need help
 

im new so bare with me.... I have a 2005 mazda 6s 3.0 liter all stock. Had a mis fire then engine was knocking had to replace engine got a used engine with 48000 miles on it installed everything was fine 900 miles later on a friday small tapping checked oil 1 quart low (not leaking) drove out that night next day 1 quart low again more of a knock now. Took it back where i got motor from he said my pre cats were bad i needed new ones. Told me $1k price and he was putting a new engine in it (im guessing something he should have caught) i told him order the pre cats. He calls me today @ 445 pm tells me he has my motor out and new one on the shop floor but im gonna need all 4 cats now so its $2k. Im kinda stuck and cant get a second opinion cause car has no engine in it. How likely is it for all 4 to be bad or is he throwing parts at it at my cost?

Engines that use oil screw up cats! The oil clogs up the cat, then bits that break from the cat are recirculated back in to engine via the EGR system and screw up your engine (Mazda Zoom Zoom Boom). Get your EGR cleaned as well!
For cats read and use the search: https://www.mazdaforum.com/forum/maz...3-0-ltr-30510/
They may be cheaper than Mazda. The front pre-cats on a 3.0 liter are actually part of the exhaust manifold and are expensive. You have to buy it as an assembly.

http://images.magnaflow.com/02product/parts/50575.jpg

but do you think all 4 would be bad at the same time?

When bits get broke in front cats they wreck the back ones, so yes!
Rear ones are cheap, front ones are expensive.

ok thanks for the info i feel a little better shelling out $2000 now. Went ahead and told him to order the parts hes putting in new engine and all 4 cats.. Should get car back back 17th or 18th will let you know how it went..