OBD-II trouble code
P0159: O2 Sensor Circuit Slow Response (Bank 2, Sensor 2)
The downstream oxygen sensor on bank 2 is responding too slowly. It's the bank-2 mirror of P0139 and the lowest-stakes of the bank-2 O2 codes: a rear sensor that has gotten lazy. An aged sensor is the usual cause, and because this sensor mainly watches catalyst efficiency rather than trimming fuel, the practical impact is small — mostly a check engine light and a failed emissions test.
Quick facts
- System
- Powertrain
- Category
- Oxygen Sensor
- Severity
- Low severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $150 – $400
- DIY difficulty
- Intermediate DIY
Browse every code in P0100–P0199, or start from the full code library.
What does P0159 mean?
The downstream (sensor 2) oxygen sensor sits behind the catalytic converter, where it monitors how effectively the converter is cleaning the exhaust by comparing its slower, steadier signal against the rapidly switching upstream sensor. P0159 sets when the bank-2 downstream sensor responds too slowly — when commanded conditions change, it takes longer than expected to react. Because the rear sensor naturally moves more slowly than the front one, the threshold for 'too slow' here is about the sensor being unable to react adequately during the PCM's monitoring tests, not about fast fuel-trim switching. 'Bank 2' is the cylinder bank without cylinder number one, and 'sensor 2' is the post-catalyst position.
Do not sort the cause by mileage. Contamination from the wrong sealant, oil, or coolant can dull the element. A heater that leaves it cool slows the response, and connector or wiring resistance can do the same. An exhaust leak near the sensor can distort the trace. The downstream sensor monitors the catalyst rather than trimming fuel, so the practical result is a blocked catalyst monitor. Replace the sensor when the response test still fails after those checks.
For the driver, P0159 is a check engine light, little change in how the car runs, and a failed emissions test. Confirm the slow result on the monitor, check the heater and the connector, and rule out an exhaust leak and contamination before the sensor is replaced.
Common causes
- Exhaust leak near the sensor distorting its response
- Corroded or loose sensor connector
- Weak sensor heater keeping the element below operating temperature
- Wiring resistance slowing the signal
- Sensor element contaminated by silicone, oil, coolant, or fuel additives
- Bank-2 downstream sensor that still fails the response test after the heater, the connector, the leak, and contamination have been checked
- Low-quality aftermarket sensor that can't meet response specs
Symptoms
- Check engine light with P0159 stored
- Little or no change in driveability
- Failed emissions test
- Disrupted catalyst-efficiency monitoring
- Sluggish bank-2 downstream sensor trace in live data
- Sometimes paired with catalyst codes like P0430
Diagnostic steps
- 1.View the bank-2 downstream O2 sensor live data and confirm it reacts too slowly during monitoring conditions.
- 2.Verify the sensor heater circuit so the element reaches operating temperature.
- 3.Inspect the connector and wiring for corrosion or resistance that would slow the signal.
- 4.Check for an exhaust leak near the sensor that could distort its behavior. This sensor sits behind the catalytic converter, so reaching it generally means going underneath: put the vehicle on a lift or axle stands with the wheels chocked, and let the exhaust cool before touching it — the converter is the hottest component under the car.
- 5.Rule out contamination from oil, coolant, or non-O2-safe sealant.
- 6.Replace the downstream sensor if it stays sluggish after the heater, wiring, leaks, and contamination are addressed.
Repair cost
$150 – $400
The low end of the printed bracket is a heater or a connector that is slowing the bank 2 downstream sensor. The high end is the sensor after the response monitor still fails. Read the bracket as a planning span, not a quoted price. There is no mileage that makes the sensor automatic.
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.
Related repairs
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.