Can I drive with P0138?

Short term yes, but a real rich condition wastes fuel and can clog the catalytic converter within weeks of constant rich running. If you smell strong fuel from the exhaust or fuel economy has dropped sharply, repair it promptly.

What causes high voltage on a downstream O2 sensor?

A genuinely rich exhaust (leaking injector, fuel pressure too high from a faulty regulator, or a regulator with a leaking diaphragm), a shorted sensor signal wire, or a contaminated sensor element. Always check fuel trims and look for companion codes before replacing the sensor.

Can a bad O2 sensor cause rough idle?

The downstream sensor alone usually does not cause a rough idle because it does not drive primary fuel mixture decisions. If you have rough idle plus P0138, look for a problem that makes the engine run rich, such as a leaking injector, excess fuel pressure or a faulty upstream sensor. A vacuum leak or a misfire normally pushes the readings lean, not rich.

How do I know if my catalytic converter is bad versus the sensor?

Compare upstream and downstream waveforms. If the downstream sensor switches as rapidly as the upstream, the converter has lost its oxygen storage and is failing. If the downstream is just stuck high regardless of operating conditions, the sensor is the likely culprit.

P0138: O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)

Quick Answer

Safe to drive (short-term)

P0138 means the downstream oxygen sensor on Bank 1 (post-catalyst, Sensor 2) is reporting a voltage that is too high for too long, above roughly 1.0V. A healthy downstream sensor reads around 0.6-0.8V steady. Persistent high voltage typically indicates a rich exhaust condition, a short to voltage in the wiring, or a contaminated sensor.

Severity: Moderate · Emissions
Estimated repair cost: From €65 to €650+, varies by vehicle

Scan your own car in about 60 seconds: see whether P0138 is really what it has, how urgent it is, and what the fix should cost. Free on iPhone and Android with an ELM327 adapter (Bluetooth LE on iPhone).

Get Skanyx free

What does P0138 mean?

P0138 is a emissions-related diagnostic trouble code classified as moderate severity. When your vehicle's ECU detects this condition, it stores P0138 and illuminates the check engine light.

Where it is on the car

Engine
Oxygen sensor before the converter (sensor 1)
Catalytic converter
Oxygen sensor after the converter (sensor 2, or 3 on some cars)
Silencer and tailpipe

What are the symptoms of P0138?

If your vehicle has triggered P0138, you may notice one or more of these symptoms:

Check engine light illuminated
Increased fuel consumption
Black smoke or strong fuel smell from exhaust in severe cases
Possible failed emissions test
May appear with P0172 or P0175 if the engine is genuinely running rich

What causes P0138?

Here are the most common causes of P0138:

Failed downstream O2 sensor (Bank 1, Sensor 2) stuck at high voltage
: high
Actual rich condition (leaking fuel injector, high fuel pressure)
: high
Short to voltage in the O2 sensor signal wire
: medium
Contaminated sensor (coolant, oil, silicone sealant)
: medium
Failed catalytic converter no longer storing oxygen
: low

Is it safe to drive with P0138?

Generally yes, for short-term driving. Safe to drive short term. A high downstream signal usually points to a rich condition reaching the sensor, a shorted sensor, or a failing catalytic converter. Long-term rich running can clog the converter, so do not ignore this for more than a few weeks.

How do you diagnose P0138?

Follow these steps to pinpoint the root cause of P0138:

Read freeze frame data and check for companion codes (P0172, P0175, P0420) that point to a real rich condition or converter problem
Use a scan tool to view downstream sensor voltage. A reading stuck above 0.9V at idle indicates a rich condition or a faulty sensor
Check long-term fuel trims. If LTFT is significantly negative (under -10%), the engine is genuinely running rich
Inspect the Bank 1 Sensor 2 signal wire for shorts to voltage, especially near heat shields and the battery positive cable
Substitute a known-good sensor. If high voltage persists, the issue is fuel mixture or wiring, not the sensor

How much does P0138 cost to fix?

Repair costs for P0138 vary depending on the root cause and your vehicle. Here are typical estimates:

Repair	Parts Cost	Labor Cost	Total Estimate	DIY Difficulty
Replace downstream O2 sensor (Bank 1, Sensor 2)	€40–€180	€50–€130	€90–€310	Moderate
Repair shorted O2 sensor wiring	€5–€30	€60–€150	€65–€180	Moderate
Diagnose and fix rich condition (injector, fuel pressure)	€30–€400	€80–€250	€110–€650	Professional

Prices estimated as of March 2026. Costs vary by region, vehicle, and shop.

Full repair guides

Oxygen Sensor Replacement Cost

P0138 on specific vehicles

P0138 behaves differently depending on your vehicle. Select your car below for model-specific causes and known issues:

P0138 on BMW

BMW 3 Series

See BMW-specific causes & cost

P0138 on Volkswagen

Volkswagen Golf

See Volkswagen-specific causes & cost

P0138 on Audi

Audi A4

See Audi-specific causes & cost

P0138 on Mercedes-Benz

Mercedes C-Class

See Mercedes-Benz-specific causes & cost

These codes are often seen alongside P0138 or indicate related issues:

Common questions about P0138

Can I drive with P0138?: Short term yes, but a real rich condition wastes fuel and can clog the catalytic converter within weeks of constant rich running. If you smell strong fuel from the exhaust or fuel economy has dropped sharply, repair it promptly.
What causes high voltage on a downstream O2 sensor?: A genuinely rich exhaust (leaking injector, fuel pressure too high from a faulty regulator, or a regulator with a leaking diaphragm), a shorted sensor signal wire, or a contaminated sensor element. Always check fuel trims and look for companion codes before replacing the sensor.
Can a bad O2 sensor cause rough idle?: The downstream sensor alone usually does not cause a rough idle because it does not drive primary fuel mixture decisions. If you have rough idle plus P0138, look for a problem that makes the engine run rich, such as a leaking injector, excess fuel pressure or a faulty upstream sensor. A vacuum leak or a misfire normally pushes the readings lean, not rich.
How do I know if my catalytic converter is bad versus the sensor?: Compare upstream and downstream waveforms. If the downstream sensor switches as rapidly as the upstream, the converter has lost its oxygen storage and is failing. If the downstream is just stuck high regardless of operating conditions, the sensor is the likely culprit.

Read our detailed guides

Scan your car with Skanyx to diagnose P0138

Your car logged P0138. Skanyx reads it on a standard OBD2 adapter (Bluetooth LE on iPhone), tells you whether it is safe to keep driving, and shows the €65-€650 repair range so you know the number before a shop quotes you. Free on iPhone and Android, and you do not need to be a mechanic to read it.