I ask is it really necessary to replace a catalytic converter (at ~ $1200 and up) after the check engine came on, and one of two mechanics said a bypass valve was plugging up, and the solution is to replace the “second” catalytic converter.
This car, a Prius, has twice failed a State inspection of auto emissions. The diagnostic codes were (at first) P 171 and P1436; but now only P1436.
The first mechanic cleaned the “mass air flow sensor” and cleared the check-engine light. However that light came on again during the second inspection.
Is there a simple alternative to replacing the catalytic converter?
If it must be replaced, can I obtain one in good condition from a junkyard or some other market?