Originally Posted by Svt5.0
What was the code? There is no code for a clogged cat.
If DTC code turns out to be the one shown below, it may simply indicate the cat is no longer doing its job of cleaning the exhaust gases; it doesn't necessarily mean the cat is clogged or physically damaged. In that case, it won't affect how the car runs but you won't pass an emissions test with it. Investigate and eliminate all other possible causes before replacing a catalyst as the part can be expensive.
P0420 - Catalyst System Efficiency Below Threshold (Bank 1)
Indicates Bank 1 catalyst system efficiency is below the acceptable threshold
Use of leaded fuel
High fuel pressure
Damaged exhaust manifold
Damaged catalytic converter
Downstream HO2S wires improperly connected
Damaged exhaust system pipe
Damaged muffler/tailpipe assembly
Retarded spark timing
Compare HO2S upstream and downstream switch rate and amplitude. Under normal closed loop fuel conditions, high efficiency catalysts have oxygen storage which makes the switching frequency of the downstream HO2S very slow and reduces the amplitude of those switches as compared to the upstream HO2S. As catalyst efficiency deteriorates, its ability to store oxygen declines and the downstream HO2S signal begins to switch more rapidly with increase amplitude, approaching the switching rate and amplitude of the upstream HO2S. Once beyond an acceptable limit the DTC is set.