P0136 — O2 Sensor Circuit (Bank 1 Sensor 2)

What it means

The ECM has found an implausible signal in the circuit of the downstream oxygen sensor on bank 1, the sensor mounted after the catalytic converter. This sensor does not control the mixture directly; it monitors the oxygen storage capacity of the catalyst and provides a slow correction to the upstream sensor. Its voltage should stay comparatively steady around 0.6 to 0.8 V on a warm engine, so a flat, stuck or missing signal sets this code and disables the catalyst monitor.

Likely causes (most common first)

1. Worn out or contaminated downstream oxygen sensor
2. Exhaust leak between the catalytic converter and the sensor
3. Damaged signal wiring or corroded connector under the vehicle
4. Open or shorted sensor heater affecting the signal
5. Deteriorated catalytic converter altering the expected signal pattern
6. Faulty engine control module

Can you drive with it?

Driveability is unaffected, so normal use is fine, but the car will not pass an emissions test until it is fixed.