2001 LaSabre check engine light


I have a 2001 Buick LaSabre limited with a check engine light on, the code reads PO420 and says Catalyst system efficiency below threshold (Bank 1) can anyone tell me what exactly that means and how to correct the problem?


This code usually indicates that there’s a problem with the catalytic converter itself. However you should first find out if there’s a reflash for the computer from the manufacturer for this problem. Also, the catalytic converter is warranted for 8 years/80,000 miles. So if you fall into this time/mileage limit and there is a problem with the catalytic converter, the dealer must replace it free of charge.



The engine computer is saying, “From the information I am being given, by the front and rear oxygen sensors, there is too much unburned fuel going out the back of the catalytic converter.” Too much unburned fuel going into the catalytic converter can result in too much coming out. To reduce that, the engine needs to be in good tune. Good tune is the result of the scheduled maintenance being up to date. Some of those items are: air filter, pcv valve, spark plugs and wires, etc. There are other causes for a failed Catalytic converter: burning motor oil, anti-freeze, silicone. If the cause isn’t fixed, a new catalytic converter, and oxygen sensors, will fail, also.


The engine runs like a top, I am maticulous about the maintenance and do it myself every 3000 miles and it does not burn or use any oil during that time. it does have 102K on it though, is there any way to check the oxygen sensors or pcv valve to know if their bad or just start by replacing them?

Thanks for the response


The best way I know of to see how a O2 sensor is performing is to look at the signal with a O-scope. Since the car is running well the problem may be with one of the sensors or, the converter is really bad.


A scan tool (more than just a code reader) can look at the voltage pattern. The patterns of front and rear, properly operating, oxygen sensors will be different. You could look at the voltages, which vary from 0.100 volt to 0.900 volts, with a good digital electrical multimeter (“voltmeter”).
If your car has less than 80,000 miles, it’s covered by emissions warranty.


So it sounds like the general consesis from the reponses I recieved is that it is the converter itself that is bad. would you say that I should start by having that replaced? I got a price from a muffler shop of $214 including tax installed, which isn’t bad considering I could only find one part house that could even order one for me for $206. but I hate to spend that much if dosent fix the problem.


I re-read all the posts. I didn’t see a “general consensus” that the catalytic converter is bad; but, if that’s what you want…


Many problems in the engine could cause excess fuel to be dumped into the cat. converter, and cause it to die. if this is the case, a new cat. convertor will soon die also, unless the underlying problem is fixed.


What would be the first things you would check Bill? Like I said before the car runs like a top, is well maintained and the only problem is the check engine light showing the code PO420 catalyst system below threshold (bank 1). what would some of the most common causes be for excess fuel while not affecting the performance of the vehicle. I really don’t want to fork out the $214 for a new converter if I could start by replacing something else less expensive.


Read the posts above. With the proper tool, you can check the O2 sensors before and after the Cat. converter. If they are reading correctly, than the problem is in the converter. How to test for excess fuel is beyond me… sorry.


You are doing your own work so start by changing the engine candy that gets chewed up the most. Forward oxygen sensor. Run the new one for a week and see if the light stays out. I would not change the catalytic converter until I tried something else. Sometimes guesswork is as good as rocket science. Sometimes it is also cheaper, but not all the time. Be optimistic; you can’t always schedule success, but as long as you have to try something, have faith. Also; you wouldn’t have a heart transplant if you only needed Alka-Seltzer. Something like that.