OBD-II trouble code
P0136: O2 Sensor Circuit Malfunction (Bank 1, Sensor 2)
The downstream oxygen sensor circuit on bank 1 isn't behaving the way the PCM expects — the generic bank-1, sensor-2 circuit fault (the specific low- and high-voltage versions are P0137 and P0138). Either the sensor itself is failing, the wiring is faulty, or the catalyst has changed how exhaust flows past the sensor.
Quick facts
- System
- Powertrain
- Category
- Oxygen Sensor
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $150 – $600
- DIY difficulty
- Intermediate DIY
Browse every code in P0100–P0199, or start from the full code library.
What does P0136 mean?
Before troubleshooting this code it's worth being precise about what a downstream oxygen sensor actually does, because most people who hit P0136 in a search bar are familiar with upstream O2 codes (P0131, P0151) and assume P0136 works the same way. It doesn't. The upstream sensor — Sensor 1 — sits before the catalytic converter and is part of the fuel control loop. Its voltage swings constantly between roughly 0.1V and 0.9V as the PCM trims fuel rich and lean. The downstream sensor — Sensor 2 — sits AFTER the catalytic converter, and its job is completely different: it monitors catalyst efficiency, not fuel trim. In a healthy system the downstream sensor's voltage should sit fairly steady, often around 0.45V to 0.7V, because the catalyst evens out the rich/lean swings before exhaust gas reaches it.
P0136 sets when the downstream sensor's circuit reads out of range — missing, stuck, or otherwise implausible — for too long. The more specific low-voltage and high-voltage versions are P0137 and P0138. That can happen for three categories of reasons. First, the sensor itself can fail — internal contamination, a worn heater element, or simple age. Second, the wiring or connector can fault — a short or an open — corrupting the signal regardless of what the sensor is actually producing. Third — and this is the one mechanics often miss — a catalytic converter that's flowing oddly (cracked substrate, missing chunks, severe overheating in the past) can change what the downstream sensor sees in a way that mimics a sensor failure.
Any diagnosis should start by ruling out wiring and confirming the sensor's heater is working before condemning the sensor or the cat. A worn sensor is the most frequently reported single answer, but wiring faults, connector corrosion and exhaust leaks together make up enough of the remainder that skipping them is exactly how people end up paying for a catalytic converter they did not need.
Common causes
- Failed downstream O2 sensor on bank 1 (worn or contaminated sensing element)
- Damaged or shorted wiring between the sensor and PCM, often near the catalytic converter
- Corroded or oil-fouled sensor connector
- Exhaust leak ahead of the downstream sensor, drawing in atmospheric air
- Catalytic converter substrate damage changing how exhaust gas reaches the sensor
- Failed sensor heater element (often sets P0141 alongside P0136)
- Coolant or oil contamination from a head gasket leak (less common but does happen)
- Aftermarket exhaust or header installation that damaged the sensor or harness
Symptoms
- Check Engine Light on, often without obvious driveability issues
- Slight loss of fuel economy
- Sometimes a faint exhaust smell on cold start if there's a leak ahead of the sensor
- Failed emissions test on inspection
- Catalyst monitor not ready on emissions readiness check
- Occasionally a faint hesitation under steady cruise (when the PCM second-guesses its closed-loop fueling because of the bad downstream reading)
Diagnostic steps
- 1.Read the live data PID for bank-1 sensor-2 voltage. Compare to the upstream (bank-1 sensor-1) voltage. The downstream should sit fairly steady around 0.4-0.7V. If it's flat, stuck, or out of range, the code is reflecting reality.
- 2.Let the exhaust cool before working near the sensor, and if the vehicle has to come off the ground put it on rated stands rather than trusting a jack — the downstream sensor sits under the floor against pipework that stays hot enough to burn through a glove long after shutdown. Then inspect the connector for oil, coolant, corrosion, or backed-out pins.
- 3.Check the wiring harness between the sensor and the PCM for chafing, melted insulation, or shorts to the exhaust pipe (common after aftermarket exhaust work).
- 4.Verify the sensor heater is working. Many platforms set a heater-circuit code (P0141) alongside P0136 when the heater fails. Check for battery voltage at the heater supply pin with the engine running, not with the key on and the engine off: on a lot of vehicles the heater feed arrives through a relay that only closes once the engine is turning, so zero volts key-on-engine-off is normal there and condemns nothing. Check the heater fuse at the same time.
- 5.Look and listen for an exhaust leak ahead of the downstream sensor. A pinhole leak in the cat or downpipe will pull in atmospheric air and pull the sensor signal low.
- 6.If the catalyst is suspect (severe overheating history, rattling can, or visible damage), the downstream sensor symptoms can be the catalyst's, not the sensor's. Cat replacement diagnosis follows its own process — don't assume.
- 7.On a V-configuration engine that has a second downstream sensor of the same part number, swap the two to confirm the diagnosis before buying parts: if the fault follows the sensor to bank 2 (causing P0156), the sensor is bad. An inline engine has no bank 2 and nothing to swap against — there the confirmation is the live-data trace plus the wiring and heater checks above.
Repair cost
$150 – $600
Most P0136 repairs are downstream O2 sensor replacement: $150-400 depending on access and platform. Wiring repair is highly variable — a single damaged pin is 30 minutes; a section of melted harness is $200-500. If the underlying issue is a failed catalytic converter, the cost jumps to $400-2,500. Diagnose carefully before quoting cat replacement.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with oxygen sensor replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.