P0166 code meaning
O2 Sensor Circuit No Activity Detected Bank 2 Sensor 3
Powertrain system Generic code P01xx codes Check engine light
What P0166 means
Sensor 3 on bank 2 is a switching oxygen sensor behind a second converter or converter brick. It is rare – documented for Jaguar Land Rover's supercharged V6 and V8 (2013–2016), where it is the 'post-cat' sensor of the second bank, and for Ford systems that monitor part of the converter volume. P0166 means sensor 3 shows no voltage change or stays in the control module's bias band – an open or high-resistance circuit, or a cold or dead sensor. For downstream sensor 2 GM sets the equivalent code when the voltage does not change after heater warm-up (within 15 s) or sits at 401–548 mV. No manufacturer document using exactly this code was found; Jaguar Land Rover does not list it and Ford uses only heater codes for sensor 3. A failed heater (P0167) can cause it. When it fails, the downstream evaluation of that converter is suspended; there is no limp mode.
P0166 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyEasy, DIY possible
- Typical labor0.3 to 1.5 hours
- Clears after repairAfter a drive cycle
Symptoms of P0166
- Check engine light on (usually after two drive cycles)
- Often together with P0167
- Usually no noticeable change in driving
Common causes of P0166
- Most likelySignal or ground wire open, connector corroded
- Most likelySensor dead
- PossibleHeater failed (sensor stays cold)
- Less likelyEngine control module
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Oxygen sensor bank2 sensor3Oxygen sensor wiringCatalytic converter
What each part of P0166 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 1 | Area: fuel and air metering. |
| 66 | The exact fault in that area. |
What to do next
- Note P0166 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P0166
What does P0166 mean?
P0166 means "O2 Sensor Circuit No Activity Detected Bank 2 Sensor 3". Sensor 3 on bank 2 is a switching oxygen sensor behind a second converter or converter brick. It is a generic code, so it means the same on every make.
Is it safe to drive with P0166?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will P0166 fail an emissions test?
Yes, it can. P0166 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P0166 to fix?
Difficulty: easy. A typical repair takes 0.3 to 1.5 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will P0166 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.