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Thread: P0421 error code

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  1. #1

    P0421 error code

    I have searched and seen post that list this as either a O2 sensor problem or Cat converter problem. I want to go the route of the O2 sensor first. I know it's Bank 1 but which sensor? There is one that is before the cat and one after the cat, but which one does the error point to?

    Also where after the cat is it? someone told me that it's close to the back of the car by the muffler. If anyone gets their car on a lift, can you please get a picture of where the sensor after the cat is?
    2004 Galant LS

  2. #2
    bank one is the oxygen sensor before the cat if it is bank one,bank two is the one after the cat.

  3. #3
    Booty Luva wetamup2k3g's Avatar
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    Brooklyn, Maryland
    Is yours a V6 (I think all 9G LS's are)? If so, the code P0421 refers to the warm-up catalyst for the rear cylinder head, or bank, according to what my Haynes manual says about OBDII codes. P0431 refers to the warm-up catalyst for the front head. So the o2 sensor in question would be the downstream one on the bank by the firewall, just under the rear warm-up cat in the downpipe.


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  4. #4
    Po421 bank 1 cat below threshold. Replace bank 1 cat. 8yrs/80k miles emisson warranty

  5. #5
    ^^ x2 had the same code
    04 GTS 9G

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  6. #6
    TGC Lifetime Patron oakrdrs187's Avatar
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    OT: Pre-Cat costs arm and leg, where's the best place to buy?

  7. #7
    What's the easiest way to get to the downstream O2 sensor? From under car when it's on ramps? I just don't remember seeing the O2 when I looked a while ago.
    2004 Galant LS

  8. #8
    1st o2 sensor is on top of the exhaust manifold, 2nd in under the middle of the car near the secondary cat. i had that code, i ended up getting the o2 sensors from for about 120 and the cat i got from for 425 i think. might be cheaper if you dont need the CA legal one. id change the o2 sensor 1st and if its still on, then get the cat since its more expensive.
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  9. #9
    Replace the cat, mitsu cats always go bad. I should know i work for them. Only them u replace an o2sensor is when it throws a heater circuit. Sometimes if its a slow or no response if no updated reflash is available.

  10. #10
    Sorry for my english. It is necessary to check sensor O2.If it is ok then,remove the cat. It is necessary to deceive ECU. This can be done in two ways. Method 1 (Sorry, inscriptions on the russian language.) Make this part for the o2 sensor. This will give a weak signal from the sensor and deceive ECU. Method 2 Simulating sensor signal by means of an electronic fraud. But this is expensive. I can find the wiring diagram.

  11. #11
    With a graphing scan. Check data list for b1s1 o2s and b1s2 o2s. When the engine is steadily rev at 2500, does the graph of s2 almost mimic s1? If it does its a failing cat convertor.

  12. #12
    I am currently having the same issue bank one CAT, this is referring to firewall side manifold? correct? My 05 v6 galant ls actually died on me the other day, but restarted and I managed to get it home. The dealer wasn't very informative when I asked about which side was bank one CAT. Is this a timely fix? (how long to R&R?)

  13. #13
    Any assistance is appreciated...

    I have taken the route of replacing both upper and down stream oxygen sensors with Bosch and the check engine light came back on with the same error code P0421. At first I felt what a waste of money buying both but then again I have 125,000 miles and time to replace anyway!

    Since the error code is back, would it be ultimately the catalytic converter? If it is would a universal or aftermarket cat converter work and clear the check engine light or it has to be direct bolt on? I know of a place that would do it inexpensively and would rather get universal and weld so long as it clears error code/check engine light!

    Thanks in advance!

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