02 Diamante DTC P0401, P0421, and P0431 - Mitsubishi Forum - Mitsubishi Enthusiast Forums

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Old 09-26-2011, 10:20 PM
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Default 02 Diamante DTC P0401, P0421, and P0431

All right, first things first, I am not the owner of this vehicle, but I wanted to get some straight answers about it. This afternoon my pastor's daughter pulls into my shop with the check engine light on in her Diamante. Its a 2002 Diamante ES (I believe) with 199,726 miles on the odo, and is setting the following DTC's: P0401 EGR Insufficient Flow Detected, P0421 Warm-Up Catalyst Below Threshold (Bank 1), and P0431 Warm-Up Catalyst Below Threshold (Bank 2). So I switch my scanner over to live data stream and start looking at the O2 sensor voltages coming from the post-cat sensors. Both of them are switching rich/lean quickly just like the pre-cat sensors, which normally would tell me that I have a problem with either the O2 sensors being bad (which upon visual inspection, they're the original ones), OR we've got a cat loosing its efficiency. I didn't have a chance to do a vacuum check on the engine to actually determine if the engine is loosing vacuum or not, but that's my next test.
Now with all that said, I know that in most situations I would work on solving the issue with the EGR valve first since if it is not opening properly then it will increase the combustion chamber temps, which will increase the amount of NoX the engine makes, which kills the cats (at least in everything else I've ever worked on, that's what happens) and would also cause the O2 sensors to read hinky. So, would my diagnosis and fix on this one be the same as "everything else"?
Any help on this one would be greatly appreciated. Like I said, its my pastor's daughter, and while I'm normally pretty good at diag-ing out most cars, I'd rather be sure on this one than guess.
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Old 10-02-2011, 01:57 AM
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Join Date: Sep 2011
Posts: 13
Default EGR Valve

I have a similar problem with my 2001 diamante LS the "P0401" code and most of them recommend to clean the EGR valve first which solve the problem and after that you have to clean the EGR valve every 20-30k miles. I already replace it once.
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02, 2001, 2002, code, diamante, diamonte, egr, flow, insufficient, mitsubishi, p0401, p0421, p0431, po401, valve

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