P0136 — O2 Sensor Circuit Fault (Bank 1, Sensor 2)

What Is P0136?

P0136 is a circuit fault on the downstream oxygen sensor — Bank 1, Sensor 2. This sensor is fitted after the catalytic converter in the exhaust flow and is used primarily to monitor catalytic converter efficiency rather than control fuelling directly. Unlike the upstream (Sensor 1) which drives closed-loop fuel control, Sensor 2 acts as a reference point.

Because Sensor 2 plays a secondary role, P0136 generally causes fewer driveability problems than P0130 (upstream sensor fault). However, it prevents the ECU from monitoring catalytic converter health, which can mask a failing catalyst. It will also trigger the engine management light, causing an MoT failure.

Common Symptoms

Engine management light on
No driveability impact in most cases
ECU cannot monitor catalytic converter efficiency
Possible failed MoT emissions test
P0420 (catalyst efficiency) may also be stored

Common Causes

Failed O2 SensorThe downstream sensor element or heater has failed, producing a signal outside the valid electrical range. Common after high mileage.

Damaged WiringThe sensor is exposed to more movement and heat than the upstream sensor. Wiring chafing, particularly where the downpipe flexes, is common.

Corroded ConnectorThe connector is located in the hot underbody environment and is prone to corrosion, especially on older vehicles with road salt exposure.

Failed Catalytic ConverterA completely failed catalyst changes the exhaust gas composition downstream significantly, which can occasionally trigger circuit-level codes in some ECU calibrations.

How to Diagnose P0136

Locate the Downstream Sensor

Sensor 2 is downstream of the catalytic converter — typically on the downpipe or mid-section of the exhaust, usually accessible from underneath the vehicle. On some vehicles it may be on the underside of the exhaust manifold after the cat.

Inspect Connector and Wiring

The sensor wiring often runs along the underside of the vehicle and is exposed to road debris and moisture. Check for corrosion at the connector and chafing along the loom, particularly at any points where the exhaust can move.

Test Heater Circuit

Measure voltage on the heater pins and resistance across the heater terminals. Infinite resistance confirms heater failure. Check fuses for the lambda heater circuit.

Monitor Sensor Signal

The downstream sensor signal should be relatively steady compared to the upstream sensor — usually 0.6–0.9V on a warm engine, indicating the catalytic converter is working and the exhaust gas is oxygen-depleted. An erratic signal or signal outside range confirms sensor fault.

Replace the Downstream O2 Sensor

Downstream sensors are typically slightly cheaper than upstream sensors (£30–£90). Apply anti-seize to the threads. Be aware downstream sensors are often more corroded and harder to remove than upstream sensors.

Verdict

P0136 is lower priority than upstream sensor faults as it does not directly affect fuelling. However, it must be repaired to clear the engine management light for MoT. Check the connector for corrosion first — it is the most common cause on older vehicles. Sensor replacement is straightforward on most vehicles.

Frequently Asked Questions

Sensor 1 (upstream) is before the catalytic converter and actively controls the air/fuel ratio. Sensor 2 (downstream) is after the catalytic converter and mainly monitors catalyst efficiency. Sensor 1 failure causes fuelling problems; Sensor 2 failure mainly prevents catalyst monitoring.

P0136 may prevent P0420 (catalyst efficiency low) from being detected, not cause it. If the downstream sensor cannot report correctly, the ECU cannot assess whether the catalyst is working — so P0420 may not set even if the catalyst is failing.

Yes — an engine management light is an automatic MoT failure regardless of which code caused it. Repair P0136 and clear the code before presenting the vehicle for MoT.

Typically 20–45 minutes, but can take longer if the sensor is corroded. Apply penetrating oil the day before if possible. Heat the exhaust pipe with a blowtorch to expand the thread if the sensor is very tight. Use a proper O2 sensor socket with a slot for the cable.

It depends on which warning light the code is triggering. Since 2018, any car presenting with an illuminated amber Engine Management Light (EML) at the MOT is a Major failure under DVSA rules — even if the car drives perfectly. A red warning light is always a Major or Dangerous failure depending on context. If clearing the fault makes the light go out and the code does not reappear during the pre-test drive, you will pass; if the code returns within minutes of clearing, the underlying fault must be fixed before MOT day. A tester is required to fail the car on the light being on, regardless of whether the underlying fault is something safety-critical or not. For codes that affect emissions specifically (catalyst, lambda, EGR), the car may also fail the actual emissions check. Fix the cause, clear the code, and drive the car for a few miles before the test.

P0136 — O2 Sensor Circuit (Bank 1 Sensor 2)