My check engine light turned on and I took it to the mechanic to get it checked out. He said I needed to replace my rear oxygen sensor. I needed to save up the money for it so I drove back home with the light still on. Two days later, the light shut off on its own. Should I still be worried?
No need to be worried.
When the Check Engine light turns on, it means there’s a problem with an emissions control component.
If the Check Engine light then turns off, it means that problem no longer exists.
Drive the vehicle and see if the Check Light turns back on.
It sounds like whatever problem exists is just on the cusp of the out of spec number for triggering the light. Change in ambient temperature, humidity, type of driving etc could cause that. Intermittent sensor or wiring problem as well. If it remains out, no need to worry, you lucked out. Be sure however to make sure the CEL turns on with the key in ‘on’ but the engine not started. If it doesn’t light up then, the dash bulb may be burned out.
The problem will return at the next failure. Then it shuts off if the problem goes away for a time. Repeat.