P0420 — Catalyst System Efficiency Below Threshold (Bank 1)

What it means

The control module compares the signals of the upstream and downstream oxygen sensors on bank 1 — the cylinder bank containing cylinder number one. On a healthy catalytic converter the rear sensor voltage stays almost flat because the ceramic stores oxygen; when the rear signal starts copying the front one, the oxygen storage capacity has dropped below the legal threshold. Usually the only symptoms are the check engine light and a failed emissions test, sometimes a sulphur smell or reduced power if the substrate is melting.

Likely causes (most common first)

1. Aged catalytic converter with lost oxygen storage capacity
2. Slow or contaminated downstream oxygen sensor
3. Exhaust leak ahead of or between the sensors (manifold, gaskets, sensor bosses)
4. Rich or lean running from misfires, leaking injectors or a faulty MAF sensor, which overheats and poisons the substrate
5. Engine burning oil or coolant, coating the catalyst with phosphorus or silicone
6. Aftermarket or universal converter with too little precious-metal loading

Can you drive with it?

You can drive, but fix the root cause: a degrading catalyst can melt and choke the exhaust.