P0136 Code: O2 Sensor Circuit Malfunction (Bank 1 Sensor 2)
The oxygen sensor behind the catalytic converter on bank 1 is sending a signal that fails the PCM's checks.
At a glance
Severity: Low
- Meaning
- O2 Sensor Circuit Malfunction (Bank 1 Sensor 2)
- System
- Fuel & Air Metering · Powertrain
- Safe to drive?
- Yes, short term
- Typical repair cost
- $0–$350
- DIY difficulty
- Easy
Bank 1 Sensor 2 is the downstream oxygen sensor, mounted after the catalytic converter on the cylinder 1 side. It doesn't steer fueling from moment to moment; its job is to watch how much oxygen makes it through the converter, which the PCM uses for the catalyst efficiency test and, on many engines, for a slow correction of the upstream sensor. With a healthy converter its voltage sits fairly steady, typically between about 0.5 and 0.8 V, drops toward 0.1 V when fuel is cut on deceleration and rises when you accelerate hard. P0136 is the general circuit code for this sensor: the signal failed a plausibility or range check without matching the more specific low, high, slow or inactive patterns. Because the sensor and its harness live under the car, road water, salt and heat from the converter are frequent culprits alongside the sensor itself.
Can I keep driving? Safe to drive; engine performance is normally unaffected. The catalyst monitor can't run while this code is active, so fix it before any emissions test.
How to read P0136
- P Powertrain
- 0 Generic (SAE)
- 1 Fuel & air metering
- 36 Specific fault
Symptoms
- Check engine light, usually with no drivability change
- Catalyst and oxygen sensor monitors stay incomplete
- B1S2 value on the scan tool jumps around, drops out or shows impossible numbers
- Occasionally a slight change in fuel economy on engines that trim from the rear sensor
- Code may appear after driving through deep water or on salted winter roads
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Downstream sensor failed internally
- Connector under the car corroded or full of water
- Harness chafed, melted or snagged along the exhaust, heat shield or crossmember
- Exhaust leak between the converter and the sensor at a flange, gasket or flex joint
- Heater not working, leaving the sensor too cold in the cooler post-converter exhaust
- Aftermarket converter, test pipe or sensor spacer altering the signal
- PCM input fault
Diagnosis step by step
Make sure you have the right sensor
Sensor 2 is the one after the catalytic converter; on a transverse four-cylinder it may sit just below a manifold-mounted converter, on trucks and rear-drive cars it is under the vehicle near the transmission. Some engines have a third sensor per bank, so confirm the position in a service diagram.
Tools: Flashlight
Graph B1S2 on a test drive
With a fully warm engine, a healthy downstream sensor holds a fairly steady 0.5 to 0.8 V at cruise, falls below about 0.2 V when you coast in gear and climbs above 0.8 V under hard acceleration. Values that jump, drop to zero for an instant or exceed about 1.1 V point to the circuit rather than the converter.
Tools: OBD-II scanner
Inspect the underbody harness and connector
With the car safely on ramps or stands, follow the pigtail from the sensor to its connector, often clipped to the transmission or floor. Look for green corrosion, water in the housing, pushed-back pins and insulation rubbed through on the exhaust or heat shield. Wiggle the harness with the engine running while a helper watches live data.
Tools: Flashlight
Check bias and ground at the harness
Unplug the sensor, key on: the harness-side signal pin usually shows the PCM bias of roughly 0.45 V, and the scan tool should read the same. With the engine running, voltage drop between the sensor ground wire and battery negative should stay under 0.1 V.
Tools: Multimeter
Look for an exhaust leak ahead of the sensor
A leaking flange gasket or a cracked flex joint between the converter and the sensor lets outside air reach the tip and makes the signal erratic. Soot streaks and a puffing sound at idle are the giveaways; repair the leak before judging the sensor.
Tools: Flashlight
Replace the downstream sensor
Downstream sensors are often badly rusted in, so soak the threads with penetrating oil, use an O2 sensor socket and a long breaker bar, and run a thread chaser through the bung if the threads are rough. Torque the new sensor to about 30 lb-ft and secure the harness in every original clip.
Tools: O2 sensor socket, Torque wrench
Where the parts are
- 1Upstream O2 sensor
- 2Catalytic converter
- 3Downstream O2 sensor
- 4Diesel particulate filter
- 5Turbocharger
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Clean, dry and seal the underbody connector | Yes | $0–$30 |
| Replace downstream O2 sensor yourself (part) | Yes | $40–$130 |
| Replace downstream O2 sensor at a shop | No | $120–$280 |
| Repair chafed or melted sensor harness | No | $80–$250 |
| Repair exhaust flange or flex pipe leak | No | $100–$350 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Emissions / smog test impact
Will fail
An OBD-II emissions test fails the car while P0136 is active, and even after the repair the catalyst monitor needs a few days of normal driving to show ready. Don't clear the code the day before the test.
Frequently asked questions
Does a bad rear O2 sensor affect how my engine runs?
Usually not noticeably. The upstream sensor does the real-time fuel control; the downstream one mainly grades the converter. On engines that use it for a long-term correction, mileage can shift slightly, but most drivers notice only the light.
My downstream sensor reading barely moves. Is that the fault?
Not by itself. A steady voltage around 0.5 to 0.8 V is exactly what a good converter produces, because it smooths out the oxygen swings. The sensor should still react to a coast-down and a hard acceleration.
Can I use an O2 sensor spacer to make the code go away?
Spacers and defoulers change the signal to hide converter problems; they don't fix a circuit fault and on street-driven vehicles they count as emissions tampering. A proper repair is cheaper than a failed inspection.
Why do I have P0136 and P0420 together?
The PCM grades the converter by comparing the upstream and downstream sensors. If the downstream signal is faulty, the catalyst test can be misjudged, so repair P0136 first and see whether P0420 returns.