I need help figuring out a problem. I have a 2004 santa fe 3.5L. For at least a month now when driving at speeds of 25 to 30mph I get a check engine light code P0421. I also noticed I was starting to feel what I thought was a slight misfire but it usually went away. The last couple of days the misfire has been feeling worse. For 2 to 3 days now no code 421 but today a constant bad misfire. I revved it a little and the light came on with code P0303. I cleared the code but the misfire is still there. I have to figure out whats causing the misfire. But I was also wondering, could this misfire have been causing the other code all this time? I was getting 421 pretty much every day with a slight misfire. Now that the misfire has gotten worse for 2 or 3 days now no code 421.

 

Do you have coil over plugs (cop)?
If you do then you can swap out cylinder three with another cylinder to see if the misfire moves
If it does then cop is faulty

 

Thanks I just came back from seeing my mechanic. On the way home the check engine light came on with multiple codes P0300, 303 & 306. The middle coil pack runs both 3 & 6. Also I don't know what my car has 3 or 4 O2 sensors but he determined that both upstream are also bad. Man, I'll tell you I will never buy a Hyundai again. I've had nothing but trouble with this car. Since the 70's I have to say this is the worst car I've had for gas millage. I don't think this car gets any better than maybe 8 to 10 MPG. My ford taurus before this also had a big 6 cylinder in it but the gas millage was better than this. Also I'm trying to find out but no one knows the answer. auto parts stores cant find the answer hyundai dealers & parts dept don't know the answer. supposedly I have I think its called a six pack of sensors in the transmission. I need to replace 1 or 2 of them the lock up & the TCC sensor. The hyundai parts dept could only tell me that the six were all the same part but each had a different function but nobody knows which sensor is which. because I keep getting code P0741 for this.

 

2004 Hyundai Santa Fe 3.5L Replaced the coil pack. Getting code P0421, right side Bank 1. Mechanic hooked up his reader and it told him both upstream o2 sensors were bad. So he replaced them. They were, all white & crusted. Then I started getting code 431, left side Bank 2. Replaced the left & right side downstream O2 sensor. Its running great, I even see a difference in the gas millage. But, the check engine light is still coming on every day. Its usually 431 again, once or twice it was both 421 & 431. What else could be causing this?

 

Cat may have a problem?
Or the O2 sensors are giving you these issues?

 

My mechanic doesn't think its the cats, I have 3. When we started this he replaced the 2 upstream. Then I started getting the codes for the other side and he replaced 1 of the downstream. I had to wait another day or two for the other side to come, it had to be ordered. by not changing the 2 at the same time, can that cause a problem? Mechanic also thinks I may have water in the fuel tank. He thinks this is what is causing the issue.

 

Water in the tank can be taken care of by fuel additives

Good luck
Hope it is an easy and cheap fix

 

you have a code , that means you know where to start looking: Converter Warehouse Explaining Trouble Codes PO420 - PO421- PO430 - PO431

 

Still having trouble! Again my mechanic still doesn't think its a cat. But now I have replaced all 4 O2 sensors & still getting a check engine light every day for the same codes P0421 & 431. After a few days suddenly a new code along with the other 2, P1193 so we replaced the TPS. I do feel a difference with the new TPS but still getting the check engine light 421 & 431. I bought a cheap code reader & everyday was erasing the codes to turn off the light. Is it possible I need to leave it alone and let it cycle through?

 

And why exactly does your mechanic think it is not the cats?
These codes mean: Warm-Up Catalyst Efficiency Below Threshold.

Threshold is 95% efficiency. If they are only 94% efficient they should throw you a code. I know how old your car is but how many miles?

Some people had luck using a lacquer thinner treatment to bring it back up over the threshold. It's temporary at best.

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