CodeXOBD

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

P0138 means the downstream O2 sensor is stuck at a high voltage, usually indicating a rich exhaust. A failed sensor or a shorted signal wire are the most common causes.

Low

The engine runs normally in most cases. If the cause is an actual rich condition, you may notice poor fuel economy and a fuel smell.

$20–$200Parts, typical range
0.3–1 hLabour
estimateNot a quote · 2026 basis

What P0138 means

P0138 sets when the downstream oxygen sensor voltage stays above 0.9V for an extended period. This can mean the sensor is shorted internally, the signal wire is touching a power source, or the engine is actually running rich enough to produce a high downstream reading. A rich condition from a leaking injector or high fuel pressure can also trigger this code.

Technical summary
SystemEmission control
How the catalytic converter is gradedExhaust leaves the engine, passes the upstream oxygen sensor, then the catalytic converter, then the downstream oxygen sensor. The module compares the two sensor signals: on a healthy converter the downstream signal is steady while the upstream one oscillates. When the downstream signal starts to mirror the upstream one, the converter is judged to have stopped storing oxygen.Engineexhaust outUpstream O₂sets fuel trimCatalytic converterstores oxygenDownstream O₂grades the converterThe module compares the two sensors — that comparison is the test

Symptoms you may notice

What causes P0138

Downstream O2 sensor shorted high

Very likelylow costeasy

The sensor's internal element has shorted, producing a permanently high voltage signal.

Signal wire touching a power source

Likelylow costeasy

Chafed insulation can cause the O2 signal wire to contact a power wire, pulling the voltage high.

Engine running rich

Possiblemedium costmoderate

A leaking fuel injector, high fuel pressure, or a purge valve stuck open can produce genuinely rich exhaust that reads as high voltage on the downstream sensor.

How to diagnose it

1

Check the upstream sensor first

If the upstream sensor also reads high, the engine is running rich and the downstream sensor is reporting accurately. Fix the rich condition first.

Tools: OBD-II scanner with live dataIf it passes: Upstream sensor oscillates normally. The downstream sensor is the fault.If it fails: Upstream is also reading high — investigate the rich condition.
2

Unplug the sensor and check the signal wire

With the sensor disconnected, check if the signal wire at the harness side still reads high voltage. If it does, the wire is shorted to power.

Tools: MultimeterIf it passes: Voltage drops to zero with sensor disconnected. The sensor is faulty.If it fails: Voltage remains high — trace the wire for a short to power.
3

Check fuel pressure

If the upstream sensor also reads rich, connect a fuel pressure gauge and verify the fuel pressure is within specification. Excessive pressure or a leaking injector can produce a genuine rich condition.

Tools: Fuel pressure gaugeIf it passes: Fuel pressure is within spec. The downstream sensor is faulty.If it fails: Pressure is high or an injector is leaking — address the fuel system fault.

Related codes

P0136P0137P0140

Common questions

Can a rich-running engine cause P0138?

Yes. If the engine is running rich enough, the downstream sensor will legitimately read high. In that case the sensor is doing its job and the rich condition needs to be diagnosed.

Last verified 2026-07-27 · Sources: SAE J2012 — Diagnostic Trouble Code Definitions and Failure Type Byte (SAE International)

This information is for general guidance and is not a substitute for professional diagnosis. Cost figures are estimates, not quotes.