O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)
The P0138 code means the vehicle's onboard diagnostic system has detected: O2 Sensor Circuit High Voltage (Bank 1, Sensor 2). This is a moderate severity code.
- Keep driving?
- Yes, but fix soon
- DIY difficulty
- easy
- Estimated cost
- $50-$200
Test O2 sensor voltage (should fluctuate 0.1-0.9V). Do not replace a $150 sensor without confirming it is actually bad first.
Check Price on AmazonAffiliate link -- we earn a commission at no extra cost to you.
Shows real-time O2 sensor readings on your phone. See if the sensor is lazy, stuck, or actually working before spending money.
Check Price on AmazonAffiliate link -- we earn a commission at no extra cost to you.
Common Symptoms
- Check Engine Light on
- Engine runs rich downstream
- Catalyst overheating risk
- Poor fuel economy
Probable Causes (Ranked by Likelihood)
- O2 sensor stuck in rich/high state Very High
- Genuine rich exhaust condition after catalyst Moderate
- Sensor contaminated with coolant or oil causing rich bias Moderate
- Short to voltage on signal wire Low
Step-by-Step Diagnostic Procedure
-
Monitor downstream O2 voltage. Should be relatively stable at 0.6-0.8V in normal operation.
-
If voltage is stuck above 0.9V constantly, suspect sensor contamination or a stuck-rich sensor.
-
Check for coolant burning: sweet smell from exhaust, white smoke on deceleration.
-
Check for oil burning: blue smoke, excessive oil consumption.
-
Replace sensor if contamination is confirmed.
Common Fixes by Vehicle
What techs usually find when diagnosing P0138 on specific vehicles — tap your vehicle for the fix and the exact part:
2009-2020 Ford F-150/Explorer 3.5L/5.0L Easy DIY
Bank 1 downstream O2 sensor reading high voltage (stuck rich above 0.9V). This often indicates the catalytic converter is failing and allowing rich exhaust to pass through. Check for P0420 as well. If the cat is not failed, the O2 sensor itself may have an internal short. Replace with Motorcraft DY-1178.
Labor: 30-45 min2010-2020 Chevrolet Silverado/Tahoe 5.3L Easy DIY
Bank 1 Sensor 2 stuck high voltage. Check for a rich-running condition first by monitoring fuel trims. If the upstream sensor shows rich, fix that issue first. If fuel trims are normal, the downstream sensor has an internal fault. Replace with ACDelco 213-4573. Also check for antifreeze contamination of the cat from a leaking intake manifold gasket.
Labor: 30 min2007-2017 Toyota Camry/RAV4 2.5L Easy DIY
Downstream O2 sensor stuck high. If the voltage stays above 0.8V constantly, the sensor is either shorted or the cat is passing rich exhaust. Monitor the upstream sensor for a persistent rich condition first. Replace the downstream sensor with Denso 234-4350 if it is confirmed faulty. Use penetrating oil before removal.
Labor: 30 minFrequently Asked Questions
Can burning oil contaminate the downstream O2 sensor?
Yes. Combustion byproducts from burning oil coat the sensor element with a silicone-like deposit, biasing the sensor toward a rich reading. Fixing the oil consumption issue and replacing the sensor resolves this.
Sources
This page is built from documented references. Verify against your own service info before repair work.
See All Codes & Fixes for Your Vehicle
Explore More
Was this helpful?
Share carcodefinder.com with a friend who needs to decode their check engine light.