# P0138: O2 Sensor Circuit High Voltage (Bank 1 Sensor 2) **Verdict:** Safe to drive Oxygen sensors - Generic OBD-II code, applies to all gasoline cars and light trucks sold in the US since 1996. - **Typical cost:** $100-$400 - **Urgency:** Keep driving - **Most likely:** Failed downstream O2 sensor or wire short - **DIY?:** Steps 1-3 ## Quick answer P0138 means the downstream oxygen sensor on bank 1, after the catalytic converter, is reading a voltage that stays too high. The sensor is claiming the exhaust is rich even when it should not be, for example when you lift off the gas. A failed sensor or a signal wire shorted to heater power is the usual cause. ## What does P0138 mean? The downstream oxygen sensor on bank 1 sits after the catalytic converter and helps the computer judge how well the converter is working. On a narrow-band sensor, a warm engine at cruise often gives a fairly steady reading in the middle to upper part of the range. When you lift off the throttle and the computer cuts fuel, the exhaust fills with oxygen and the reading should fall low within a few seconds. P0138 sets when the signal stays above a high limit for too long, often around 1.0 to 1.2 volts depending on the maker, or on some cars when it stays high during deceleration fuel cut, when it should read lean. That points to an electrical short, a sensor stuck high, or less often a very rich engine. It usually needs the fault on two trips. Bank 1 is the side with cylinder 1, and sensor 2 is the second sensor in line, after the converter. ## What are the symptoms of P0138? - Check engine light on - Car usually runs normally - Catalyst monitor not ready, or a failed emissions test - Fuel smell or black smoke only if the engine is truly running rich ## What causes P0138? Ranked from most to least likely. Check them in this order: the cheap, common causes first. 1. **Failed downstream oxygen sensor** (most common). An aging sensor can short inside or become contaminated and stick at a high voltage, so it no longer drops when the computer cuts fuel. It is more common on high-mileage cars and those that have burned oil or coolant. A reading that stays high during a closed-throttle deceleration, with good wiring, points to the sensor. 2. **Signal wire shorted to heater power or other voltage** (common). The sensor plug carries a 12-volt heater feed beside the signal wire. Melted insulation, a crushed harness or an internal sensor short can leak heater voltage onto the signal, pushing it far above normal. Damage often appears where the harness passes near the exhaust or a heat shield, or after previous repairs. 3. **Water or corrosion in the connector** (less common). The downstream connector sits under or along the floor of the car, where water, slush and road salt reach it. Moisture between pins can lift the signal voltage. Connectors with missing seals or cracked housings are most at risk, especially in snow-belt states. Cleaning may help, but corroded terminals usually need replacing. 4. **Engine running rich** (less common). A leaking injector, high fuel pressure, a stuck-open purge valve or a fouled upstream sensor can push the mixture rich enough to keep sensor 2 high. Rich codes such as P0172, negative fuel trims, black smoke or a fuel smell point here. Fix the rich condition first, because it can also overheat the converter. 5. **Engine computer fault** (rare). The engine computer reads the downstream signal and supplies a bias voltage. An internal fault can make a normal signal look high. It is rare and is normally confirmed only after the sensor, wiring and connector test good. Some makers have released software updates for downstream sensor codes, so a shop may check bulletins. ## How do you diagnose P0138? 1. Read all codes; P0172 or rich fuel trims point to a real rich condition rather than a sensor fault. 2. Warm the engine and graph bank 1 sensor 2; a reading stuck above about 1 volt is not normal. 3. Coast with your foot off the gas from about 2,500 RPM; during fuel cut a good downstream sensor should drop low within a few seconds. 4. Unplug the sensor and check whether the computer-side signal falls toward zero; if not, look for a wire shorted to voltage. 5. Inspect the connector for water and corrosion, then replace the sensor if the circuit tests good. ## How do you fix P0138, and what does it cost? | Fix | Typical cost | What it involves | | --- | --- | --- | | Replace the downstream oxygen sensor | $150-$400 | Fixes a sensor that is shorted inside or stuck high. | | Repair shorted wiring or the connector | $100-$250 | Separates the signal from heater power or replaces a corroded plug. | | Fix the cause of rich running | $100-$800 | Covers a leaking injector, purge valve or regulator, after testing confirms it. | Full price breakdowns: [oxygen sensor replacement cost](/cost/oxygen-sensor-replacement), [evap purge valve replacement cost](/cost/evap-purge-valve-replacement). ## Is it safe to drive with P0138? > **Safe to drive.** Yes, it is generally safe to drive if the car runs normally. If you notice a fuel smell, black smoke or rough running, the engine may truly be rich, which can overheat the catalytic converter, so get it checked soon. ## Will P0138 fail an emissions test? P0138 keeps the check engine light on, which fails an OBD-II emissions test in most states. A downstream sensor stuck high stops the catalyst monitor from producing a valid result, so readiness usually shows not ready. After the repair and clearing the code, drive several days, including steady cruising and some coasting, until the monitors complete. ## After the repair: clearing P0138 After the repair, clear the code. On a warm engine, bank 1 sensor 2 should read fairly steady at cruise and drop low quickly when you coast with your foot off the gas. After several trips, the oxygen sensor and catalyst monitors should be complete with no pending code, confirming the fix. ## How to prevent P0138 - Keep the downstream sensor connector sealed and clipped up, away from water and road salt. - Make sure the sensor harness cannot touch the exhaust pipe or heat shields after repairs. - Fix rich running, leaking injectors and purge valve problems promptly to protect the converter. - Repair oil burning and coolant leaks, which contaminate oxygen sensors over time. ## Codes often seen with P0138 - [P0136](/codes/p0136): O2 Sensor Circuit (Bank 1 Sensor 2) - [P0137](/codes/p0137): O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2) - [P0139](/codes/p0139): O2 Sensor Circuit Slow Response (Bank 1 Sensor 2) - [P0141](/codes/p0141): O2 Sensor Heater Circuit (Bank 1 Sensor 2) - [P0172](/codes/p0172): System Too Rich (Bank 1) - [P0420](/codes/p0420): Catalyst System Efficiency Below Threshold (Bank 1) ## Frequently asked questions ### Can I drive with a P0138 code? Yes, P0138 is usually safe to drive with if the car runs normally. The downstream sensor mainly checks the converter. You will fail an emissions test, though, and if you smell fuel or see black smoke, the engine may truly be rich and should be checked soon. ### What causes P0138 most often? A failed downstream oxygen sensor is the most common cause of P0138, followed by a signal wire shorted to the heater feed. Water in the connector and a truly rich engine are less common. A simple unplug test shows whether the high voltage comes from the sensor or the wiring. ### How much does it cost to fix P0138? Most P0138 repairs cost $100 to $400 at an independent shop. A downstream oxygen sensor usually runs $150 to $400, and wiring or connector repairs $100 to $250. If the engine is truly rich, fixing an injector, purge valve or regulator can add $100 to $800. ### What is the difference between P0132 and P0138? Both mean high oxygen sensor voltage on bank 1, but on different sensors. P0132 is the upstream sensor ahead of the converter, which controls fuel and can affect how the car runs. P0138 is the downstream sensor after the converter, which mainly checks the converter. ### Can P0138 cause poor gas mileage? Usually only slightly. On many cars the computer uses the downstream sensor for small fuel corrections, and a false rich reading may lean the mixture a bit. A large drop in mileage, a fuel smell or black smoke suggests the engine is really rich, which needs its own diagnosis. ### Can I fix P0138 myself? Often, yes. You can inspect the wiring under the car, check the connector for water and replace the downstream sensor with an oxygen sensor socket. Support the car safely on stands and let the exhaust cool first. Diagnosing a rich engine or fuel pressure problem is usually a shop job. ### Does P0138 mean my catalytic converter is bad? No, P0138 is a sensor circuit code, not a converter code. A worn converter usually sets P0420 instead. However, a very rich engine can damage the converter, so if P0138 comes with rich codes or a fuel smell, fix the cause before the converter overheats. ### Why do I get P0138 after a long drive? Many cars test the downstream sensor during deceleration, when fuel is cut and the sensor should read lean. Long drives with plenty of coasting give the test more chances to run. If the sensor stays high during those coasts, the computer sets P0138, often on the second trip. ### Should I use an OEM sensor for P0138? An OEM or quality direct-fit sensor with the correct connector is the safest choice. Downstream sensors are generally less fussy than upstream air-fuel sensors, but a wrong or poorly made part can still cause repeat codes. Avoid splicing universal sensors into a wet area under the car. ### Will P0138 go away by itself? It may disappear if water in the connector dries out, but it usually returns, and the computer stores a history of the code. A sensor that has shorted inside will not recover. If the light goes off on its own, still inspect the connector and wiring. ## Sources - [SAE J2012: Diagnostic Trouble Code Definitions](https://www.sae.org/standards/content/j2012_201812/) - [EPA: On-Board Diagnostics (OBD)](https://www.epa.gov/state-and-local-transportation/vehicle-emissions-board-diagnostics-obd) ## Other oxygen sensors codes - [P0130 O2 Sensor Circuit (Bank 1 Sensor 1)](https://diagxcar.com/codes/p0130) - [P0131 O2 Sensor Circuit Low Voltage (Bank 1 Sensor 1)](https://diagxcar.com/codes/p0131) - [P0132 O2 Sensor Circuit High Voltage (Bank 1 Sensor 1)](https://diagxcar.com/codes/p0132) - [P0133 O2 Sensor Circuit Slow Response (Bank 1 Sensor 1)](https://diagxcar.com/codes/p0133) - [P0134 O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 1)](https://diagxcar.com/codes/p0134) - [P0135 O2 Sensor Heater Circuit (Bank 1 Sensor 1)](https://diagxcar.com/codes/p0135) - [P0136 O2 Sensor Circuit (Bank 1 Sensor 2)](https://diagxcar.com/codes/p0136) - [P0137 O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)](https://diagxcar.com/codes/p0137) - [P0139 O2 Sensor Circuit Slow Response (Bank 1 Sensor 2)](https://diagxcar.com/codes/p0139) - [P0140 O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 2)](https://diagxcar.com/codes/p0140) ## Symptoms - [Why does my car smell like gas?](https://diagxcar.com/symptoms/car-smells-like-gas) - [Why is black smoke coming from my exhaust?](https://diagxcar.com/symptoms/black-smoke-from-exhaust) ## Repair costs - [Oxygen sensor replacement cost](https://diagxcar.com/cost/oxygen-sensor-replacement): $150-$450 - [EVAP purge valve replacement cost](https://diagxcar.com/cost/evap-purge-valve-replacement): $120-$350 --- Source page: https://diagxcar.com/codes/p0138 Updated: 2026-11-03 General information, not a substitute for a qualified mechanic.