The first one is blocked by carbon particles. At this time, the signal voltage of the oxygen sensor will be very high, so the engine computer will issue instructions to reduce the fuel injection and make the mixture too lean.
The second reason is that dust and oil block the through hole between the oxygen sensor and the atmosphere. At this time, the signal voltage of the oxygen sensor will be low, and the engine voltage will instruct the fuel injector to inject more fuel, resulting in a rich mixture.
Oxygen sensor is an important part of electronically controlled fuel injection engine. Once it fails, it will not only make the emissions exceed the standard, but also make the engine work worse.