ODB-II Code P0420 on 2004

Mach5

Mach5

2004 Merlot
Bronze Member
Last seen
Joined
May 17, 2018
Thunderbird Year
2004
Her check engine light lit yesterday. My code reader shows P0420 (and no other codes at this time). She started idling just a little rough and shifting a bit harsher (spring allergies?). Otherwise she seems to run perfectly smooth on our daily commute. I tried searching our Forums but couldn't find any reference to this particular code. I found the info below on the web (repairpal.com).

Has anyone else experienced (and hopefully resolved) this?
(Replacing the COPs sounds much better than replacing the Cats!)

P0420 Symptoms
  • Check Engine Light will illuminate
  • In most cases, there are no adverse conditions noticed by the driver
  • In some cases, there may be some performance problems noticed by the driver such as a lack of power from a restricted and/or damaged Catalytic Converter
Common Problems That Trigger the P0420 Code
  • Inefficient Catalytic Converter(s)
  • Defective Front or Rear Oxygen Sensor(s)
  • Misfiring engines
 

This site contains affiliate links for which I may be compensated. As an eBay Partner, and Amazon Associate I may be compensated if you make a purchase at no cost to you.

That code would lead me to a cat issue or a O2 sensor issue. A COP issue would cause a cylinder misfire in my opinion
 
Mach5 said:
Her check engine light lit yesterday. My code reader shows P0420 (and no other codes at this time). She started idling just a little rough and shifting a bit harsher (spring allergies?). Otherwise she seems to run perfectly smooth on our daily commute. I tried searching our Forums but couldn't find any reference to this particular code. I found the info below on the web (repairpal.com).

Has anyone else experienced (and hopefully resolved) this?
(Replacing the COPs sounds much better than replacing the Cats!)

P0420 Symptoms
  • Check Engine Light will illuminate
  • In most cases, there are no adverse conditions noticed by the driver
  • In some cases, there may be some performance problems noticed by the driver such as a lack of power from a restricted and/or damaged Catalytic Converter
Common Problems That Trigger the P0420 Code
  • Inefficient Catalytic Converter(s)
  • Defective Front or Rear Oxygen Sensor(s)
  • Misfiring engines
Click to expand...
Mach5 said:
Her check engine light lit yesterday. My code reader shows P0420 (and no other codes at this time). She started idling just a little rough and shifting a bit harsher (spring allergies?). Otherwise she seems to run perfectly smooth on our daily commute. I tried searching our Forums but couldn't find any reference to this particular code. I found the info below on the web (repairpal.com).

Has anyone else experienced (and hopefully resolved) this?
(Replacing the COPs sounds much better than replacing the Cats!)

P0420 Symptoms
  • Check Engine Light will illuminate
  • In most cases, there are no adverse conditions noticed by the driver
  • In some cases, there may be some performance problems noticed by the driver such as a lack of power from a restricted and/or damaged Catalytic Converter
Common Problems That Trigger the P0420 Code
  • Inefficient Catalytic Converter(s)
  • Defective Front or Rear Oxygen Sensor(s)
  • Misfiring engines
Click to expand...
I had the same thing happen to me 6 years ago when I got my 2004. It turned out to be the O2 sensors front and rear and the COP's. As it turned out the COP's were never replaced by the original owner when the tech bulletin was released.
I would go with that first. If you are handy you can do them yourself, its not hard to do. I'd replace the spar plugs as well while you are at it. I would start there. Good Luck.
 
If I recall correctly from my past experiences working in MB dealerships p0400 codes are egr related issues, which o2 sensors would fall in with. Misfire codes would be p0300 as a random misfire and p03xx (01, 02,etc) as a misfire at that individual cylinder. OBDII is good to give you a direction to diagnose the problem so if it was myself I would get to the root of what caused the fault you have first.
 
fdf3280 said:
I had the same thing happen to me 6 years ago when I got my 2004. It turned out to be the O2 sensors front and rear and the COP's. As it turned out the COP's were never replaced by the original owner when the tech bulletin was released.
I would go with that first. If you are handy you can do them yourself, its not hard to do. I'd replace the spar plugs as well while you are at it. I would start there. Good Luck.
Click to expand...
Make sure there's NO oil in the plug wells before you pull the plugs...I found an old window cleaner bottle works great...take the sprayer off put the tube down into the well then pump out MOST of the oil...the rest you you can sop up with paper towels and some sort of prod...make sure you have it dry so none of the oil gets into your cylinders. Threebird
 
Update (long over due). After researching numerous sources on the Interweb and talking to my mechanic I tried some easy, low cost steps to solve my problem as follows: replaced air filter (was due anyway); cleaned MAI sensors; and, ran through two cans/tanks of Seafoam. And, low and behold, code was cleared and has not returned! She's running smooth and breathing easy.
(FYI - I had replaced my COPS and plugs earlier this year.)
 
You must log in or register to reply here.
Back
Top