Exhaust gas detection but the cause of elimination

Tail gas detection but the reason for the detection of 1.CO than low air fuel, check whether the failure of oxygen sensors, and then too much fuel supply, too little air supply. To check the fuel supply system and air supply system is normal. To replace the air filter, replace the oxygen sensor. 2. HC over-standard air-fuel ratio is too high, the same check whether oxygen sensor failure, to check