Mazda Demio DY3W E/G Check Lamp Lit

2005 Mazda Demio DY3W Engine Model ZJ-VE

Situation
Engine Check Lamp illuminted.
The car run smoothly.

Cause
Faulty O2 sensor

Maintenance
Diagnosis noticed cord P0031that meant faulty of O2 sensor system.
Then I changed new O2 sensor.