What Does P0159 Mean?
The downstream O2 sensor on Bank 2 (Sensor 2, post-catalytic converter) is responding too slowly to changes in exhaust oxygen content. The PCM has determined the sensor's voltage switching rate is below the expected threshold. While downstream sensors are expected to be more stable than upstream sensors, they should still respond to commanded fuel enrichment and enleanment tests within certain time parameters. The driver is unlikely to notice any symptoms.
Common Causes
45%
Aging or contaminated downstream O2 sensor with degraded response time
20%
Catalytic converter issues affecting exhaust gas chemistry at the sensor
15%
Exhaust leak near the downstream sensor location allowing air intrusion
10%
Sensor wiring degradation or high-resistance connections slowing the signal
10%
Oil or coolant contamination on the sensor element
Diagnostic Steps
1
Monitor Bank 2 Sensor 2 voltage response during a forced rich-then-lean event using scan tool fuel trim control — the sensor should respond within 1-2 seconds of the upstream sensor showing the change.
2
Compare Bank 2 Sensor 2 response time to Bank 1 Sensor 2 — both downstream sensors should behave similarly; a significant difference isolates the problem to the Bank 2 sensor.
3
Inspect the sensor tip (if accessible) for contamination — white deposits indicate coolant, black soot indicates rich running, and a glassy appearance indicates silicone contamination.
4
Check for exhaust leaks between the catalytic converter and the sensor — even a small leak can introduce enough air to skew the sensor readings.
5
Verify the sensor heater circuit is functional (heater resistance 2-25 ohms, 12V supply present) — a sluggish heater can cause the sensor to operate at sub-optimal temperature.
Estimated Repair Cost
$100 - $350
Parts + labor, varies by vehicle and location
The downstream O2 sensor on Bank 2 (Sensor 2, post-catalytic converter) is responding too slowly to changes in exhaust oxygen content. The PCM has determined the sensor's voltage switching rate is below the expected threshold. While downstream sensors are expected to be more stable than upstream sen...
The most common cause of P0159 (Oxygen Sensor Slow Response, Engine Bank B, Downstream of Catalytic Converter) is: Aging or contaminated downstream O2 sensor with degraded response time
Typical repair costs for P0159 range from $100 to $350, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Safe to continue driving. The downstream sensor monitors catalyst efficiency and does not directly control fuel delivery. The vehicle will fail emissions testing and the MIL will remain illuminated, but there are no safety or driveability concerns.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0159 to identify the root cause.
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Quick Info
Category
Powertrain
System
Emissions / Exhaust System
Difficulty
Type
Generic (SAE)
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