Catalyst Efficiency Below Threshold

[NCexplorer05]

1996 Explorer V-6 139,000 miles. Over the past 3 months, the "Check Engine" light has come on several times. Use my OBD-II reader and get a Powertrain 0420 - "Catalyst System Efficiency Below Threshold (Bank 1)" reading. It is the only code in the computer. I've cleared the code, everything seems fine for 3-4 weeks and then the Check Engine light comes on and its the same code. It has happened 4 times now.

Engine performance seems normal. Gas mileage is normal. There is a very rare rough idle.

Do I need a new catalytic converter? How can my converter be checked? Are there other sensors that should be checked/replaced first? Thanks for any help/insight you can give me.

 

[410Fortune]

this is a driveability issue, it could be the gas you are using, it could be your cat is getting old, it could mean the cataylst monitor 02 sensor is going out......
It could be alot of things, but unless you feel like throwing money at it now, just continue to clear the code and drive.

basically you have an 02 sensor before the cat and after the cat, the computer has a set of limits its looking for, telling it how much 02 is in the exhaust before and after the cat, for one reason or another its going beyond the set limits, long enouhg to throw a code.

The 96 trucks had some PCM issues, you might see if Ford has a update for your computer.

 

[Glacier991]

the post cat 02 sensor (which is responsible for this code) expects NOT to see any variation in the gases going by it, essentially outputting a flatline voltage - meaning the CAT is doing its job. The upstream pre-cat sensor will fluctuate many times a minute as it does its job. Your computer will periodically run a check and intentionally vary the mixture and looks to see if the post cat O2 sensor remains flatline, or whether it fluctuates too. If it does, you get your code.

I'm curious, do you have a scanner or a code reader. I'd want to know what your fuel trims are.

 

[NCexplorer05]

I just have a code reader. I may be able to go to the local college where I used to take some automotive classes. If I can use their scanner, what am I looking for in terms of fuel trims? Thanks.

 

[Glacier991]

I'd be interested in both Short term (ST) and long term (LT) I am going to hazard a guess based on a theory I have. LT FT 16-18%.

 

[NCexplorer05]

I'll try to find out. It will be next week before I can even see if I can use the scanner.

 

[Glacier991]

No prob. Just don't go and replace the sensor. This is not a driveability issue. Go ahead and drive it. Also, watch the output voltage on that sensor.. watch for fluctuations..compare to upstream output.

 

[410Fortune]

Glacier what do you mean this is not a driveability issue?
Its an intermittant CEL correct?
Obviously you know more about this stuff then I do, I jsut wanna learn
Driveability issue I guess would mean somethng that effects the performance, and this is not, just throwing a code but not really effecting performance?

Do you know anything about the 96 PCM's throwing flase codes? I have heard this from a few sources, that Ford had an update for those particular computers due to false 02 codes....

 

[Glacier991]

A potentially dead CAT will not affect vehicle driveability, it will affect emissions, but from an operational standpoint, the driver won't notice any difference in vehicle operation. IF his rear O2 sensor is in fact fluctuating with changes in AF ratio, then his cat is probably shot and the sensor is telling him the truth - like I said this is why I'd be curious to know some of the things a scanner can show, especially the rear O2 voltage output. In the meanwhile, with that trouble code, no harm in continuing to drive it.

I recall hearing something about late 90's Explorers and a reflashing of the PCM, but have no specific recollection. Eneurb might know.