Can't figure this out..............

[Erich Zeppelin]

I keep getting a P0420 on my 1999 Expedition, 5.4L. I changed out all 4 O2 sensors, it came back after a few days. Took her to the shop for a full scan, thought maybe a stuck open fuel injector, but the scan showed everything functioning perfectly except the downstream passenger side O2 sensor, so I replaced it again. The one I pulled out was covered in black soot, dry, not wet. The SES came on again, same code, now what?

 

[stamp11127]

P0420 Error Code: Catalyst System Efficiency Below Threshold (Bank 1)

 

[Erich Zeppelin]

Yeah, I know what it is, how do I fix it?

 

[stamp11127]

The code is telling you the cat isn't working according to the reading from the sensors. I would verify the sensor readings prior to changing the cat.

 

[Erich Zeppelin]

I know what the code means, what I don't know is why two new rear passenger side O2 sensors are coated in black soot. The soot causes the sensor to fail, giving the P0420 code.

 

[stamp11127]

http://www.autotap.com/techlibrary/catalytic_converters_and_o2_sensors.asp
What is the condition of the front sensor, coated also?

 

[Erich Zeppelin]

Lol! There is no way I'm going to pull that upstream sensor. I had to jack the Expedition up, remove the tire, pull out the wheel well liner, buy a crows foot O2 sensor tool and an 18" extension. Serious PITA........the SES light has been lighting up about four days after I replace the O2 sensor, around 30 miles, yesterday, I drove 150 miles round trip to pick up a WAAG brush guard and the SES never came on...........wonder if short trips are having to do with it.

 

[stamp11127]

From the searching I did I ran across one thread where the guy had a massive carbon buildup in the cat. He ended up shooting the intake with a few cans of MAF cleaner @3500 rpm then "drove it like he stole it". He did say a large amount of carbon did blow out during the process. If you decide to give that a shot I would also check the spark plugs on that bank for carbon buildup.

 

[FordandPolaris]

Sounds like you have an exhaust leak somewhere. I always see black soot near exhaust leaks before the cats.

 

[JUST4FUN]

my 97 taraus gets that same code. I fix it by warming it up and hold it in 1st gear then drive it down the street at the rev limiter for about a mile. its a 17 year old car with 121000 miles. I get it to pass the e-test then after a while it goes back on. im not spending anymore money on this car, The kids drive it