Motor Vehicle Expert
OBD oxygen sensor fault guide

P0138 Code Meaning UK

P0138 means “O2 Sensor Circuit High Voltage Bank 1 Sensor 2”. In plain English, the engine computer is seeing a high voltage signal from the downstream oxygen sensor after the catalytic converter. This can be caused by a faulty sensor, wiring short, poor connector contact, rich-running condition, exhaust issue, sensor contamination or catalytic converter monitoring problem.

📅 Updated June 2026 🇬🇧 UK diagnostic guide 🛠 Mechanic-style O2 sensor advice
Quick answer Diagnosis steps Fault Codes Hub
✓ Downstream O2 high voltage explained ✓ UK repair cost guide ✓ MOT emissions advice ✓ Heavy internal linking included
Quick answer Meaning Symptoms Causes Can you drive? Repair costs Diagnosis MOT impact Related guides FAQs
Quick answer

What does P0138 mean?

P0138 means the ECU has detected a high voltage signal from the oxygen sensor circuit for bank 1 sensor 2. This is normally the downstream oxygen sensor fitted after the catalytic converter.

The downstream sensor mainly monitors catalytic converter performance. A high voltage signal can be caused by a faulty sensor, shorted wiring, poor connector contact, rich-running issue, exhaust fault or catalyst monitoring problem.

Most common area

Downstream oxygen sensor, wiring, connector, short to voltage, rich running or catalyst fault.

Main risk

Emissions monitoring may be unreliable and the engine warning light may stay on.

Best first check

Check live downstream O2 voltage, wiring, connector condition and related catalyst codes.

Code meaning

P0138 — O2 sensor circuit high voltage bank 1 sensor 2

Bank 1 is the side of the engine containing cylinder 1. Sensor 2 is the oxygen sensor after the catalytic converter. This sensor checks catalyst efficiency by comparing exhaust oxygen behaviour after the converter.

P0138 is stored when the ECU sees a downstream oxygen sensor signal that is higher than expected for too long. This may be a sensor issue, wiring short, rich-running condition or catalyst/emissions monitoring fault.

Symptoms

Common symptoms of a P0138 code

P0138 may not always make the car drive badly because bank 1 sensor 2 mainly monitors catalyst performance, but it can still affect emissions monitoring.

Engine management light

A steady engine warning light is the most common symptom.

Emissions warning

Some cars may show emissions or catalyst-related warnings.

Normal driving feel

The car may still drive normally even with the code stored.

Fuel smell

If the high signal is linked to rich running, there may be a stronger fuel smell.

Poor fuel economy

Some vehicles may use substitute values or adapt differently when O2 data is unreliable.

Related catalyst codes

P0138 may appear with P0136, P0137, P0141, P0172 or P0420.

Common causes

What causes P0138?

P0138 can be caused by the downstream sensor, its circuit or a condition making the sensor signal stay high.

Sensor

Faulty downstream oxygen sensor

The sensor may be stuck high, contaminated, slow or internally faulty.

Electrical

Short to voltage

Damaged wiring near the exhaust can short and pull the sensor signal high.

Connector

Poor connector contact

Water ingress, loose pins or corrosion can cause abnormal sensor signal behaviour.

Fuel mixture

Rich-running condition

Too much fuel can affect downstream oxygen readings and catalyst behaviour.

Catalyst

Catalytic converter issue

A weak or contaminated catalyst can create abnormal downstream sensor readings.

Exhaust

Exhaust or sensor contamination

Oil, coolant, fuel contamination or exhaust faults can affect sensor operation.

Safe to drive?

Can you drive with P0138?

Short careful driving may be possible if the car runs normally and there are no serious symptoms. However, P0138 should not be ignored because it can affect emissions monitoring and may be linked to rich running, exhaust faults or catalytic converter issues.

✅ Lower risk: steady engine light only, normal running and no smoke or fuel smell.

⚠️ Medium risk: repeated downstream O2 codes, fuel smell, poor economy or P0420 catalyst code.

🚫 Higher risk: flashing engine light, misfire, black smoke, strong fuel smell or catalyst overheating smell.

Can You Drive With Engine Light? Engine Light Guide
Catalyst warning

High downstream voltage can point to more than the sensor

Because bank 1 sensor 2 sits after the catalytic converter, a high voltage reading should be checked alongside fuel trims, rich-running codes and catalyst performance.

Do not replace the catalytic converter without first checking the downstream sensor, wiring, exhaust leaks and upstream mixture faults.

P0420 Catalyst Code P0172 Rich Code
UK repair costs

Typical UK repair costs for P0138

Costs depend on sensor access, wiring condition, exhaust condition and whether rich-running or catalyst-related faults are also present.

Diagnostic scan and live data

Typical range: £40–£120.

Downstream oxygen sensor replacement

Typical range: £100–£350+.

Wiring or connector repair

Typical range: £60–£300+.

Exhaust leak repair

Typical range: £80–£400+.

Rich-running diagnosis

Typical range: £60–£180+.

Catalyst diagnosis

Typical range: £80–£180+ before any catalytic converter repair.

Repair Costs Guide UK
Diagnosis flow

How to diagnose P0138 properly

✅ Scan all stored, pending and permanent codes.

✅ Record freeze-frame data before clearing anything.

✅ Check whether P0138 appears with P0136, P0137, P0141, P0172 or P0420.

✅ Inspect downstream oxygen sensor wiring near the exhaust.

✅ Check connector condition, corrosion and pin fit.

✅ Read live downstream O2 sensor voltage.

✅ Compare upstream and downstream oxygen sensor behaviour.

✅ Check fuel trims and look for rich-running symptoms.

✅ Inspect for exhaust leaks around the catalytic converter and sensor area.

✅ Confirm sensor or circuit fault before replacing the sensor.

Real experience

What I would check first

On a real P0138 fault, I would first inspect the downstream oxygen sensor wiring and connector. A wiring short near the exhaust can easily cause a high signal fault.

Then I would check live downstream sensor voltage and fuel trims. If the car is genuinely running rich, replacing the downstream sensor may not fix the cause of the high voltage reading.

P0137 Sensor 2 Low Voltage P0141 Sensor 2 Heater P0420 Catalyst
MOT impact

Will P0138 fail an MOT?

P0138 itself is not normally what directly fails an MOT. The concern is what it causes. If it triggers an emissions-related engine warning light, high emissions, catalyst problems, smoke, fuel smell, misfire or poor running, it can become an MOT issue.

⚠️ Higher risk: engine warning light, emissions failure, smoke, misfire, fuel smell or catalyst issue.

⚠️ Medium risk: repeated downstream O2 code, rich-running code, exhaust leak or P0420 catalyst code.

✅ Lower risk: repaired fault, no warning light and normal emissions behaviour.

Engine Light MOT Guide Emissions MOT Guide
Used car buying advice

P0138 on a car you want to buy

If a used car has P0138 stored, check for oxygen sensor history, exhaust leaks, catalyst codes, rich-running codes, fuel smell and poor wiring repairs around the exhaust.

A downstream oxygen sensor can be a manageable repair, but if the code is linked to rich running, catalyst damage or wiring faults, the cost can rise.

Used Car Buying Guides Inspection Checklist
Frequently asked questions

FAQs about P0138

Common questions about P0138 oxygen sensor high voltage bank 1 sensor 2, safe driving, repair costs and MOT impact.

What does code P0138 mean?

P0138 means the ECU has detected high voltage from the oxygen sensor circuit for bank 1 sensor 2.

Can I drive with P0138?

Short careful driving may be possible if the car runs normally, but emissions and catalyst monitoring can be affected.

What is bank 1 sensor 2?

Bank 1 sensor 2 is normally the downstream oxygen sensor fitted after the catalytic converter.

Can P0138 mean rich running?

Yes. A high downstream oxygen sensor signal can sometimes be linked to rich running, fuel smell or catalyst monitoring issues.

Will P0138 fail an MOT?

It can contribute to an MOT issue if it causes an engine warning light, emissions failure, smoke, fuel smell, misfire or catalyst problems.

Should I replace the oxygen sensor first?

Not before checking live data, wiring, connector condition, exhaust leaks, fuel trims, rich-running symptoms and related catalyst codes.

About this guide

Written for practical UK fault finding

Motor Vehicle Expert explains diagnostic trouble codes in clear, mechanic-style language for UK drivers. This P0138 guide is designed to help you understand downstream oxygen sensor high voltage faults, likely causes, safe driving advice, repair costs and MOT implications before replacing parts.

Fault codes should always be treated as a diagnostic starting point. P0138 can involve the downstream oxygen sensor, wiring, connectors, exhaust leaks, rich-running faults, fuel trims and catalytic converter performance, so proper testing is better than guessing.