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Daryl, Shop Owner, Master Mechanic
Category: Buick
Satisfied Customers: 12549
Experience:  28 years experience, Master Mechanic
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Buick Century 3800 2001 buick century, p0404 code was cleaned

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3100  2001 buick century, p0404 code was cleaned , throttle body dirty, cleaned, replaced egr, cleaned out the vent to egr, car now quiet idle NO codes but car bogs, wont down shift, wont go hard, acts like lost vaccuum, or intake fuel in plenum really carboned up, put new plugs in, car runs so quiet dont hear, but wont get up and go. do I need to remove plenum and clean out? putting fuel injector cleaner in tank 2 tanks now.
The catalytic converter is the most likely cause of this problem, you can test this by removing the exhaust in front of the converter and taking the vehicle for a little drive, the P0404 will probably come back if this is the case because of to much exhaust back pressure.
Customer: replied 6 years ago.
After morning ride on way to work , the P0404 has shown up on check light again. my thought was the plenum into the valves were caked with carbon, and a valve or valve chamber entry area has lot of carbon, the plenum entry was real dirty, clean as far as could reach. \

Cat sounds possible, Would this not show up as a pressure difference on exhaust as wrong on computer. I would like to just put in a test pipe and leave it, would this show as an error or check light if a test pipe is put in.
This vehicle will not run correctly with a test pipe in replacement of the converter + it is illegal to do so. Eventually more codes would show up indicating a problem with the converter but the vehicle probably can not be driven enough to get a good testing process for the computer.
Customer: replied 6 years ago.
I hate computers, wish gm would go back to proven horsepower of carbs and better intakes, with dual exhaust.

Ok, If the intake throttle was cleaned and the plugs were replaced (Platinum) , The throttle butterfly was truely black caked and was cleaned car runs real nice in idle now. But if you take throttle in hand under hood and raise rpm, the motor gets a bog time and releasing throttle car comes back.

This means the CAT is plug in front of the ceramic stone inside cat, right?, due to carbon blow thru from poor plugs and fuel ignition being poor. The motor is breathing but when RPM's raise the exhaust causes a back pressure on the motor? P0404 usually is bad EGR or dirty throttle area. Doesn't the computer send a different code for back pressure from Cat? Mechanic with out seeing I know is hard.

What about intake plenum? off , easy? clean out? etc, will fuel injector cleaner help in gas?

If CAT is remove front of Cat to motor side what code will show if run for test? I know federal law says no CAT is illegal and you must stand by that as ASE mechanic but taking out will give 10hp right off the bat. How will computer "in so call test state with out cat" how will it run, what codes would arrive, and other problems caused by computer ECM compensation?

please describe the P0404 code as GM states search issue. Now knowing I cleaned throttle and EGR, and put in new plugs.
P0404 EGR Open Position Performance - With Automatic Transmission
Possible CausesSetting Conditions
  • EGR sensor connector is damaged (inspect pins for damage, and moisture)
  • EGR sensor signal circuit is open, shorted to ground or shorted to voltage
  • EGR sensor low reference circuit is open
  • EGR sensor high reference circuit is open
  • EGR valve seat or pintle contains debris or plugged with carbon
  • PCM has failed


The computer is thinking that the EGR is getting stuck in the open position because it is opening when it should not, the back pressure from the exhaust is known to cause this. If the converter is removed the computer will turn on a check engine light because it thinks the converter is bad, the o2 sensors can not get the right reading so the system will run lean most of the time and over time that is not good.

Daryl and other Buick Specialists are ready to help you
Customer: replied 6 years ago.
Talk with diesel mechanic on the Cat, and the mechanic told us the rear O2 and front O2 must read the same and the computer will be good, thus if is plug take out the cat , interior metals. So we remove the cat metals as was plugged bad by Jan 7th evening. Put back together, sealed with pipe and sealer/clamps. Motor good, NO computer signal or check lights. Thanks for telling about the cat being plugged.
I would be surprised if the light does not come on eventually for a bad catalytic converter but give it a try and see how it goes. Thanks for using JustAnswer.