Resetting the Check Engine Light

Videos is the easy way.

Soounds like it to me.... But to be sure, knowing the make, model & year of the car would be a huge help!

My first question would be, what was the original trouble code that directed the mechanic to replace the sensors ? You seldom need more than one O2 sensor at a time. Its possible the exhaust manifold is leaking and that might cause a problem that would turn on the warning light. Or it could be something else.
A valve cover gasket will leak oil if it fails. No check engine light.

Is the valve cover leakage coming from a crack on cover or valve gasket (between the cover and engine)?
If it is coming between the valve cover and engine, then you will need a new gasket. They range between 20-50 bucks depending on vehicle.
If you can not afford one, for a quick fix, but needs to be corrected as soon as can afford. Your able to make your own quick fix gasket using high heat gasket silicone. It would have to cool off completely before adding silicone making your own bead around the contour of bottom of valve cover then sit for a few hours before starting it.
If you can afford a new gasket; remove the old one. It might take a scraper.
Take some sand paper and clean the where the gasket was really good. Wipe clean then add some high heat gasket silicone. then place the gasket on the silicone. Add a bead of high heat gasket silicone to the engine where the valve cover would go, that helps keep it in place when screwing the screws back on.
If you leakage is coming from a cracked valve cover, i would advise replacing it. I am sure you could find a used cheap one at a local junk yard.

Good luck and hope this helped.

Probably a valve cover gasket leaking . You need to get the code for the light.

You have a bad valve cover gasket. If I remember correctly the spark plug wires go thru the valve cover? If that is true a loose valve cover will leak motor oil on the plug wires possibly causing the check engine/maint light. I would start with the valve cover and a tune up. That may solve all your issues.

You will have to remove the valve cover and replace the gasket and seals on the cover and the spark plug tubes. It is not to bad unless it is the rear cover which can be some work.

White smoke from exhaust indicates burning coolant. Blue smoke from the exhaust indicates burning oil. In replacing the valve cover gasket for an oil leak, some smoke from residual oil is normal but it would be excess oil burning off on the manifold from the engine compartment, not the exhaust. Hope this helps with your question.
Greg

Possible failed bank 1 sensor 1 oxygen sensor..