I owned a 2005 4Runner with the 4.7 L V-8. At about 35k I started getting a "tinging" coming from the exhaust. Toyota service department said the baffles inside the catalytic converters were broken loose. They said on each side of the engine both converters were a one piece unit with the exhaust manifold. Both sides were replaced. It was covered under my extended Toyota warranty. They said they cost $3500 a pair...

Then about 4 years later (2012) they let loose again with about 50k miles on the vehicle. Again the dealer replaced them under warranty. The bumper to bumper / $0 deductible was a Toyota one. The 4Runner was used to tow my 5500 lb RV trailer. My thinking was when going down hills I dropped the tyranny into a lower gear to use the engine to help slow the trailer. Nothing crazy, but I'm thinking that this back pressure caused the converters to heat up.

I did have a number of "other" non-engine warranty issues... (heater fan switch, steering U joint, Alternator, OX sensor, AC compressor, tranny wiring issue...) Thank God for the extended warranty...

Got check engine light and VSC light on. Would not pass inspection. Catalyst system below threshold bank 1. Toyota wanted over $1500 to replace the ones on the side with the problem. Bank 1 is same side as cylinder 1, in this case the passenger side. There are 4 catalytic converters on this vehicle and 4 oxygen sensors. 2 oxygen sensors on each side of the vehicle. 1 sensor is located before the first catalytic converter and 1 after. There are no other sensors past the second catalytic converter. Local shop had converters with 25000 mile warranty. I purchased from Autozone because of a 5 year warranty. I only purchased a replacement for the first catalytic converter because it is the only one that is monitored by sensors. The computer is looking for a difference between the first oxygen sensor before the first catalytic converter and the second oxygen sensor after. It is moderately difficult to change but the new converter came with new exhaust connection bolts which is good because the old ones break off. It also comes with a new gasket for each end. Too soon to fail and too many $500 catalytic converters and too many $300 oxygen sensors.