P0138 Code: O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)
The oxygen sensor behind the converter on bank 1 reads too high, often from an internal short in the sensor.
At a glance
Severity: Low
- Meaning
- O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)
- System
- Fuel & Air Metering · Powertrain
- Safe to drive?
- Yes, short term
- Typical repair cost
- $0–$280
- DIY difficulty
- Easy
P0138 is set when Bank 1 Sensor 2, the downstream sensor after the catalytic converter, reports a voltage above the PCM's upper limit for too long. Most PCMs check this during deceleration fuel cut: with the injectors off, the exhaust is full of oxygen and the rear sensor must fall close to 0.1 V within a few seconds. A sensor that stays at 0.8 V or more while no fuel is being injected is not seeing a rich mixture; it is being held up by an internal short between its heater and signal, by moisture bridging the pins in the connector, or by a harness wire touching the heater feed. A truly rich engine can also keep the rear sensor high, but then the upstream sensor and fuel trims show it clearly. In everyday practice P0138 is one of the codes most often cured by the sensor itself, provided the wiring checks out.
Can I keep driving? Safe to drive, and the engine usually runs as before. If the front sensor and trims also show a rich mixture, have that looked at soon to protect the catalytic converter.
How to read P0138
- P Powertrain
- 0 Generic (SAE)
- 1 Fuel & air metering
- 38 Specific fault
Symptoms
- Check engine light with no change in drivability
- B1S2 stuck above about 0.8 V, sometimes over 1 V
- Rear sensor doesn't drop when you coast in gear
- Catalyst monitor incomplete
- Occasionally paired with the rear heater code P0141
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Downstream sensor shorted internally between heater and signal, or element fouled
- Signal wire touching the heater feed or another voltage source in a damaged harness
- Engine genuinely running rich from injectors, fuel pressure or purge valve
- Sensor coated with oil or fuel residue from a rich or oil-burning engine
- Water and corrosion in the connector bridging the heater pin to the signal pin
- Universal sensor spliced with heater and signal wires swapped
- PCM input fault
Diagnosis step by step
Run the fuel-cut test
With the engine warm, drive at about 45 mph, then lift completely off the throttle in gear for five seconds. Fuel is cut, the exhaust fills with oxygen and B1S2 should fall below about 0.2 V within a few seconds. If it stays high with no fuel going in, the fault is in the sensor or its circuit, not the mixture.
Tools: OBD-II scanner
Unplug the sensor and watch the reading
Key on, engine off, disconnect the sensor and watch B1S2 on the scan tool. If the value drops to the PCM bias or near zero, the high voltage was coming from the sensor side. If it stays high, measure the harness-side signal pin: a reading near battery voltage means a short in the harness.
Tools: OBD-II scanner, Multimeter
Measure the sensor for a heater-to-signal short
On the sensor side of the connector, measure resistance between each heater pin and the signal pin; the result must be open. Internal shorts often appear only when the sensor is hot, so repeat the test right after a drive if the cold reading looks fine.
Tools: Multimeter
Open and dry the connector
Downstream connectors sit low and collect road water. Look for moisture, white or green deposits and corrosion between pins; a film of water between the 12 V heater pin and the signal pin is enough to pull the signal up. Clean, dry and seal it, then retest.
Tools: Flashlight
Confirm the engine isn't rich
If the sensor and wiring test fine, look at B1S1 and fuel trims. A front sensor that also leans toward rich with long-term trim below about -10% means the engine is really rich, and that has to be fixed first.
Tools: OBD-II scanner
Install a direct-fit sensor
Use a direct-fit sensor with the factory connector to avoid splice errors, remove the old one with penetrating oil and an O2 sensor socket, and torque the new one to about 30 lb-ft. Repeat the fuel-cut test to confirm the rear voltage now drops normally.
Tools: O2 sensor socket, Torque wrench
Where the parts are
- 1Upstream O2 sensor
- 2Catalytic converter
- 3Downstream O2 sensor
- 4Diesel particulate filter
- 5Turbocharger
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Clean, dry and seal the sensor connector | Yes | $0–$30 |
| Replace downstream O2 sensor yourself (part) | Yes | $40–$130 |
| Replace downstream O2 sensor at a shop | No | $120–$280 |
| Repair shorted harness near the exhaust | No | $80–$250 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Notes by brand
Toyota
Many Toyota and Lexus engines combine a wideband air-fuel sensor up front with a conventional narrowband sensor behind the converter. P0138 on these vehicles refers to that rear narrowband sensor and is commonly resolved by replacing it once the harness has been checked.
Field data
| Chevrolet | 4 | Malibu Eco, Equinox | |
|---|---|---|---|
| Toyota | 4 | Corolla, Avalon, Venza |
Source: NHTSA Complaints API (US government open data), analyzed by DTC mention.
Emissions / smog test impact
Will fail
P0138 with the check engine light on fails an OBD-II emissions test. After the repair, drive a few days so the catalyst monitor can run; it needs a working rear sensor to complete.
Frequently asked questions
Why test the rear sensor while coasting?
When you lift off the throttle in gear, the PCM shuts the injectors off completely. With no fuel there is nothing rich for the sensor to report, so a high reading at that moment has to be electrical.
Could my engine be running rich instead?
It can, but then the front sensor and fuel trims show it too, and you would normally see P0172 or P0132 as well. If only the rear sensor is high and it ignores the fuel-cut test, the sensor or its wiring is at fault.
Why does the code come back after rain or a car wash?
Water in the underbody connector can bridge the heater supply to the signal pin and push the voltage up. Once it dries the code may disappear for a while, which is a strong hint to reseal or replace the connector.
Is P0138 bad for the catalytic converter?
The code itself isn't; the rear sensor doesn't control fueling on most engines. Only if a rich mixture is behind it, or the PCM uses the rear sensor to trim the front one, is there extra load on the converter.