Is it safe to drive with this code?

In many cases, you can drive for a short period, though fuel economy may worsen and emissions will likely increase. Because this sensor typically monitors catalytic converter efficiency rather than directly controlling fuelling, immediate engine damage is uncommon—but the underlying issue should be addressed promptly. Refer to our full disclaimer for MOT and safety guidance.

It may affect your MOT test if the malfunction indicator lamp is illuminated at the time of the test. The emissions system must be functioning correctly, and an active engine warning light can result in a failure. Always check our full disclaimer on MOT implications.

Roughly how much does it cost to fix?

Costs vary widely depending on the root cause. If the sensor itself needs replacing, expect low three-figure costs (parts and labour) at an independent garage; main dealers may charge mid three-figure sums. If the fault lies in wiring or connectors, repair costs might remain in the low two-figure to low three-figure range. Diagnostics typically add an hour of labour.

Can I just clear the code and ignore it?

Clearing the code without addressing the underlying fault will typically result in the code returning once the engine control module re-checks the sensor. Proper diagnosis and repair are needed to prevent the warning light from reappearing and to maintain legal compliance for emissions testing.

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← All fault codes | Electrical & Sensors

P0166

Powertrain

O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 3)

The engine control unit has detected that the third oxygen sensor on Bank 1 (typically the rear sensor after the catalytic converter) is not providing any signal activity.

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ⓘ Information only. This page provides general educational information about fault code P0166. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.

Recommended next steps

Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.

What does P0166 mean?

P0166 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 3).

This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.

About Electrical & Sensors codes

Codes in the Electrical & Sensors category.

P0166 is one of 4,010 codes in this category. Browse all Electrical & Sensors codes →

What we know about P0166 specifically

We record the SAE J2012 standardised definition and the system this code belongs to, but we have not yet published detailed driver-reported symptoms, common causes, or step-by-step diagnostic guidance for P0166.

Rather than invent that information by generating generic content from a template, we leave it out. For specific guidance, the most reliable sources are:

•your vehicle's own service manual or manufacturer-specific technical bulletins
•a qualified mechanic with a manufacturer-level diagnostic tool
•a marque specialist familiar with your make and model

Codes nearby

Codes adjacent to P0166 in the P-code series. Many fault codes appear together with their neighbours because they relate to the same circuit or component.

P0163 O2 Sensor Circuit Low Voltage (Bank 2 Sensor 3)

P0164 O2 Sensor Circuit High Voltage (Bank 2 Sensor 3)

P0165 O2 Sensor Circuit Slow Response (Bank 2 Sensor 3)

P0167 O2 Sensor Heater Circuit Malfunction (Bank 2 Sensor 3)

P0168 Fuel Temperature Too High

P0169 Incorrect Fuel Composition

Other Electrical & Sensors codes

Other fault codes in the same system as P0166. The ones with detailed guides have plain-English explanations, symptoms, causes, and diagnostic steps.

B1257

Air Temperature External Sensor Circuit Short To Ground Detailed guide

B1363

DBPT Circuit Short To Ground Detailed guide

B2100

Ignition Run/Start Input Circuit Performance Detailed guide

C2000

Steering Angle Sensor Not Calibrated Detailed guide

P0030

HO2S Heater Control Circuit (Bank 1 Sensor 1) Detailed guide

P0031

Oxygen (A/F) Sensor Heater Control Circuit Low (Bank 1 Sensor 1) Detailed guide

Spotted an issue or have something to add? Mechanics, technicians, and drivers who have first-hand experience with P0166 can submit a correction or expansion. We review every submission before publishing, mechanic submissions are prioritised.

Information only, not professional advice

The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →

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