Will P0136 hurt my engine if I ignore it?

Not directly. But the computer can no longer monitor the catalytic converter, so a failing converter could go undetected and eventually cause more expensive damage.

Can P0136 and P0420 appear together?

Yes, and you should fix P0136 first. A faulty downstream sensor can cause a false P0420, and replacing a converter before fixing the sensor wastes money.

Bank 1 Sensor 2 -- which side is that?

Bank 1 is the side of the engine with cylinder 1. Sensor 2 is the downstream sensor, after the catalytic converter.

Codes › P0136

P0136: The downstream oxygen sensor on the driver side has a circuit fault

O2 Sensor Circuit Malfunction (Bank 1, Sensor 2)

Moderate severityCan you keep driving?

Yes, you can drive for now. The downstream sensor mainly monitors the catalytic converter, so engine operation is not immediately affected. Get it checked within a week or two.

What it means

The downstream oxygen sensor on Bank 1 sits after the catalytic converter. Its job is to watch converter efficiency, not directly control fuel trim the way the upstream sensor does. P0136 means the computer saw an unexpected voltage, or no signal at all, from this sensor's circuit. Because the sensor comes after the converter, losing it will not make the engine run noticeably rough. But it will blind the computer to converter health and can lead to undetected converter damage, so the code is worth fixing.

Common causes

Failed downstream O2 sensorDownstream sensors run cooler than upstream ones and tend to fail gradually. Common past 100,000 miles.
Corroded or damaged sensor connectorRoad salt and heat cycles crack the connector boot and oxidize the pins.
Wiring harness damage from exhaust heatThe harness runs close to the exhaust pipe. Sagging or melted insulation causes open or short circuits.
Exhaust leak near the sensorAn upstream leak changes the oxygen content the sensor sees, causing out-of-range readings.

What it costs

Wiring or connector repair$80 to $200
Downstream O2 sensor (Bank 1)$130 to $300 installed

Symptoms

Check engine light on, steady
No obvious change in how the car drives
Possible related P0420 (catalyst efficiency) if the fault is long-standing
Failed emissions test

What to do next

Check for other stored codes first. If P0420 is also set, the sensor fault may be masking a converter problem. Inspect the connector and wiring at the sensor before buying a part. Downstream sensors are often cheaper than upstream ones because they are easier to reach. If the wiring is clean and the connector is good, replace the sensor.

Lewis logs the downstream sensor voltage over time, which shows whether the reading is stuck, lazy, or just a wiring fault. Download Lewis.

Related codes

FAQ

Will P0136 hurt my engine if I ignore it?: Not directly. But the computer can no longer monitor the catalytic converter, so a failing converter could go undetected and eventually cause more expensive damage.
Can P0136 and P0420 appear together?: Yes, and you should fix P0136 first. A faulty downstream sensor can cause a false P0420, and replacing a converter before fixing the sensor wastes money.
Bank 1 Sensor 2 -- which side is that?: Bank 1 is the side of the engine with cylinder 1. Sensor 2 is the downstream sensor, after the catalytic converter.