O2 Sensor Circuit (Bank 1, Sensor 2)

What it means

Bank 1 Sensor 2 is the downstream lambda sensor fitted after the catalytic converter. Its job is to monitor catalyst efficiency, so with a healthy catalyst its signal should be comparatively steady, typically 0.5-0.8 V at cruise, with far less switching than the upstream sensor. P0136 is the general circuit code: it sets when the signal is out of the valid 0.0-1.0 V range, stuck at bias voltage, or fails the ECU's response test when it deliberately drives the mixture rich and lean. On most vehicles it does not affect fuelling because sensor 2 only trims the upstream target slightly.

Symptoms

• Check engine light with no noticeable drivability change in most cases
• Catalyst monitor never runs, so readiness is incomplete for inspection
• Slightly higher fuel consumption on vehicles that use sensor 2 for fuel trim
• Occasionally stored with P0420 if the signal was implausible for a long time
• Possible exhaust smell if an exhaust leak is the real cause

Common causes

1. Aged downstream sensor with a slow or offset signal
2. Wiring damaged under the car: chafed on heat shields, cut by debris, corroded connector from road salt
3. Exhaust leak between the catalyst and the sensor or at the sensor thread drawing in air
4. Heater circuit failure keeping the sensor cold (see P0141)
5. Water in the connector, common on sensors mounted low under the floor
6. Incorrect universal sensor or poor splice repair

How to diagnose

1. Locate the sensor: bank 1 (cylinder 1 side), sensor 2 is after the catalyst; on many inline engines it sits under the floor
2. Warm the engine and read B1S2 at steady 2500 rpm: expect a slowly varying 0.5-0.8 V; a flat 0.45 V indicates an open circuit or cold sensor, a line that mirrors the upstream sensor indicates a dead catalyst instead
3. Force the mixture rich (propane or briefly cover the intake): voltage must rise above 0.8 V; force lean (vacuum leak or fuel cut on overrun): must fall below 0.2 V within 2-3 seconds; no response means a dead sensor
4. Measure the heater: 2-15 ohm at 20 °C (most 4-wire sensors 3-9 ohm), with 12 V supply present on the harness with the engine running
5. Inspect the full harness run from the sensor to the body connector for chafe, melt or corrosion; check signal wire continuity (under 1 ohm) and ground under 0.1 V
6. Smoke-test or listen for exhaust leaks between the catalyst and sensor; a leak here gives a permanently lean (low) reading

Terms to know

Related codes

Nearby codes in this group

• P0135O2 Sensor Heater Circuit (Bank 1 Sensor 1)
• P0137O2 Sensor Circuit Low Voltage (Bank 1, Sensor 2)
• P0134O2 Sensor Circuit No Activity Detected (Bank 1, Sensor 1)
• P0138O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)
• P0133O2 Sensor Circuit Slow Response (Bank 1 Sensor 1)
• P0139O2 Sensor Circuit Slow OBDResponse
• P0132O2 Sensor Circuit High Voltage (Bank 1, Sensor 1)
• P0140O2 Sensor Circuit No Activity Detected (Bank 1, Sensor 2)