Is it Important to Fix Code P0420?

[LanceGillette]

I understand code P0420 could be caused by failed oxygen sensors, an exhaust leak, or failed catalytic converters. I intend to at least replace the front oxygen sensor. I've read that one controls some engine functions and could result in poorer gas mileage. If I replace both sensors and make sure there is no exhaust leak and code P0420 still shows up, is it important to replace the catalytic converters right away or can I wait a year. My next emissions test is in about 18 months.

[Soutthpaw]

I understand code P0420 could be caused by failed oxygen sensors, an exhaust leak, or failed catalytic converters. I intend to at least replace the front oxygen sensor. I've read that one controls some engine functions and could result in poorer gas mileage. If I replace both sensors and make sure there is no exhaust leak and code P0420 still shows up, is it important to replace the catalytic converters right away or can I wait a year. My next emissions test is in about 18 months.

 

If you keep getting the code after doing the sensor, you really need to see what the datastream is showing from the computer, it will tell you if you are running rich or lean.... rich will cause poor mileage, lean will cause the engine to run hot. if the catalytic converter is plugged it will cause back pressure and exhaust gasses will not clear the combustion chamber properly. again causing drivability problems.

[xrturbo]

the car depending how bad it goes into meltdown could basically run power wise like you pulled a couple plug wires

[Snowman]

What exactly does PO420 supposedly stand for?

 

I have been told, and my personal experience with two different cars indicates, that if you're getting a code for "catalytic converter inefficiency" or " #2 oxygen sensor" or anything like that, most likely the second oxygen sensor is worn out, which seems to happen after a while on these cars. It seems that cat problems are pretty rare on EFI soobs, but they do happen. As long as your first oxygen sensor is working fine and the cat isn't plugged, it will NOT affect fuel economy or performance. The second oxygen sensor is only there to make sure the cat is doing its job.

 

I would bet that if you replaced both sensors, the code would go away.

[cookie]

On my car PO420 was just a slow front sensor. It took me awhile to make up my mind what to try so I cleared it a couple times and it would go away for months.

Did not seem to make any driveability or fuel issue on my car, just that glaring light. I would not have gotten through and emissions test here with it. Keep in mind mine was a small issue compared to a plugged or broken cat.

[OB99W]

P0420 is the code for "Catalyst system efficiency below threshold ". The ECU determines that by monitoring the voltage output of the second (rear) O2 sensor. The output of the front sensor is supposed to oscillate (once the engine is warm enough for the system to be running closed-loop). If the catalyst is working sufficiently well, the rear sensor output should be pretty constant; if it's not, the ECU flags that as catalyst inefficiency, and sets the P0420 code.

 

Unfortunately, the problem can have a few causes. It could be due to a bad O2 sensor (often the front one, but the rear ones do fail as well), or sometimes the cat itself. A cat doesn't have to be in a state of meltdown to be defective; it can be poisoned by various things in the exhaust, most of which shouldn't be there if the engine is running well, has no internal leaks, no one has used inappropriate "cleaners", etc.

[cookie]

or an exhaust leak if we failed to mention that. 99 pretty much has it. The only one that would really scare me is a clogged cat and that is easy to determine. A bad ox sensor might cause cat problems also if you let it go too long if it caused a rich mixture.

[LanceGillette]

I found a website that says one can test a sensor like this:

 

"Next step is to measure the voltage at the oxygen sensor on Bank 1 (the rear sensor, or the sensor after the converter). The voltage should be low and steady (~200mV, depends on the vehicle). If the voltage varies (e.g. 100mV to 600mV) then it's time to replace the catalytic converter. In fact, it would be a good idea to test each oxygen O2 sensor while you're at it."

 

I'm looking on comments on this test. I've read that if the cat. converter is bad, that one should put in new sensors with the converter. So I'm tempted to start with a rear sensor replacement as the prudent approach.

[xrturbo]

it actually is fairly common for efi subarus to have cat failures, and especially on newer phase 2 vehicles, i have done quite a few at work in the last couple months.

[nipper]

Actually i cant recall a cat failure on this board in ages. Most cat failures when investigated are o2 sensor failures.

 

Not replacing the senosr can cause

poor performance

poor gas mileage

too rich a condition resulting in a fried catalytic converter

emission failure

 

 

nipper

[OB99W]

or an exhaust leak if we failed to mention that. 99 pretty much has it. The only one that would really scare me is a clogged cat and that is easy to determine. [...]

True, an exhaust leak is another possibility. A clogged cat causes what any exhaust restriction would -- poor engine performance (limited power, especially at higher RPM, and reduced gas mileage); a vacuum gauge can be useful in making the diagnosis.

 

 

I found a website that says one can test a sensor like this:

 

"Next step is to measure the voltage at the oxygen sensor on Bank 1 (the rear sensor, or the sensor after the converter). The voltage should be low and steady (~200mV, depends on the vehicle). If the voltage varies (e.g. 100mV to 600mV) then it's time to replace the catalytic converter. In fact, it would be a good idea to test each oxygen O2 sensor while you're at it."

 

I'm looking on comments on this test. I've read that if the cat. converter is bad, that one should put in new sensors with the converter. So I'm tempted to start with a rear sensor replacement as the prudent approach.

As I said before, if rear sensor output varies much, that's what sets the P0420 code. So measuring that sensor's output voltage can verify that the ECU is accurately reporting what the rear sensor is doing, but not necessarily determine whether the cat is the cause.

 

Cookie mentioned that P0420 was the result when his front sensor became "slow". That's because its poor response allows more-sustained swings in mixture to occur, resulting in exhaust variations which the cat can't sufficiently cope with, and some of the variation will therefore be seen at the rear sensor.

 

If you're going to measure O2 sensor response, I'd start with the front one. However, be aware that a voltmeter probably won't give easily-interpreted results, since a properly functioning front sensor's output will be constantly cycling at a rate of about twice a second (when installed; they can switch in less than a tenth of a second on a test bench). Therefore an oscilloscope or "scopemeter" is the better tool for the job.

 

If you have access to the equipment, then doing some further troubleshooting makes sense. However, if you have to pay someone to run the tests, replacing sensors might actually be less expensive.

[cookie]

That's a good point 99. A place here offered to put it on a test machine for $120. I decided since the ox sensors were old anyway I'd try the front. $60 and fixed. Definately more cost effective.

[xrturbo]

in the situations i dealt with it deffinately was not an issue of mis-diagnosis. the ones we pull you can put a coat hanger half way or all the way through the body of one of the cats before you hit anything.

[OB99W]

in the situations i dealt with it deffinately was not an issue of mis-diagnosis. the ones we pull you can put a coat hanger half way or all the way through the body of one of the cats before you hit anything.

I'm not suggesting that cats never fail; obviously, they sometimes do, but the more-modern honeycomb type are less prone to than the older pelletized kind. When they do go, the honeycombs tend to develop holes, rather than plugs like the pellets.

 

This is where I expect Nipper to suggest that the cat be banged on (not too hard !) to see if it rattles. Honeycomb-type shouldn't (the pellet-bed ones should somewhat, or it usually indicates the pellets have fused together).

[ron917]

The honeycombs can break up, big chunks can then turn crosswise and block the flow. That's exactly what happened on one of my Volvos. Jamming a steel bar through the cat and dumping out the pieces solved my drivability problem (wouldn't pass emissions, of course).

[LanceGillette]

Replacing the front oxygen sensor did the trick. I replaced the rear one for good measure since the car has 133,000 miles on it. Hopefully they last for the rest of the cars life. Thanks all for the advice.