You do not need to go under car to change oil you can buy special pump which can suck old oil thru dipstic pipe ( cost about $20) some guys sell it for 100 ( check harbor fright )
When you remove co2 sensor you do not need to drill holes and then you can use that hile to check back pressure pressure after catalic converter is not likely to be high so you do not need to check it also you can clean catalic converter with chemicals you can buy it at any major auto store
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