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2003 Hyundai Elantra - Code 0420 and strange light on Dash Board

I have a 2003 Hyundai Elantra. It has 165000 miles on it. A few weeks back I got a check engine light on. Checking the code it turns out it was P0420 Catalyst System Efficiency Below Threshold (Bank 1) code. I checked with a few shops and estimates varied from $500 to $1300 to replace catalytic converter.

However, I am reading on web that it can be catalytic converter, but it can also be O2 sensor or something else as well. What if after spending all the money the problem doesn’t go away. Is it really Cat Converter? Shops were not sure as there can be one of two cat converters that may be the culprit here. How do I make sure that they replace the right part.

I disconnected the battery and the code went away for about a month and came back on. Again disconnected battery and it again went away. What is the down side? Can it pass Emissions Inspection if the Check Engine Code is wiped out by disconnecting battery?

Also a strange thing started happening around the same time and I had never ever noticed this in all 9 years of owning this car. Now, when I put my foot on brake pedal, even when the key is out, the dash board light will illuminate. When I take the foot off, the light goes away. It is strange. What can it be? Could it be connected to the code mentioned above?

I am stumped. Please advise.

Thanks,
Andy

Have you recently changed any of the tail light, brake light, or turn signal bulbs on the vehicle?

They are not connected, the brake light issue is probably due to a ground issue. Check to see if the brake lights are working at night, you might find that with your headlights and tail lights on, when you hit the brakes, all the lights in the rear go out instead of getting brighter.

The P0420 code is a difficult code to troubleshoot. The most common cause is a leak in the exhaust pipe before the rear cat. Second on the list is the rear O2 sensor, and since your light stayed out for a month, that is a good indication that that is the problem here. It can also be caused by ignition problems (bad plug) but usually there will be another code associated with that.

The only definitive test is with a scanner that can monitor both O2 sensors and graph them at the same time, or a dual trace oscilloscope connected to the front and rear O2 sensor wires at the computer.