1999 Honda Accord Check Engine Light

HENRI62

MEMBER
1999 HONDA ACCORD
6 CYL
2WD
AUTOMATIC
165,000 MILES

My check engine light came on and I had it checked by the local auto store and it read a cat 420 which he said was a catalytic converter code. I had the converter changed the light was turned off but I traveled only 20 miles or less and the light came on again. Took it back to check out light and he said it was a cat code again and it showed the forward oxygen sensor. I had the sensor replaced, the light was turned off again and 20miles or more later the light came on again. Took it to get diagnosed and the guy at the auto store said that maybe the code was stuck in the computer and the computer keeps reading the cat code.

I hope you can give me some sort of explanation before I spend anymore money.

1 Reply

RASMATAZ

MECHANIC
75,992 POSTS

Hi henri62, Welcome to 2carpros and TY for the donation

Other causes for the catalytic converter to failed and has to be identified otherwise the code will keep coming back

Causes Of Converter Failures

Fouling, clogging, melt-down and breakage of the ceramic substrate inside a converter are common conditions that can cause problems. Plugging is usually the end result of a melt-down, which occurs because the converter gets too hot. This happens because the engine is dumping unburned fuel into the exhaust. The excess fuel lights off inside the converter and sends temperatures soaring. If it gets hot enough, the ceramic substrate that carries the catalyst melts.
The unburned fuel may be getting into the exhaust because of a bad spark plug or valve, but an overly rich air/fuel mixture is another possibility. In older carbureted engines, a heavy or misadjusted carburetor float may be the underlying cause. But on newer engines with "feedback" carburetion or electronic fuel injection, the engine may not be going into "closed loop" (the normal mode where the computer regulates the air/fuel mixture to minimize emissions).
A bad oxygen sensor or coolant sensor may be giving the computer bogus information. A sluggish or dead O2 sensor will make the computer think the exhaust is running lean, so the computer will try to compensate by making the fuel mixture rich. A coolant sensor that always indicates a cold engine will also keep the system in open loop, which means a steady diet of excess fuel. But it might not be the sensor's fault. A thermostat that's stuck open or is too cold for the application can prevent the engine from reaching its normal operating temperature. So if your converter has failed and needs to be replaced, the engine should be diagnosed for any underlying problems before the new converter is installed.
Another cause of converter clogging and contamination is excessive oil consumption. Worn valve guides or seals can allow oil to be sucked into the engine's combustion chambers. The same goes for worn or damaged rings or cylinders. Oil can form a great deal of carbon, and metals present in the oil can contaminate the catalyst. A compression check or leak-down test will tell you if the rings are leaking, while a fluttering vacuum gauge needle will help you identify worn valve guides