I have a 98 Honda Accord with 177k on engine. Have no history on this car only owned it a short time. Check Engine light on. Checked with OBDII and it reads P0420 Catalyst System Efficiency below threshold (bank 1). I am fairly handy with cars but don't know what this code means. I believe it is Catalytic Converter related but I don't know what needs to be replaced or repaired. Any help would be appreciated. Thank you. Dom12

. Check the Engine Coolant Temperature (ECT) Sensor, not the ECT Gauge or the ECT Switch. It may not be working properly.

. Check the exhaust manifold and converter for leaks.

. Check the operation of both the upstream and downstream oxygen sensors. A scan tool, connected to the under dash data link connector, should show the upstream sensor's waveform fluctuating, while the downstream sensor's waveform is steadier. The readings should not be similar.

Thanks txGreasemonkey will look at these possibilities, I do get exhaust fumes inside car while car is sitting, idling warming up

On an old Toyota I once owned, there was a port on top of the catalytic converter. The plug fell out.

P0420 is Catalytic Converter efficiency below threshold.

In other words, you're going to need to have your Catalytic Converter replaced. From experience, I would highly recommend that you replace it with a genuine Honda converter. Suggested retail price for parts alone will be close to a thousand. Find a dealer that is willing to make you a deal. If you are suffering from sticker shock, you can roll the dice with an aftermarket. I have seen plenty of aftermarket converters turn the light back on within a short amount of time. After which you'll get to argue with the manufacturer's engineer about how much more substrate their converters have compared to OE, and blah blah blah. Eventually you will learn that only a Honda genuine converter will do.