2001 Hyundai Santa Fe OBD II Code P0421 Problem

JORGE10928

MEMBER
2001 HYUNDAI SANTA FE
4 CYL
FWD
AUTOMATIC
111,720 MILES

I've been having trouble with code P0421. I had the catalytic converter replaced and it went away. Two days later the check engine light came back on so I went back to the mechanic and he cleared the code. Then 3 days later the check engine light came back on again with the same P0421 code. The scanner says it has two codes but they are both P0421. I don't know whats going on. Could it be that the oxygen sensors are faulty? Also when I come to a stop at a red light somewhere or anywhere where I stop and hold the brakes. Sometimes the car feels like it jumps. It doesn't move forward but it just feels like some sort of bump. It runs fine but the only problem that comes up is code P0421. Anyone have any suggestions? Thanks.

I also understand that there are two cat. Converters; one on the front of the car and one on the bottom center of the car. The one I had replaced was the bottom center one. Will I need to replace the front cat?

5 Replies

KHLOW2008

MECHANIC
41,814 POSTS

Hi jorge10928,

Thank you for the donation.

I have attached a description and diagnosis procedure and you most probably need to replace the front catalytic converter as well.

DTC P0421: WARM-UP CATALYTIC CONVERTER EFFICIENCY BELOW THRESHOLD

NOTE: ECM/PCM compares front and rear HO2S output signals to determine whether rear HO2S output signal is beginning to match the front HO2S output signal. As catalytic converter efficiency declines, the switching frequency of front HO2S and rear HO2S signals become more and more alike.

Failure Conditions

ECM/PCM monitors catalytic converter efficiency for 140 seconds when the following conditions are met:
Engine speed is 3000 RPM or more.
Closed throttle position switch is off.
ECM/PCM is operating in closed loop.

ECM/PCM determines fault exists when rear HO2S switching frequency becomes like front HO2S switching frequency.

Probable Cause
Catalytic converter deteriorated.
HO2S failed.
ECM/PCM failed.

Test Procedures
1. Check if DTC P0133, P0135, P0136, and/or P0141 is set in addition to DTC P0421. If DTC P0133, P0135, P0136 and/or P0141 is set, repair that DTC first. If DTC P0133, P0135, P0136 and/or P0141 is not set, go to next step.

2. Start engine and allow to idle. Inspect for exhaust leaks upstream and downstream of catalytic converter. Inspect for exhaust leaks around front HO2S and rear HO2S. If no leak is found, go to next step. If leak is found, repair exhaust leak as necessary. When repairs are completed, clear DTC and verify operation.

3. Check rear HO2S operation. If problem does not exist, go to next step. If problem exists, perform repairs or replace rear HO2S as necessary. When repairs are completed, clear DTC and verify operation.

4. Turn ignition off. Replace catalytic converter and clear DTC P0421. Road test vehicle and check for DTCs. If DTC P0421 does not reset, repairs and inspection are complete. If DTC P0421 resets, ensure ECM/PCM connector and terminals are clean and tight. If ECM/PCM connector is okay, replace ECM/PCM. When repairs are completed, clear DTC and verify operation.

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Thursday, December 18th, 2008 AT 12:00 PM

JORGE10928

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4 POSTS

How much will it cost me to replace the front cat. Converter? Any educated guess would be fine. Thanks.

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Sunday, December 21st, 2008 AT 1:23 AM

KHLOW2008

MECHANIC
41,814 POSTS

Hi jorge10928,

After checking through again, I realised I made a mistake regarding the catalytic converter. I had provided you with info for a V6 engine. Yours being a 4 cylinder engine, only has 1 catalytic converter and it is listed as OEM $ 834.41.

When you replaced the the cat converter, did you use generic or OEM part?

You could have a catalaytic converter that is inefficient or the O2 sensors are not functioning as required.

You should do the diagnostic as per procedures above to verify before doing any replacement.

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Sunday, December 21st, 2008 AT 2:53 AM

JORGE10928

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4 POSTS

I went to the dealer I bought the car from and I explained to them what the problem was and I was told that it shouldn't be so expensive to fix. They said maximum I would have to pay is around $400. You're not the first person to mention a price over $800 thought. May I ask where you're getting you're price from please?

Does the cat. Converter HAVE to be an OEM part?

And I was told my car had 2 converters. One under the engine and the other underneath around the area of the driver's and front passenger's seat.

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Sunday, December 21st, 2008 AT 3:28 AM

KHLOW2008

MECHANIC
41,814 POSTS

Hi jorge10928,

The price I got is from my estimator courtesy of but the actual price have to be enquired at the dealer as these prices are sometimes not up to date.

Generic cat converters are applicable but sometimes the do not meet the requirements of the one on the vehicle so it might not be accurate.

Depending on models and market, you might have got 2 cats but the info in my database only shows one. The only way to find out would be to get under the vehicle to have a look. It should be joined from the front of exhaust manifold to the rear exhaust system.