Howdy all. I finally got to Autozone and got the computer read to see what the CEL was on for. It puled a code PO420-Catalyst Efficiency below threshold Bank 1. Can anyone tell me what that means? The counter guy thought that it maybe meant one of the O2 sensors was bad but couldn't be for sure that was it, nor which one it was. Anyone tell me for sure?

 

Honestly you should have a technician plug in a MasterTech or Tech2 analyzer into the OBDII port and read the output from each of the O2 sensors. It could very well be the sensor in the driver's side exhaust pipe but the best way to tell is with a proper analyzer. The guys at AutoZone should be able to read output from each O2 sensor on their machine. There are pretty tight limits on voltage output from each, anywhere from 0.4 to 1.2 volts and 0.0 volts from the rear sensor. If the rear sensor is showing volts it means the mixture is bad coming out of the cat, but that can also be caused by a bad O2 sensor prior to the cat telling the PCM to richen the fuel charge or not, screwing up the Cat output. If you have a Chiltons manual it will also mention this about the O2 sensors and give you specs to look for. You can get that software from OBD2.com and use a laptop to do your own analyzing too. It works very well.


Mike
'96 EB 5.8L



Auto Enginuity OBDII software

 

That's right, it's hard to tell if an O2 sensor is bad or the catalytic converter is not doing it's job. I'm having the same problem on a '98 half-ton pick-up and had a shop pressure test the cat. It did find an exhaust leak in the y-pipe, but I still get the code. Speaking from experience, if you change one O2 sensor, go ahead and change all of them at the same time. If you only do one, it throws off another one and it goes out. Since this truck has 4 of them, I'm looking into an aftermarket replacement cat that will work. The factory cat is over $600 and I ain't buying that. You can choose to ignore it, but watch out for a random misfire code caused by this problem.

 

So it is an O2 sensor for sure on this code? I don't want to go through replacing all three of them to find out that it is the cat. Does anyone know which O2 it is referring to? "Bank 1"?

 

Rear o2 is the ONLY one (or ones) that can tell the pcm if everything is working as it should--11 is bank 1 front sensor 12 is bank 1 sensor 2 (rear) 21 is bank 2 sensor 1, 22 is bank 2 sensor 2 (rear),yours only has one rear so it says bank 1 But it is BOTH!!Cat effi. codes are ussaully cured by programing pcm to latest calibration if availible for your 96(OBDII)OR replacement of your cat ASSEMBLY(pipes and two cats Although you said 3????) will be the fix .Most aftermarket cat companies will not tell you that thier cat will fix cat effic. code prob.You might want to get someone to moniter oxy. snsor to make sure they are switching as they should- rear have to be warmed up before they switch-get someone familar with this- I think cure might be expensive- do a lot of homework on this first , Good luck- PS to justshootme84 call aftermarket cat company before throwing away your money or ???? latest calibration is much cheaper if availible. Also this does not cause random misfire codes- plug or coil over plug causes that in 98 WDS mode 6 is the place to look at this.

 

Yeah, it's the O2 sensor after the cat giving this code PO420, but like i said, the sensor can be O.K. but the cat isn't working or the sensor is bad. I have not had the sensor checked yet either, been dealing with multiple coolant leaks. I recently had a tune-up done, with new plugs and wires, etc. I cannot detect a miss in the engine when driving, but the scanner showed about 200 misfires on cyl #8 in the last month. Even a couple of mechanics I've talked to say the same thing about this code, so I don't have a definite answer on what to replace to stop it. Like stated, some aftermarket cats like the universal models, will still give you the cat.eff. code, so that's good advice to check out the stats on a certain one. I've had the PO420 code for about 6 months, and it doesn't seem to adversely affect the gas mileage. But whne I get a new cat and exhaust system installed, I will go ahead and replace the O2 sensors to start new.

 

So if I'm reading this correctly, Bank 1 (this code)is only the rear sensor? Also, my truck only has two cats, not three. But it does have three O2 sensors. Two upstream and one downstream.

 

Does anyone know? I'm driving myself mad trying to figure this out. BTW, is there some sort of website that I can see all of the error codes?

 

12 and 22 are rear oxy sensors(downstream) -call yours 12 or 22 -does not matter-you only have one and it is reading off of flow of bank 1 and bank 2 (first year OBDII) 97 and up ALL have two upstream and two downstream.

 

I guess I'm reading this wrong, or I'm just thick skulled. You're saying that both of my front sensors are bad? Which one specifically is the one causing the fault?

 

--Blue, the code that is being pulled is referring to the post cat o2 sensor only. This sensor monitors cat efficiency and does not have any significant role in being an input to the pcm to adjust for a lean or rich condition.

More than likely, and id put money on this, in your case only that rear o2 is bad. Dont get me wrong, though 99% of the time its the sensor you could have a wiring issue in the power and ground circuit for the 02 heater or the signal circuit to the pcm. Fyi, the autozone scanner, ive seen those guys in the parking lot scanning people cars. Their scanner is a code flasher only. They dont have the ability to read actual o2 voltage and properly diagnose the problem. Hence why they are there to sell parts first before fixing cars. Mind you a very lean or rich condition or even an exaust leak can trigger this code also in your case i would steer away from that idea being more than likely missfire and sensor 1 bank 1 and 2 codes would have set.

If you want to do your own research, www.alldata.com You can subscribe to alldatas version of your service manual for about 20 bux. Its alot cheaper then getting the full subscription on dvd's and fairly complete. The only thing it lacks are all of the tsb's from the factory, all data only lists a few, and their schematics are missleading on occasion, wrong wire color, location, etc.


--shootme, just my opp but if you were driving with a missfire dtc for a long period of time, missfire dtc meaning you had a missfire, your cat could very well be carbon fouled due to unburned fuel or damaged by the raw fuel itself. I think you would be wasting your money replacing all of your o2's just being you have a cat efficiency code. Even if the bank 1 and 2 sensor 1 o2's are carbon fouled from the missfire, typically they are cleaned enough to opperate normally as soon as the missfire is repaired and they get hot enough to burn off deposits. Also wanted to add, not sure of your milage, and technically if you ran with a missfire for that long the cat failure wouldnt really be a manufactures defect, but if your problem truly turns out to be and internal cat failure, remember lots of those were covered under the emissions warranty. 5/50, 10/100, i think the newer cars may even have a 12/120 but not sure. Maybe worth a try, besides if it is within the warranty period, as long as the dealer didnt know you drove it in a failed state for a period of time, my guess is they would just replace it no questions asked.

My 2 c's, hope it helps.

 

Joboo, thanks for the clarification! I was getrting lost in all of the bank discussions, and really didn't follow what was trying to be said. I think that the internet is terrific, but something is always lost when it is written rather then verbal.

I'll be repacing the rear O2 sensor, even though it hasn't set off the CEL in the past few days just to be safe and make my wife feel more at ease when driving it.

Thanks again for all of your help all!

 

Never said ANY sensors were bad-IF they are not switching ,it is very poss. that the one or ones not switching are bad ,you Must moniter them with a NGS , Snap-on or some piece of equip. them equal to them.Front oxy. sensor can be swapped left to right and reread to see if bad one swaps sides.Cat effic. codes are not generated by a BAD rear sensor- a code for oxy sensor not swiching will be generated.The rear sensor tells the PCM how clean(or good) a job it is doing at the tailpipe end(EPA loves low hydrocarbons-read that as fuel) JOBOO cats do not get damaged by too much fuel-FORDs strategy to cool down cats is to dump more fuel- ever seen a cat turn red - always from being too lean!

 

Oh, now I'm back to being confused. Basically I don't have access to such a code reader, and I'll be paying out the hind end for a local shop to investigate. What should I do otherwise?

 

You can live with it,( seen them come in with big piece of tape on intrument glass covering up light- I laugh every time) I know of a couple trucks running around here & been running fine for about 3 years- only drawback is if something else goes bad and code is set, you just assume the light is on for cat code SOOO you have to scan every once in awhile to stay on top of it.ONLY 96s will set a cat effic. code(OBDII) 95 and down EECIV- only one oxy sensor used.Sorry to have confussed you earlier.Wish I could tell you something good like it is in warranty (8 years or 80,000 miles) OR YOU HAVE JUST WON THE LOTTERY-- BUT I can't!!!!