KlaroFault codeP0166
MODERATEO2 Sensor Circuit No Activity Detected (Bank 2 Sensor 3)
What this code means
O2 Sensor Circuit No Activity Detected (Bank 2 Sensor 3).
“Bank 1” is the side of the engine that holds cylinder number 1; “Bank 2” only exists on V and flat engines. An in-line engine has a single bank.
“Sensor 1” is the oxygen sensor upstream of the catalytic converter, the one that regulates the mixture; “Sensor 2” sits downstream and monitors the converter.
P0166 is a generic code: it means the same thing on every make. P0166 belongs to family P01xx — Fuel and air metering. Its page explains how to read a code and groups all its codes by part.
Oxygen sensor (lambda sensor)
The oxygen sensor is screwed into the exhaust and measures how much oxygen is left after combustion. The upstream sensor (sensor 1) lets the control unit keep the mixture right on the ideal ratio, correcting it many times a second; the downstream sensor (sensor 2), after the catalytic converter, checks that the converter is doing its job.
What you notice
An upstream sensor that fails often raises fuel consumption and can make the idle uneven; a downstream sensor fault is usually felt not at all. In both cases the car may fail the emissions test.
How to check it
Inspect the sensor's connector and wiring, which run close to the hot exhaust and often melt or corrode. An exhaust leak upstream of the sensor lets in outside air and fools it. A sensor coated in soot, oil or white deposits points to a combustion problem to solve first, or the new sensor will die the same way.
Watch it live in Klaro
With an OBD2 adapter plugged in, these live readings show the part at work, when your car reports them:
- O2 sensor voltage, bank 1 sensor 1
- O2 sensor voltage, bank 1 sensor 2
- Short term fuel trim bank 1
- Long term fuel trim bank 1
- Commanded equivalence ratio (lambda)
What to do
Have it checked this week. The car remains usable, but the fault will not clear itself and will fail the roadworthiness test.
Clearing a code switches the light off but repairs nothing: if the fault remains, the code comes back. Clearing a fault code yourself →
How to read this code
- PPowertrain
- 0standardised code
- 1Fuel and air metering
- 66P0166
Other codes for the same part
- P0152O2 Sensor Circuit High Voltage (Bank 2 Sensor 1)
- P0153O2 Sensor Circuit Slow Response (Bank 2 Sensor 1)
- P0154O2 Sensor Circuit No Activity Detected (Bank 2 Sensor 1)
- P0156O2 Sensor Circuit Malfunction (Bank 2 Sensor 2)
- P0157O2 Sensor Circuit Low Voltage (Bank 2 Sensor 2)
- P0158O2 Sensor Circuit High Voltage (Bank 2 Sensor 2)
- P0159O2 Sensor Circuit Slow Response (Bank 2 Sensor 2)
- P0160O2 Sensor Circuit No Activity Detected (Bank 2 Sensor 2)
- P0162O2 Sensor Circuit Malfunction (Bank 2 Sensor 3)
- P0163O2 Sensor Circuit Low Voltage (Bank 2 Sensor 3)
- P0164O2 Sensor Circuit High Voltage (Bank 2 Sensor 3)
- P0165O2 Sensor Circuit Slow Response (Bank 2 Sensor 3)
