I had the converter replaced and mech turned off light. Drove 20 miles and the light returned. Replaced the oxygen sensor mech turned off light and the light returned. What would you suggest? Someone said that the coolant sensor may be the problem, but the car is not over heating. The car is a 1999 Honda Accord Ex v-6

. Replace the other oxygen sensor (Denso only), which is likely the downstream heated oxygen sensor. The upstream and downstream sensors need to record varying waveforms to not throw a P0420.

. Replace the ECT Sensor, which has a two-wire connector going to it. The ECT Sending Gauge Unit has only a one-wire connector. They are located next to one another. ECT Sensors get corroded by not changing the coolant frequently enough and by not using distilled water.

The Engine Coolant Temperature (ECT) Sensor affects fuel injector pulse width. It's a temperature dependent resistor (thermistor), whose resistance decreases as the engine coolant temperature increases. It provides a richer mixture when cold, leaner mixture when hot. They become corroded, due to not changing the coolant frequently enough, flood the engine, and waste a tremendous amount of fuel.

These actions will likely solve the problem.