P0138 Rear O2 Sensor High Voltage (Bank 1)
Drive normally - check soon
The rear oxygen sensor on bank 1, after the catalytic converter, is stuck reading high. A failed sensor or a wiring short is the usual cause, but a rich-running engine can do it too. Usually safe to drive.
- Safe to drive?
- Yes
- Typical cost at a shop
- $440-$523 RepairPal national averages, parts and labor
- Emissions test
- Yes, while the check engine light is on, most OBD emissions tests fail the car.
Watch the rear oxygen sensor live during a coast to tell a bad sensor from a rich engine.
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What it means
The rear oxygen sensor on bank 1 sits after the catalytic converter and tells the computer how well the converter works. P0138 means its signal stays too high, as if the exhaust there were very rich. The sensor or its wiring may have failed, or the engine may really be running rich.
The technical version
A narrow-band rear sensor works between about 0.1 and 0.9 V. P0138 sets when the signal stays above its high limit, often during a test where the computer leans the mixture and expects the rear sensor to drop. A short between the signal wire and the heater power wire, a contaminated sensor, or a truly rich mixture can hold it high.
Official name: O2 Sensor Circuit High Voltage (Bank 1 Sensor 2).
Symptoms
- Usually only the check engine light.
- Sometimes a fuel smell or worse fuel economy if the engine runs rich.
Causes, most likely first
| Cause | How likely | How to tell |
|---|---|---|
| A failed or contaminated rear oxygen sensor | most common | The sensor reads high even on a coasting, lean engine. |
| Wiring shorted to voltage (often the heater wire) | common | Melted wires or a damaged connector. |
| The engine really running rich | less common | Negative fuel trims and P0172 with it. |
| Coolant or oil burning in the engine (contaminates the sensor) | rare | White smoke, coolant loss or high oil use. |
How likely is our ranking from repair data and common practice, not a published rate.
What the live data shows
On a warm engine, let off the throttle at speed: during fuel cut, a healthy rear sensor drops close to 0.1 V. With P0138 it stays high, near or above 0.9 V. Check fuel trims: strongly negative long-term trim means the engine really is rich.
What to check first, in order
-
Inspect the sensor wiring and connector.
A short to the heater wire is common.
What the data shows: No reading needed.
Cost: free to look
-
Watch the rear sensor during a coast with your foot off the throttle.
A good sensor drops when fuel is cut.
What the data shows: It should fall close to 0.1 V.
Cost: no cost if you have a scanner
-
Check fuel trims for a rich condition.
A rich engine holds the sensor high.
What the data shows: Long-term trim near 0% points back to the sensor.
Cost: no cost if you have a scanner
-
Replace the rear oxygen sensor.
If the wiring is good and the engine is not rich.
What the data shows: The sensor drops and rises normally again.
Cost: $440-$523 at a shop (Oxygen sensor replacement, RepairPal)
Is it safe to drive?
Drive normally - check soon
Yes. The rear sensor mostly checks the converter; it does little to steer fueling.
Usually safe to drive. If the engine also runs rich (fuel smell, black soot, rich codes), get it checked soon: extra fuel can overheat the catalytic converter. You will fail an emissions test until it is fixed.
Stop driving and get help if
- The check engine light flashes.
- You smell strong fuel or see black smoke.
What it costs
| Repair | Typical cost |
|---|---|
| Oxygen sensor replacementRepairPal, updated September 3, 2026 | $440-$523 |
National averages for the full job at a shop, parts and labor, from RepairPal (updated September 3, 2026). Your price depends on the car and the area. Many of the first checks on this page cost nothing with a scanner. Oxygen sensor replacement was CarMD's number 2 check engine repair in 2025, at an average of $287. CarMD averages come from a different method (repairs reported by its users), so they can sit below RepairPal's shop estimates.
Sources for the notes: CarMD 2026 Vehicle Health Index (press release, April 22, 2026).
Can I clear it? Will it come back?
After the repair, clear the code and drive. The oxygen sensor and catalyst monitors should complete within a few drives.
Will it fail an emissions test?
Yes, while the check engine light is on, most OBD emissions tests fail the car.
Related codes
Also related, no guide yet: P0136, P0158.
How Obie handles P0138
Obie reads the rear sensor during a coast and the fuel trims and tells you whether it looks like the sensor, its wiring or a rich engine. Obie is part of ScanMyCar Pro. Codes and live sensors are free.
Questions people ask
Is P0138 the upstream or downstream sensor?
Downstream: the rear sensor on bank 1, after the catalytic converter.
Can I drive with P0138?
Usually yes. If the engine also runs rich, get it checked soon.
Does P0138 mean my catalytic converter is bad?
No. It points to the rear sensor, its wiring or a rich engine. A bad converter sets P0420.
Sources
- RepairPal: P0138 code page repairpal.com
- RepairPal: Oxygen sensor replacement (updated September 3, 2026) repairpal.com
- CarMD 2026 Vehicle Health Index (press release, April 22, 2026) prnewswire.com
- Texas DPS emissions testing FAQ dps.texas.gov
- Arizona myazcar.com: OBD and the check engine light myazcar.com
A guide, not a diagnosis. This page describes what P0138 generally means across OBD2 cars. What is actually wrong with your car depends on its live data and its history, and any repair decision deserves a proper diagnosis, whether that is you with a scanner or a professional with a lift.