What Does P0138 Mean?
The ECM has detected that the downstream oxygen sensor (bank 1 sensor 2) voltage is stuck high, consistently reading above the expected threshold. For a post-catalyst sensor, a persistently high reading suggests the catalytic converter may not be storing oxygen effectively (degraded converter), the sensor is biased rich, or there is a wiring fault creating an artificially high voltage.
Common Causes
30%
Failed or degraded catalytic converter no longer storing oxygen properly, causing the downstream sensor to mirror upstream rich/lean cycling
30%
Failed downstream O2 sensor with internal rich bias or shorted sensing element generating excessive voltage
20%
O2 sensor signal wire shorted to the heater supply or battery voltage through harness damage in the underbody area
20%
Rich running condition on bank 1 overwhelming the catalytic converter's oxygen storage capacity
Diagnostic Steps
1
Monitor B1S2 voltage alongside B1S1. If the downstream sensor is closely mimicking the upstream sensor's rich-lean cycling, the catalytic converter has lost its oxygen storage capacity and is failing.
2
Disconnect the B1S2 sensor and check signal wire for short to voltage. Measure voltage at the ECM pin with sensor disconnected; it should read near 0.45V (ECM bias voltage), not near battery voltage.
3
Check fuel trims for bank 1. If short-term fuel trim is significantly negative, the engine is running rich and the downstream sensor may be correctly reporting excessive fuel in the post-catalyst exhaust.
4
Verify the heater circuit wires are not cross-connected to the signal wires at the sensor connector or at any splice points in the underbody harness.
5
If the catalytic converter is suspected, perform a converter efficiency test using Mode 06 data or by comparing upstream and downstream sensor switching rates. A healthy converter should show B1S2 switching at less than 20% of the B1S1 rate.
Estimated Repair Cost
$75 - $1,200
Parts + labor, varies by vehicle and location
The ECM has detected that the downstream oxygen sensor (bank 1 sensor 2) voltage is stuck high, consistently reading above the expected threshold. For a post-catalyst sensor, a persistently high reading suggests the catalytic converter may not be storing oxygen effectively (degraded converter), the ...
The most common cause of P0138 (Oxygen Sensor High Voltage, Engine Back A, Downstream of Catalytic Converter) is: Failed or degraded catalytic converter no longer storing oxygen properly, causing the downstream sensor to mirror upstream rich/lean cycling
Typical repair costs for P0138 range from $75 to $1,200, depending on the vehicle, location, and whether you do it yourself or go to a shop.
A high downstream O2 voltage may indicate a failing catalytic converter, which will increase tailpipe emissions significantly. While not a direct safety concern, a degraded converter can overheat if the engine is running rich, potentially creating a fire risk in extreme cases.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0138 to identify the root cause.
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Quick Info
Category
Powertrain
System
Emissions Monitoring System
Difficulty
Type
Generic (SAE)
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