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
OBD Fault Code Plain English Guide
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- A severity rating for every code so you know what can wait
- Causes and next steps listed for each fault
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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.
Mechanic's Corner - The Referee Behind The Cat
The downstream sensor has one real job: judging the catalytic converter. That makes P0136 more important than its "no driveability impact" reputation suggests, because while this fault is stored the ECU is refereeing the cat with a blind referee - and on plenty of cars it will quietly suspend the catalyst monitor entirely. The practical consequence: fix P0136 promptly and cheaply, or risk it maturing into a P0420 catalyst efficiency verdict you cannot trust. I never let anyone condemn a cat while a downstream sensor code is live or recent; the £40 sensor always goes in first, then the monitor gets a week of mixed driving to re-run before any cat money changes hands.
The physical job has one honest warning: downstream sensors live in the wettest, saltiest spot under the car and they seize into the exhaust boss harder than almost anything else you will touch. Soak the threads the night before, use a proper lambda socket rather than an open spanner, and work it back and forth rather than hauling in one direction. If it will not move, a garage with an induction heater will have it out in minutes - cheaper than the snapped-sensor extraction job, and far cheaper than the new exhaust section that follows a torn boss.
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.
