P0138 code: O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)
P0138 means o2 Sensor Circuit High Voltage (Bank 1, Sensor 2). The code says where to look, not what failed — the fix depends on which cause your car actually has.
What P0138 means
The downstream oxygen sensor on Bank 1 sits after the catalytic converter and normally reads a fairly steady, moderate voltage when the converter is doing its job. P0138 sets when that signal is pinned high — a constant rich reading — beyond what the computer accepts. The classic causes are a failing sensor, a wiring short to voltage (a chafed harness touching a power feed), or contamination on the sensor tip from oil, coolant, or excess fuel. An engine running very rich can also push the downstream reading high, but that usually brings companion codes with it. Since Sensor 2 is a catalyst monitor rather than a fuel controller, the car typically drives normally; the concern is that a stuck-high rear signal corrupts the catalyst-efficiency check, so P0420 diagnoses cannot be trusted until this is fixed.
P0138 symptoms
- Check-engine light with no drivability change
- Failed emissions test
- Fuel smell only if the engine really is running rich
- Unreliable or false catalyst-efficiency (P0420) results
What it costs to fix P0138
Typical range: $100–$450 · about ~0.3–1 hrs of labor
| If the cause is… | Part | Labor |
|---|---|---|
| Downstream oxygen (O2) sensor | $30–$250 | ~0.3–1 hrs |
| Wiring / connector repair | $10–$120 | ~0.5–1.5 hrs |
| Exhaust leak repair near the sensor | $20–$200 | ~0.5–2 hrs |
Usually an inexpensive-side O2 job since rear sensors are accessible on most cars. If contamination killed the sensor, budget for fixing whatever is fouling it — an oil or coolant problem — or the replacement will meet the same fate.
The price swings on which cause it turns out to be — so confirm the cause before paying. Diagnose P0138 for my exact vehicle →
How to fix P0138
Moderate — doable at home with common tools and patience
- Scan for companion rich-condition codes
Check whether rich codes like P0172 are set alongside P0138. A downstream sensor pinned high can be truthfully reporting a rich exhaust stream — if the engine is rich, solve that first and this code may clear itself.
- Inspect the harness for a short to voltage
Follow the Bank 1 Sensor 2 wiring under the car, looking for chafe points where the signal wire could rub through against the body or a heat shield and touch a power feed. A short to voltage pins the signal high electrically, no exhaust involved.
- Check the connector for water and contamination
Downstream sensor connectors live under the floor where they collect road spray; unplug it and look for moisture, mud, or green corrosion bridging terminals. Clean and dry it, add dielectric grease, and see if the reading returns to normal.
- Watch the signal on live data
A healthy rear sensor behind a working converter shows a fairly steady mid-range voltage (roughly 0.5–0.7 V) with gentle movement. A signal glued at the top of its range that never dips, even during deceleration fuel cut when the exhaust goes lean, confirms the sensor or its circuit is at fault.
- Replace the downstream sensor
With wiring ruled out, replace Bank 1 Sensor 2 — it's behind the catalytic converter, so plan on working under the car with jack stands. Coat the threads with anti-seize and verify a normal steady reading after a warm-up drive.
DIY or shop? The scan checks and connector inspection are easy DIY, and downstream sensors are often less seized than upstream ones since they run cooler — but you will be under the car, so proper jack stands or ramps are non-negotiable. A shop makes sense if you find evidence of a short to voltage, because tracing where a chafed harness touches power is time-consuming without a wiring diagram and experience. The fix is rarely urgent, but don't ignore it: the rear sensor is your catalytic-converter monitor.
Tools for the DIY steps: OBD2 scanner with live data · Jack and jack stands or ramps · O2 sensor socket · Penetrating oil · Dielectric grease · Anti-seize compound
Not sure which step applies to your car? Get the likely cause for your exact vehicle before buying anything for P0138.
Start a case with P0138 and your car. Redline weighs these causes against what your car shows, and names the check that would settle it before you buy parts.
Can I drive with P0138?
Redline can’t determine from a symptom or code alone whether your car is safe to drive.
P0138: common questions
Can I drive with P0138?
Not something to judge from a code alone. The downstream sensor does not control the engine, so you may not notice any change. Fix it before an emissions test, and sooner if you also notice fuel smell or other codes suggesting a rich-running engine.
What causes high voltage on a downstream O2 sensor?
Three main things: a failing sensor, a wiring short putting battery or reference voltage onto the signal wire, or contamination coating the sensor tip. A truly rich engine can do it too, but that usually sets additional codes.
Will P0138 damage my catalytic converter?
The code itself will not — but if the reading is high because the engine genuinely runs rich, the unburned fuel can overheat the converter. And while the rear sensor misreports, the computer cannot accurately monitor converter health at all.
Do I need to replace both O2 sensors for P0138?
No — only the Bank 1 downstream sensor is implicated. Replacing pairs is optional preventive maintenance on high-mileage cars, not something this code requires.
Related trouble codes
Don’t have a scanner yet? A ~$25 Bluetooth adapter reads codes like P0138 from your phone. See our guide to the best OBD2 Bluetooth adapters. Run a repair shop? See Redline for shops.
Generic OBD-II reference. Manufacturer-specific behavior varies. Confirm with a scan tool and, for safety-related codes, a professional inspection. Cost ranges are reference ranges, not quotes.
Seeing P0138 on your car?
Every problem becomes a Case. Tell Redline what the car is doing: it shows what the evidence supports, what’s still missing, and what to check next, before you approve a repair.
“Inner pad ~1 mm. Outer pad 3 mm.”
Next: check the slide pins move freely before buying parts.