Vauxhall vectra lambda sensor help!?

[Whytey-3]

Hello new to these forums so do bare with me!

Recently bought a 2006 plate vectra Sri sat nav model,
Engine management light is on the one that looks like and engine (not one with spanner) and apparently referes specifically too emissions.

I think it came on when the previous owner had a new exhaust put on the car, but not a fact. Car runs fine but seems to be lacking power and possibly in idle mode, I manage to get the post cat sensor off and give it a bit of a clean and decided to turn engine on whilst still unplugged and the light went off! But once I put sensor back in the light came back on after firing it up again.

Does the confirim it is the post cat sensor causing the problem if the light came of and on ?

What next step should I take, should I get the codes read via mechanic only £20 at my local garage apparently, or should I just go ahead and replace the sensor with a new or working sensor?

Also my dad seems to think it's possible to plug/seal the hole that is left once the sensor has been removed, thus removing the light being on and the exhaust wouldn't blow due to it being plugged but the sensor would no long be doing its job and just stuck to the side of the car therefore I wouldn't have to pay to fix problem and car would work perfectly fine ?

Thanks!