P0431 and P0421 codes?

Tiny
MATTSTORMS42
  • MEMBER
  • 2008 MITSUBISHI GALANT
  • 2.4L
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 271,018 MILES
Have dtc codes p0431/p0421. Put new "cat" and new 02 sensors. Cleared codes. Car drove fine before and after new parts were put on. Codes returned after a day of new parts were put on. What else could be the problem? What other test can I do to solve the codes?
Saturday, November 8th, 2025 AT 2:25 PM

21 Replies

Tiny
KEN L
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  • 54,137 POSTS
Strange that is has bank 2 on a 4 cylinder since there is only bank 1, hmm anyway I would check the engine cooling system thermostat, does the car have a engine temperature gauge, and if so does it stay on the cold side for a long time? This can mean the engine is not warming up fast enough and it will give you these codes.

P0421 - Warm-Up Catalyst Efficiency Below Threshold (Bank 1)
P0431 - Warm-Up Catalyst Efficiency Below Threshold (Bank 2)

If you find the thermostat stuck open replace it and then clear the codes which should fix the problem. Please upload a short video in your response of the problem, so I can see what's going on :)
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Monday, November 10th, 2025 AT 9:44 AM
Tiny
MATTSTORMS42
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1 hour of driving. Back roads and minimal highway.
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Monday, November 10th, 2025 AT 3:08 PM
Tiny
MATTSTORMS42
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After riding for an hour in morning 29 degrees
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Tuesday, November 11th, 2025 AT 5:01 AM
Tiny
KEN L
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Hmm sorry this website is having an issue so I cant see the images can you just tell me where the needle is, or you can upload a short video those are working.
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Tuesday, November 11th, 2025 AT 9:55 AM
Tiny
MATTSTORMS42
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Picture of scanner readings. Before and after pics of fuel gauge.
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Tuesday, November 11th, 2025 AT 2:39 PM
Tiny
MATTSTORMS42
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Temp took from 4:21pm to 4:43 to reach mid point of temp gauge on dash
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Tuesday, November 11th, 2025 AT 2:40 PM
Tiny
KEN L
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Yep, way to long, it should be at operating temperature with in 5-7 minutes, I would install a new thermostat and then clear the codes. Can you just tell, (type) me the codes?
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Wednesday, November 12th, 2025 AT 11:36 AM
Tiny
MATTSTORMS42
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P0431 and p0421
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Wednesday, November 12th, 2025 AT 4:46 PM
Tiny
KEN L
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Lol sorry, I thought here were more codes then what was listed, after doing more research I found that the Galant 2.4 is notorious for the front cat (manifold cat) going bad. When both P0421 and P0431 show together, that’s classic. So you replaced the front or rear cat?
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Thursday, November 13th, 2025 AT 10:37 AM
Tiny
MATTSTORMS42
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Replaced front cat. I thought it only has 1? I replaced all 4 02sensors and cat
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Thursday, November 13th, 2025 AT 3:42 PM
Tiny
MATTSTORMS42
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All 1 piece
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Thursday, November 13th, 2025 AT 3:46 PM
Tiny
KEN L
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Sorry, the site is still having an issue, so the front cat is made into the exhaust manifold, is that what you replaced? If not this will be the problem.
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Saturday, November 15th, 2025 AT 9:12 AM
Tiny
MATTSTORMS42
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Yes sir
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Saturday, November 15th, 2025 AT 11:58 AM
Tiny
KEN L
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So is this a v6 engine?
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Monday, November 17th, 2025 AT 8:53 AM
Tiny
MATTSTORMS42
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2.4 inline 4
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Monday, November 17th, 2025 AT 2:35 PM
Tiny
MATTSTORMS42
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Picture of catalytic converter
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Monday, November 17th, 2025 AT 2:48 PM
Tiny
KEN L
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It is so strange that you have bank 2 on a four cylinder, some generic scan tools will label downstream sensors as "Bank 2" or assign “Bank 2” codes to rear/secondary oxygen sensors. If so if you had these codes before the cat swap and you still have them I would say the PCM is bad. I would remove it and send it out for testing.
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Tuesday, November 18th, 2025 AT 10:21 AM
Tiny
MATTSTORMS42
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Ok thanks
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Tuesday, November 18th, 2025 AT 12:31 PM
Tiny
KEN L
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Please let me know what happens.
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Wednesday, November 19th, 2025 AT 8:21 AM
Tiny
MATTSTORMS42
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Yessir
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Wednesday, November 19th, 2025 AT 10:38 AM

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