is failing? +
Not necessarily. P0136 is specifically a rear sensor circuit fault, not a cat efficiency code (that's P0420 / P0430). A failed rear sensor often appears alone, with the cat itself perfectly healthy. Replace the sensor and the issue clears in most cases. If P0420 appears alongside P0136, that's a stronger hint that the cat is also degrading.
Can I just unplug it and ignore the code? +
No, the ECU sets a code for an unplugged sensor too, and modern engines that fail to monitor cat efficiency may eventually go into limp mode. The code also fails MOT post-2018 if the warning light is on. Fix it properly rather than working around it.
Why does the rear sensor matter if it doesn't control fuel mixture? +
It monitors emissions compliance, comparing front and rear sensor signals tells the ECU whether the cat is converting exhaust gases properly. Without a working rear sensor, the ECU can't be sure the cat is doing its job, which matters for emissions law and for protecting the cat from being run with conditions it shouldn't tolerate.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →