OBD-II trouble code
P0138: O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)
The downstream oxygen sensor on bank 1 is reporting a voltage that's stuck high — pinned near the rich end of the scale and staying there. Usually the sensor signal wire has shorted to a power source, the sensor is contaminated, or the engine genuinely is running rich.
Quick facts
- System
- Powertrain
- Category
- Oxygen Sensor
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $150 – $600
- DIY difficulty
- Intermediate DIY
Browse every code in P0100–P0199, or start from the full code library.
What does P0138 mean?
P0138 is the high-voltage counterpart to P0137. Both involve the same part — the downstream oxygen sensor on bank 1 (Sensor 2), which sits after the catalytic converter — but they describe opposite electrical symptoms, and the causes barely overlap. P0137 sets when the signal is stuck low (near 0V). P0138 sets when the signal is stuck high, toward the rich end of the scale, and refusing to come back down for longer than the PCM tolerates.
A zirconia oxygen sensor produces a higher voltage when it sees a rich (low-oxygen) exhaust stream and a lower voltage when it sees a lean (high-oxygen) one. A healthy downstream sensor on a car with a good catalyst should sit fairly steady in the middle of its range — often 0.4V to 0.7V — because the catalyst smooths out the rich/lean swings before exhaust ever reaches it. When the downstream reading climbs and stays near the top of the scale, the PCM has three suspects.
The first is an electrical fault: the signal wire shorting to a voltage source, which forces a false-high reading no matter what the exhaust is actually doing. There is a structural reason this is so common on an oxygen sensor rather than bad luck — the heater element's 12V supply runs in the same four-wire pigtail as the signal, a few millimetres away, and that pigtail is routed past hot exhaust. Melt or chafe it and the signal pin is handed battery voltage by the very wire bundled against it. The second is contamination — coolant from a small head-gasket or intake leak, or silicone from the wrong RTV sealant used during a past repair, will coat the sensing element and skew it rich. The third is a genuinely rich condition: leaking injectors, high fuel pressure, or a saturated evap purge dumping fuel vapor. Because the downstream sensor doesn't directly control fuel trim, P0138 rarely changes how the car drives — but the catalyst monitor depends on that sensor, so it will sit incomplete while the code is set, and the code shouldn't be ignored if the underlying cause is a rich-running engine slowly poisoning the catalyst.
Common causes
- Signal wire shorted to voltage — most often to the sensor's own heater supply, which runs alongside the signal in the same pigtail
- Coolant contamination of the sensor from a small head-gasket or intake-manifold leak
- Silicone contamination from the wrong RTV sealant used in a prior repair
- Genuinely rich exhaust — leaking fuel injector, excessive fuel pressure, or a stuck-open evap purge valve
- Failed downstream O2 sensor reading high internally
- Corroded or water-intruded sensor connector pulling the signal high
- Melted or chafed sensor pigtail where it passes the exhaust, bridging the signal to the heater feed
- Aftermarket exhaust work that pinched or melted the sensor harness
Symptoms
- Check Engine Light on, usually with no obvious change in how the car drives
- Failed emissions test, or catalyst monitor stuck 'not ready'
- Possible faint fuel smell or black-tinged exhaust tip if the engine is actually running rich
- Slightly reduced fuel economy in the rich-condition cases
- Occasionally a rough cold start if a leaking injector is the root cause
- No symptoms at all in the pure wiring-short cases
Diagnostic steps
- 1.Read live data for the bank-1 sensor-2 voltage, and note how high it actually goes, because the number narrows the cause before you touch anything. Bosch describes a conventional zirconia sensor from about 0.1 V lean to a maximum of about 0.9 V rich. A reading near 5 V or battery voltage is voltage fed in from outside the sensor. A steady reading at the top of the sensor's own range keeps contamination and a genuinely rich exhaust in play.
- 2.Inspect the sensor's signal wire along its full run, especially where it passes near the exhaust. A chafe point shorting to power is the single most common cause of a stuck-high downstream reading.
- 3.Check the connector for coolant, water, or corrosion. Green crust or moisture inside the connector can bridge the signal pin to voltage.
- 4.If the wiring is clean, check fuel trims and look for a rich-causing fault — a leaking injector, high fuel pressure, or a purge valve that won't close. Strongly negative long-term fuel trims point to a genuinely rich engine.
- 5.Look for coolant loss or a recent overheat. Coolant intrusion into the exhaust will both contaminate the sensor and skew it rich.
- 6.On a V-engine with a downstream sensor on each bank, swap-test against the bank-2 unit where both are the same part number and the pigtail reaches. If the high reading follows the sensor to bank 2 and sets P0158, the sensor itself is bad. On an inline engine there is no second downstream sensor to swap with, so unplugging the sensor and watching whether the PCM still reports a high voltage on that pin is the equivalent test — if it does, the voltage is coming from the harness.
Repair cost
$150 – $600
Most P0138 repairs are a downstream O2 sensor replacement at $150-400 depending on access. A wiring short repair can be as little as $50 for a single chafed wire or $200-500 for a damaged harness section. If the root cause is a leaking injector or a coolant leak, costs run higher and follow those repairs. Diagnose before replacing — a clean wiring inspection often saves the sensor.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with oxygen sensor replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.