i have a 4 banger not a truck or a ford for that matter but most engine systems are very close to being the same so i pulled a p0421 code off of my car and its is very intermittent the code only comes on when the idle gets rough and then goes away for a few hundred miles the idle gets rough like the egr is stickin. so my question is can a stickin egr cause a p0421 engine code

 

P0421= Bank 1 Warm Up Catalyst Below Efficiency.

Check wiring and 02 sensors. Reset PCM to see if the code comes back. Test the 02s FIRST brfore replacing cats.

What kind of vehicle?

 

Don't just test the O2s, check the fuel trim. The trim is shown as a percentage. It should be less than 5%, ideally less than 2%. If the fuel trims are not near zero, there is a fuel management problem that much be resolved before replacing the cat. It is possible that the cat may begin working again if the condition is corrected. Fix the cause first.

 

its actually a hyundai i bought second hand i replaced o2 sensors both pre and post cat cause it had some miles and the wires looked bad on the post cat o2 i am led to believe that it is something other than the cat though due to the fact that it is intermittent how it comes and goes

 

What year Hyundai?

 

its an 03 2.4/auto with cali emissions

 

i gave it a seafoam treatment a few days ago i know its not a fix but more of a maintenance item and i reset the computer and WOW it made a huge diff oh and i drove it 300 miles and no MIL/CEL came on. i feel as though perhaps i should use it more as a maintenance item every other oil change or every other other oil change

 

Good idea. And my notes don't show a problem with that year of Hyundai. The only ones I see problems with are the Accent and the Elantra with the 1.5L, 1.6L or 2.0L motors from '99'-02

 

does it have less than 80,000 miles?

 

lol no it just went over the light actually came just 63 miles over 80k it is workin great though right now its now at 84k