Question about 2004 Pontiac Grand Prix

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PO 420 and PO 455 codes

I have had 2 new gas caps and am still getting the 455 code. I've also replaced the cat, O2 censors, plugs, and wires and am pulling a 420 code. What the hell? I have an 05 grand prix gtp and out of nowhere I've got this hesitation that is the worst at high speed acceleration and wide open throttle. Driving normally there isn't a problem....HELP

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  • Anonymous Mar 25, 2014

    check the code with code scout 700

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There are three things that can cause this that is unrelated to the cap, if the valve on the carbon canister is defective it would cause this it would also cause a vacuum leak making the car run lean so check the carbon canister check valve first. Next your EGR valve could be dirty or not functioning, depending on the model and engine size it may have two EGR valves one a mechanical and the other the digital EGR and it to may be dirty, if it's dirty it can be taken apart and cleaned with carburetor cleaner and clean the pintal. And lube the pintal valve. If you’re careful and not tear the gasket you can reuse it. If this does not seam to fix the problem then you have a vacuum leak or the device that pumps pressure into the gas tank may be defective.

Posted on Jun 27, 2009

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P0420 is catalyst efficiency means that most like it needs a catalytic converter. P0455 most likely needs a EVAP vent solenoid both items are very common.

Posted on Jun 27, 2009

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1 Answer

Po 420 ?


code p0420 refers to cat converter system, bank 1 efficiency below threshold===causes --cat converter--wiring HO2S sensor 2

check that the cat converter is ok and replace the sensor. ( it is a HO2S sensor which will be different to a O2 sensor so get the right one )

Sep 12, 2014 | 1998 Chevrolet Astro

2 Answers

Po 171, po 305, po 441


Hi, I know from my past experiances the spark plug wires at the boot ends than slide onto the sparkplugs usually over time short out due to the metal tubes that protect the sparkplugs from heat kind of ark across from the metal tube through the plug boot causing an error code. Sometimes in the dark you can actually see them arking at the tube on the engine, and also hear a crackling noise. As to the code for the O2 sensor that's a normal wear and tear item that may need replaced. As all vehicle age over time electrical connections, and things have issues develop and cause havick with sensors and computers in these vehicles too.


As with the 441 code and then the other code for the O2 sensor showing up may mean both O2 sensors are going bad if your system has 2 sensors in the exhaust, usually one before the cat converter and one behind it. A lot of this also is due to what gasoline you use in the vehicle over time. Lower octane gas may be what is causing things to act up.

Jan 03, 2014 | Dodge Ram 1500 Cars & Trucks

1 Answer

Nissan Altima Check Engine Light - P0420 Code


You have a fail exhaust converter on bank # 1

Google the PO420 code & read up

Feb 05, 2013 | Nissan Altima Cars & Trucks

1 Answer

Engine misfire and knock sensor bank 1 codes on 1999 Nissan frontier 3.3 v6


I would want to see the actual codes, but if it is one or two cylinders you could have a failing injector and it not show up as a trouble code.
If one of the injectors is failing it can have a high resistance and affect the other injectors in that circuit.
The O2 sensors and cat would not affect a power loss in one or two cylinders, and the O2 sensor codes would be present.
The knock sensor only measures vibration, so it was probably ok before you replaced it.
There is a chance the cap is shorting out internally which would be tough to see, and the optic sensor in the bottom of the distributor could be causing problems too. Some of the cylinders may not be firing when you think they are.

Sep 21, 2012 | 1999 Nissan Frontier

1 Answer

O2 sensors keep failing and a evap leak code i put new gas cap did not cgehan ssomers11@gmail.com


Are you sure it's 02 sensors? Just because code says 02 sensors doesn't necessarily mean they're bad. Bad 02 sensor code can also indicate bad cat. converter. If 02 sensors read exact same reading before and after converter they will trigger OBC to "sensor fault" - they should be reading 2 different readings. (Pre CAT/POST cat) I'd suggest replacing cat converter before replacing sensors.

Mar 14, 2011 | 1999 Toyota Camry Solara

2 Answers

My 1998 gmc envoy 130,000 has failed inspection. i had 1 sensor replace,oil filter,air filter, ac spark plug,ignition wire set, and ignition coil all replace. code#p0302 n p0453 seem to be my problem....


remove the spark plug and see if it is wet. See if it smells like gas. Put the plug in the plug wire and crank the car to see if it sparks. If so, get a compression gauge and screw it into the plug hole and crank the engine. Good compression 100lbs or better. If good suspect injector.

Jan 03, 2011 | 1998 GMC Envoy

2 Answers

Inconsistent/rough run at idle & poor starting...new plugs, new wires, new O2 in front of cat conv,,,no codes


Have you tested the alternator and battery? Maybe be the throttle position censor as well.

Jan 26, 2010 | 1996 Cadillac DeVille

1 Answer

1997 gmc jimmy


Sounds like the cpi injector assembly. The fuel pressure regulator may be leaking. Or the poppet valves on the injectors may be sticking or shot. I replaced almost every part on my truck and that is what it ended up being the whole time. It's a common problem on these models. Hope this helped

Mar 01, 2009 | 1997 GMC Jimmy

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