P0138: The downstream oxygen sensor on the driver side is reading too high
O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)
The downstream oxygen sensor voltage is too high (bank 1). Get it checked within a week or two. It can trigger a false P0420, so fix this before anyone replaces a converter.
Stop driving and get help if
- The check engine light is flashing
- The temperature gauge runs high, or you see steam (overheating)
- The car loses power, hesitates badly or stalls
- You smell burning or see smoke
- A brake warning light comes on, or the brakes feel different
Pull over as soon as you can, turn the engine off, and call a tow or roadside help.
What it means
The downstream oxygen sensor output should stay relatively stable, usually below 0.6 volts on a healthy system. P0138 means the sensor stayed above about 0.9 volts longer than normal. A downstream sensor stuck high usually means it is seeing low oxygen in the exhaust, which happens with a rich running engine or a contaminated sensor that cannot switch properly. The critical insight here: a sensor stuck high is the most common cause of a false P0420. Before spending over a thousand dollars on a new converter, confirm this downstream sensor is actually working.
Common causes
- Failing downstream O2 sensor stuck at high voltageThe sensing element ages and loses its ability to switch, staying near the rich end of its range.
- Rich-running engine conditionIf fuel trim codes like P0172 are also present, excess fuel in the exhaust keeps the sensor high.
- Short to voltage in the signal wireDamaged wiring can supply external voltage to the signal line, pinning it artificially high.
- Coolant or oil entering the exhaustContaminated exhaust coats and ruins the sensor element, causing a stuck-high reading.
What the fix could involve
From the cheaper fix to the bigger one. Prices depend on your car, your area and the shop, so ask for a written quote that names the part and the labor before you approve it.
- Wiring repair
- Downstream O2 sensor (Bank 1)
Symptoms
- Check engine light on, steady
- May accompany P0420 or rich-running codes (P0172)
- Usually no obvious drivability change from this code alone
- Failed emissions test
What to do next
Check for other codes, especially P0172 (rich fuel trim). If the engine is running rich, address that first. If no fuel trim codes are present, inspect the sensor wiring and then replace the sensor. Confirm the repair clears both P0138 and any accompanying P0420 before assuming the converter is bad.
Lewis reads P0138 alongside any related oxygen sensor codes and shows your live fuel trims, so you can see whether a rich condition or the sensor is the more likely cause. Get Lewis on the App Store or Google Play.
Related codes
FAQ
- Can P0138 cause P0420?
- Yes, and this is the most common misdiagnosis path. A downstream sensor stuck high reports unhealthy converter output even when the converter is fine. Fix P0138 first, then re-evaluate P0420.
- Bank 1 Sensor 2 -- is that upstream or downstream?
- Downstream. Sensor 1 is before the catalytic converter, Sensor 2 is after it.
- Could a bad catalytic converter cause this code?
- A completely failed converter can let rich exhaust pass through, holding the downstream sensor high. But check the sensor first because it is far cheaper.
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