Emissions

P0138 — O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)

Severity: low Safe to drive

What P0138 means

The downstream oxygen sensor sits after the catalytic converter and helps the ECM judge converter efficiency. Its signal normally moves between roughly 0.1 and 0.9 V. P0138 sets when the signal stays too high, around 1 V or more. The cause can be the sensor itself, a short to voltage in the wiring, or a very rich mixture.

Symptoms

  • Check Engine Light on
  • Slightly higher fuel consumption
  • Failed emissions test
  • Occasional rough idle if the mixture is rich
  • May appear with P0420 or fuel trim codes

Possible causes

  1. Failed Bank 1 downstream oxygen sensor
  2. Signal wire shorted to voltage or damaged harness insulation
  3. Corroded or damaged connector pins
  4. Rich fuel mixture from injectors, fuel pressure or engine sensor faults
  5. Sensor contaminated by oil, coolant or fuel
  6. Exhaust leak upstream of the sensor
  7. ECM fault (rare)

How to diagnose

  1. Scan for codes and freeze frame data, noting any mixture or upstream sensor codes
  2. Inspect the sensor harness and connector for chafing, corrosion or heat damage
  3. Watch the sensor signal on a scan tool: it should not sit near 1 V with the engine running
  4. Check fuel trims; strongly negative values point to a rich condition
  5. Unplug the sensor: if the ECM still sees high voltage on the signal wire, look for a short to power in the harness
  6. Test the signal wire and sensor ground for continuity and shorts
  7. Check the exhaust for leaks ahead of the sensor
  8. Look for oil or coolant contamination on the sensor

Common fixes

  • Replace the Bank 1 downstream oxygen sensor
  • Repair the wiring or connector
  • Fix the cause of the rich mixture (injectors, fuel pressure regulator, engine sensors)
  • Repair the exhaust leak
  • Reflash or replace the ECM only after everything else is ruled out

Questions and answers

What does P0138 mean?

The ECM detected abnormally high voltage in the Bank 1 downstream oxygen sensor circuit. The usual culprits are the sensor, its wiring, or a rich mixture.

Can I drive with P0138?

Yes, for a short time. Still, find the cause soon, since a rich mixture hurts the catalytic converter and fuel economy.

Will replacing the sensor fix it?

Not always. Check the wiring, connector and fuel trims first, or the new sensor may set the same code again.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.