Please help with exhaust

[malibuguy]

Doesnt matter what motor it is...changing the muffler can make a large difference. Its all up to the original design. On a yaris with stock midpjpe...there aint much to be gained. But when you do the whole system...they pick up ALOT. My car in particular makes 107/110 at the wheels with a header, full custom exhaust, tooter spacer and a SRI.

No all mufflers have fiberglass nor restrictive. There is so many different designs out there. Some use rockwool. Some use ceramic filler. Some are baffled, some have multiple tubes.

Late model toyotas are especially sensitive to throwing a p0420. Your secondary cat and resonator (that was deleted) have no influence on that code. Also the downstream o2 sensor should fluctuate...thats perfectly normal operation.

I would check you donut connection at the header/manifold. Make sure the gasket/donut is in good shape...if not replace with a genuine toyota part. Most aftermarket ones are too soft and wear out fast...or not fit right. Make sure the spring bolts are in place which allow for engine movement.

Next is the primary cat original? It could be worn out if you got high mileage. If it was replaced...it would need to be replaced with a heavy loaded converter. Most standard converters do not do as much as stock and will throw a code. Or you can space out the rear o2. I run a 100cell metal core race unit on mine with a spacer.