P0138 Generic (SAE)

O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)

Updated 2026-10-02 · Fuel & air metering

Severity

Low

Typical repair cost

40 – 300 EUR

System

Powertrain

What it means

P0138 sets when the downstream bank 1 sensor signal stays above about 0.9-1.0 V for an extended period, or above 0.8 V while the ECU is deliberately running lean during its catalyst test. A zirconia sensor cannot produce more than about 1.0 V, so a reading of 1.2 V or higher is always an external voltage on the signal wire; a steady 0.85-0.95 V indicates either a genuinely rich exhaust after the catalyst or a contaminated sensor with a high offset.

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Symptoms

Check engine light, often the only symptom
Possible rich running symptoms if the cause is fuelling: fuel smell, black soot at the tailpipe
Catalyst monitor fails to complete
On some vehicles slight hesitation as the ECU trims lean in response
Melted or chafed wiring smell near the exhaust if a short is the cause

Common causes

Signal wire shorted to the heater 12 V inside the sensor pigtail or harness (melted insulation against the exhaust)
Downstream sensor contaminated by coolant (head gasket leak), oil or silicone, producing a high offset
Genuinely rich exhaust: leaking injector, high fuel pressure, upstream sensor fault causing rich fuelling
Sensor internal failure
Water in the connector bridging heater and signal pins
Catalyst dead so the rich pulses from upstream reach the rear sensor (usually with P0420)

How to diagnose

Read B1S2 at warm idle and 2500 rpm: a steady value above 0.9 V, or any reading of 1.2 V or more, confirms the fault; also note upstream B1S1 behaviour and fuel trims
If the reading is 5 V or 12 V, unplug the sensor: the harness signal pin should show about 0.45 V bias; if it still shows 12 V the harness is shorted; if it drops to 0.45 V the short is inside the sensor pigtail
Create a lean condition (vacuum leak or lift off at 3000 rpm with fuel cut): a healthy downstream sensor drops below 0.3 V within 2-4 seconds; if it stays high the sensor is contaminated or dead
Check fuel trims: strongly negative long-term trim (-10 % or more) with B1S1 also reading high means a real rich condition; check fuel pressure (3.5-4.0 bar typical) and injectors first
Inspect the pigtail where it passes the exhaust and the heat shields for melted insulation; measure heater resistance (2-15 ohm) to confirm the sensor itself is still intact
Check coolant level and look for white deposits on the sensor tip indicating coolant contamination from a head gasket leak

Frequently asked questions

My scan tool shows 1.27 V on the rear sensor. Is that possible?

Not from the sensor itself. That is an external voltage, almost always the heater supply leaking into the signal wire through melted insulation in the sensor pigtail.

Can a head gasket leak cause P0138?

Yes. Coolant contains silicates and glycol that coat the sensor element and skew its output rich. White crusty deposits on the sensor tip are the tell-tale sign.