Emissions code
P0138: Rear oxygen sensor reads high (bank 1, after converter)
How this range is built
| Part of the job | Low | High |
|---|---|---|
| Parts | $40 | $180 |
| Labor: 0.6 to 1 hours × $125 to $175 an hour | $75 | $175 |
| Fees: shop supplies, disposal, diagnostics | $0 | $15 |
| Total, rounded | $110 | $370 |
The hourly rate is the BLS Occupational Employment and Wage Statistics, May 2025 average pay for this trade in Boston, $31.54 an hour, times 4 to 5.5 to cover what a shop bills (overhead, tools, insurance, profit). Parts are a band from budget aftermarket to the manufacturer part. Full method.
Not a repair shop, estimates only. Typical range, estimate. Not a quote. Your car, parts and shop change the number.
This is the range for the most common fix, oxygen sensor replacement (one sensor). Which repair your car needs depends on a proper diagnosis.
The oxygen sensor after the catalytic converter is stuck sending a high voltage that stays high. This can mean the sensor failed or its wire touches a power source, but it can also mean the engine really is running rich and unburned fuel is passing through the converter.
- Safe to drive?
- Usually yes, for now
- Most common cause
- Failed rear sensor
Safe to drive for now; check if it smells like fuel.
What you may notice
- Check-engine light on
- Fuel smell from the exhaust
- Worse fuel economy
- Possible black soot on the tailpipe
Common causes, most likely first
- Failed rear sensor
- Shorted wire
- Engine running rich
- Coolant or oil contaminating the sensor
- Leaking injector
Check this first
Look at your list for a rich code such as P0172 stored at the same time. If one is there, fix the rich condition first, because a new rear sensor will just read high again.
Other repairs that sometimes fix P0138
| Repair | Typical range, Boston |
|---|---|
| Diagnostic scan (check engine light) | $60 to $290 |
| Vacuum leak repair | $100 to $530 |
Before you pay
- Ask the shop what test showed this part is the cause, not just that the code appeared.
- Ask for parts and labor listed separately, and for the hours.
- Ask for a written OK before any extra work. Massachusetts rules require written authorization before repairs.
Got a quote already? Check it against the range. Want to see if your car has a recall tied to this problem? Check recalls by VIN.
Need to read a code yourself? OBD-II scanners on eBay (affiliate link, disclosure).
P0138 questions
Why does a rich engine cause it?
When the engine runs rich, leftover fuel travels into the exhaust. The sensor behind the converter reads that as a high signal, so it can look like a sensor failure when the root cause is upstream.
Could coolant be the problem?
Rarely, but coolant leaking into the combustion chamber can burn on the sensor and ruin it. If you also lose coolant with no visible leak, tell the shop so they can check the head gasket.
Are rear sensors costly?
They land in the moderate range, with price depending on your make. The sensors are usually reachable from under the car and labor is small, but a rusted sensor can need extra time to remove.
How much does it cost to fix P0138?
The most common fix, oxygen sensor replacement (one sensor), has a typical range of $110 to $370 in Boston. It is an estimate built from parts prices and local labor wages, not a quote. Your car and shop change it.
Related codes
- P0137: Rear oxygen sensor reads low (bank 1, after converter)
- P0172: Engine getting too much fuel (bank 1)
- P0420: Catalytic converter not working well (bank 1)
- P0141: Oxygen sensor heater fault (bank 1, after converter)
More emissions codes
Code meaning written in our own words. Some codes mean different things on different makes, so check your scan tool or manual. Not a repair shop, estimates only.