2003 Toyota corolla 180,000 miles. The starter went up. I replaced that by removing the intake manifold, replacing starter, replacing manifold gasket, and putting manifold back on.
Car ran just as normal, then P0420 signal appeared. Is it a coincidence that it came on after messing with the intake manifold or could they be related?
Is this a plastic manifold? Could have been cracked.
Many things can cause P0420, most cheaper than the cat converter.
Its kind of a known issue on this car and it is usually caused by the rear O2 sensor, but there are other causes including the intake manifold gasket, but more often the exhaust manifold gasket or doughnut seal on the exhaust pipe. If you replace the O2 sensor, get it from the dealer based on the cars VIN code.