P0138 Code: O2 Sensor Circuit High Voltage — Bank 1 Sensor 2
Quick Answer
The downstream oxygen sensor on the driver's side is consistently showing high voltage, indicating it may be stuck or the engine is running rich.
At a Glance
What this code means
The downstream O2 sensor on Bank 1 (after the catalytic converter) should show a relatively stable, low voltage if the catalytic converter is doing its job. A high voltage reading on this sensor means it's detecting a rich exhaust condition — too much fuel, too little oxygen. This can be caused by a rich-running engine or a stuck/faulty sensor itself.
Sensor 1 (upstream) measures air/fuel mixture before the cat. Sensor 2 (downstream) monitors catalyst efficiency after it.
Common causes
- Faulty downstream O2 sensor stuck in rich (high voltage) state
- Engine running rich (excess fuel) — possibly from a stuck-open fuel injector or failing fuel pressure regulator
- Short to voltage in the O2 sensor signal circuit
- Catalytic converter issues affecting downstream exhaust composition
Symptoms you might notice
- Check engine light on
- Possible black smoke from exhaust if the engine is genuinely running rich
- Reduced fuel economy if the engine is running rich
- No symptoms if the sensor itself is faulty
Can you still drive?
Yes. If the sensor is faulty (and the engine isn't actually running rich), there's no immediate mechanical risk. If the engine is genuinely rich, address it sooner.
How serious is this code?
Low to moderate. Determine whether the engine is actually running rich or the sensor is the issue — these require different fixes.
Before you replace parts
Parts replacement is often not the first step. Before buying anything, it's worth checking these basics:
- Check for related codes — a companion code may point to the true root cause
- Inspect wiring, connectors, and grounds around the affected sensor or component
- Look for obvious physical damage: cracks, disconnected hoses, or corrosion
- Clear the code, drive the vehicle, and see if it returns — intermittent codes can come from loose connections
- Verify the part is actually faulty by testing it before replacing it
How to troubleshoot it
- Compare live fuel trim data from both banks — if Bank 1 shows high negative fuel trim, the engine may genuinely be running rich
- Inspect for stuck-open fuel injectors or leaking fuel pressure regulator
- Test the downstream O2 sensor signal with a scan tool — it should show relatively stable low voltage on a properly functioning catalyst
- Inspect the signal wire for a short to voltage
- Replace the downstream Bank 1 O2 sensor if it's confirmed faulty
Tools that may help
These are the types of tools commonly used when diagnosing this code. Having the right tool can save time and help confirm a diagnosis before spending money on parts.
- OBD-II scanner with live O2 data and fuel trim
- Multimeter
Estimated repair cost range
Downstream O2 sensor: $80–$250. Fuel injector testing/cleaning: $150–$300.
Costs vary significantly by vehicle, location, and whether you use OEM or aftermarket parts. These are general ranges for reference only.
When to call a mechanic
If you've completed the basic troubleshooting steps and the code keeps returning, or if you don't have access to the proper diagnostic tools, it's worth consulting a professional. A qualified technician can perform a full diagnosis and confirm the root cause before any parts are purchased. When in doubt, get a professional opinion — it can save you from an expensive misdiagnosis.
Real-world note
A high-voltage reading from the downstream O2 sensor can sometimes be confused with P0420 (catalyst efficiency). It helps to watch live O2 sensor data: a lazy, high-voltage sensor that barely moves is a failing sensor; one that oscillates rapidly at a high average voltage may indicate a real rich condition.
Informational purposes only. This guide is for educational reference and is not a substitute for diagnosis by a qualified technician. Repair costs, causes, and symptoms may vary by vehicle make, model, year, and condition. Always consult a licensed mechanic before performing major repairs.