What Does P1138 Mean?
This code indicates the downstream heated oxygen sensor on Bank 1 (Sensor 2, after the catalytic converter) is stuck indicating a persistent rich condition. The sensor voltage remains above the expected threshold and is not switching properly. This can indicate a catalytic converter that is not functioning correctly, a faulty downstream sensor, or a rich exhaust condition that the catalyst cannot fully process.
Common Causes
30%
Deteriorated or overloaded catalytic converter on Bank 1
25%
Faulty downstream O2 sensor biased rich or contaminated
20%
Persistent upstream rich condition overwhelming the catalyst's capacity
15%
Downstream O2 sensor contaminated with oil, coolant, or silicone
10%
Wiring issue causing false high voltage reading on downstream sensor
Diagnostic Steps
1
Monitor the Bank 1 downstream O2 sensor voltage at steady cruise; a healthy catalyst should produce a stable voltage around 0.5-0.7V rather than a consistently high reading.
2
Review Bank 1 upstream fuel trims to determine if there is an ongoing rich condition feeding excess fuel to the catalyst.
3
Compare upstream and downstream O2 sensor waveforms to assess catalyst efficiency.
4
Inspect the downstream O2 sensor for contamination (oily residue, white deposits from coolant).
5
Check the downstream O2 sensor wiring for shorts to power or damaged insulation.
6
If upstream trims are normal and the sensor tests good, evaluate the catalytic converter for internal failure.
Estimated Repair Cost
$100 - $1,200
Parts + labor, varies by vehicle and location
This code indicates the downstream heated oxygen sensor on Bank 1 (Sensor 2, after the catalytic converter) is stuck indicating a persistent rich condition. The sensor voltage remains above the expected threshold and is not switching properly. This can indicate a catalytic converter that is not func...
The most common cause of P1138 (Lack of Downstream Heated Oxygen Sensor Switch - Sensor Indicates Rich - Bank No. 1) is: Deteriorated or overloaded catalytic converter on Bank 1
Typical repair costs for P1138 range from $100 to $1,200, depending on the vehicle, location, and whether you do it yourself or go to a shop.
No direct safety hazard under normal conditions. A catalyst saturated with fuel can overheat if the rich condition worsens, potentially posing a fire risk near flammable materials under the vehicle.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P1138 to identify the root cause.
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Quick Info
Category
Powertrain
System
Emissions / Catalyst Monitoring
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