now can this be a clogged cat or a bad o2 sensor?? someone even told me maybe oil is getting past the rings and on the o2.. (doubt it) but hey! i have nooo idea!!!!!!!

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o4 Comp G
Polo Green Metallic
22x9 wheels[painted inserts]
245/30/ZR22 Lexani Tires
Avic D-3
(4)12"Premiers
K&N CAI
slp3.5pulley(not on now)
green hid lows/fogs
KelleyGreen Inlays

It can be both. More than likely its your downstream O2. If its your cat your car would be running like crap and it was your upstream O2 then your car would get pretty bad gas mileage. I had the same problem and bought an O2 sim.

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00 GTP Daytona

well the car does run like crap.. recently i did a tune up to try and fix it.. it idled a little rough sounds like a miss so i put new plugs (.55 gap) (tr6's) new wires and new coils (colder plugs for a 3.5 pulley i have yet to put on) and no change.. the car gets crappy gas milage and falls on its face every now and then! im confused! but when i race my buddy with the same exact car his even has less miles and runs better i smoke hiM!?!?! i have 77,000 he has 56,000 and i've driven his car alot it idles soo smooth runs awesome.. and its a terd. HELP!

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o4 Comp G
Polo Green Metallic
22x9 wheels[painted inserts]
245/30/ZR22 Lexani Tires
Avic D-3
(4)12"Premiers
K&N CAI
slp3.5pulley(not on now)
green hid lows/fogs
KelleyGreen Inlays

So its running like crap but does it feel choked? If the car feels really choked up then that would be the cat. If its just idling bad and bad gas mileage but runs fine then that is more than likely your upstream O2, I would think.

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00 GTP Daytona

Lost both cats on my Formula and it ran great. Runs even better without them

Fot the longest time GM had a special policy for replacing the cats on the 3.8 motors. I know because I replaced Hundreds of them at the dealership. They would melt the front part of the convertor causing low flow, lack of power. They would come apart and rattle to hell and back. They would throw the p0420 code and not have any other problems. Take it to the dealership and see if they would do anything for you. Ask about the special policy. It should not cost you anything. Or you can just take the cat out and put an o2 sim there like I did. I can run leaded race gas and not worry about blowing my cat up.

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99 Gtp XP cam, Map intercooler, PRJ fuel rail, Pro magnum 1.6 rockers, Titanium retainer, Wizaird air box. 4 inch Intake pipe, SLP headers, Cat delete, Paceset 3inch Exhaust, GMPP front rear sway bars and rear trailing arms,
HID lights, zzp 3.4 10 PSI. 13.269 @ 106.939 mph.

i took it in already and they replaced it for free! i was happy i still want to open the exhaust up so i can eventually put on my smaller pulley.... 3"..
where can i get an o2 simulator at? with that i can run a off road pipe right?

_________________
o4 Comp G
Polo Green Metallic
22x9 wheels[painted inserts]
245/30/ZR22 Lexani Tires
Avic D-3
(4)12"Premiers
K&N CAI
slp3.5pulley(not on now)
green hid lows/fogs
KelleyGreen Inlays

DTC P0420

Circuit Description

A three-way catalytic (TWC) converter controls emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx). The catalyst within the converter promotes a chemical reaction which oxidizes the HC and the CO that are present in the exhaust gas. This process converts the HC and the CO into water vapor and carbon dioxide (CO2), and reduces the NOx, converting the NOx into nitrogen. The catalytic converter also stores oxygen. The powertrain control module (PCM) monitors this process by using a heated oxygen sensor (HO2S) that is in the exhaust stream after the TWC. This HO2S 2, also referred to as the catalyst monitor sensor, produces an output signal that the PCM uses to calculate the oxygen storage capacity of the catalyst. This indicates the ability of the catalyst to convert the exhaust emissions efficiently. The PCM monitors the efficiency of the catalyst by allowing the catalyst to heat, then wait for a stabilization period while the engine is idling. The PCM then adds and removes fuel while monitoring the HO2S 2. When the catalyst is functioning properly, the HO2S 2 response to the extra fuel is slow compared to the response of the HO2S 1, which is located before the TWC. When the HO2S 2 response is near that of the HO2S 1, the oxygen storage capability and efficiency of the catalyst may be degraded below an acceptable threshold. If the PCM detects the degraded condition, DTC P0420 sets.

Conditions for Running the DTC

DTCs P0030, P0036, P0068, P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0120, P0125, P0128, P0130, P0131, P0132, P0133, P0134, P0135, P0136, P0137, P0138, P0140, P0141, P0171, P0172, P0201, P0202, P0203, P0204, P0205, P0206, P0220, P0300, P0325, P0327, P0336, P0341, P0401, P0403, P0404, P0405, P0442, P0443, P0446, P0449, P0455, P0496, P0502, P0503, P0506, P0507, P1111, P1112, P1113, P1114, P1115, P1125, P1133, P1134, P1351, P1352, P1361, P1362, P1374, P1404, P1516, P1518, P2101, P2107, P2108, P2119, P2120, P2135, P2138 are not set.
The engine has been running for more than 10 minutes.
The engine speed is within 200 RPM of the desired idle.
The engine coolant temperature (ECT) is between 75-126°C (167-259°F).
The barometric pressure (BARO) is more than 74 kPa.
The vehicle is in Closed Loop.
The intake air temperature (IAT) is between -20 and +100°C (-4 and +212°F).
The battery voltage is more than 10.7 volts.
The throttle position (TP) is 1.5 percent or less.
The short term fuel trim (FT) is between -10 and +10 percent.
The engine load must be stable.
Warm the catalyst by performing the following procedures:
Fully open the hood.
Place the transaxle in PARK for automatic transaxle.
Place the transaxle is NEUTRAL for manual transaxle, with clutch pedal depressed.
Set the parking brake.
Press and hold the service brake.
Each time the engine is started the diagnostic can run up to 18 times. The first test may be run after the initial warm up period of 10 minutes. Maintain engine speed between 1,500-2,500 RPM in park or neutral for 1 minute. Allow the engine to return to a stabilized idle. Shift into drive for an automatic transmission, without touching the accelerator in order for the test to run. Between each subsequent test, maintain engine speed between 1,500-2,500 RPM in park or neutral for 1 minute. Allow the engine to return to a stabilized idle. Shift into drive for an automatic transmission, without touching the accelerator in order for the test to run.
Test the catalyst by performing the following procedures:
Idle the engine and place the vehicle in Drive, or depress the clutch pedal for a manual transmission vehicle, to activate the diagnostic. Within 60 seconds, the air fuel ratio will transition lean, below 14.1, for up to 7 seconds, and then may transition rich, above 15.3, for up to 7 seconds.
Verify if DTC P0420 has passed or failed this ignition cycle with the scan tool.
Conditions for Setting the DTC

The PCM determines that the efficiency of the catalyst has degraded below a calibrated threshold.

Action Taken When the DTC Sets

The control module illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.
The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Freeze Frame/Failure Records.
Conditions for Clearing the MIL/DTC

The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
Clear the MIL and the DTC with a scan tool.
Diagnostic Aids

These conditions may cause a catalytic converter to degrade. Inspect for the following conditions:
An engine misfire
High engine oil or high coolant consumption
Retarded spark timing
A weak or poor spark
A lean fuel mixture
A rich fuel mixture
A damaged oxygen sensor or wiring harness
The catalyst test may abort if the vehicle falls outside the conditions for running the DTC.
The catalyst test may abort due to a change in engine load such as the A/C or engine cooling fan cycling.
If 18 tests have been attempted, and the DTC has not run or passed during this key cycle, turn the key to OFF for 30 seconds. Perform the conditions for running a second time.
If an intermittent condition cannot be duplicated, the information included in the Freeze Frame Records can be useful in determining the vehicle operating conditions when the DTC was set.
The catalyst may have been temporarily contaminated with a chemical from a fuel additive, fuel contamination, or any of the above conditions.

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1. cut cat out.
2. take to scrap. gain 50 monies
(if you can weld skip step 3 and 4)
3. pay $20 to have someone weld stright pipe in
4. pocket $30
(applies only if you caa weld)
5. pocket $50 and be happy
6. start car. enjoy the smell of not cat. enjoy the loudness.

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'05 WRX STi - COBB Stage 2|Invidia DP|3" Borla XR-1|Coilovers|Camber Plates|Swaybars|Endlinks|Bushings|Braces|Anti Lift Kit|DBA4000s|SS lines|
Sold: 06 Cobalt SS/SC, 99 GTP 245WHP/326 Ft-lbs, 06 R6

wouldnt i definatly get a check engine light? lol

_________________
o4 Comp G
Polo Green Metallic
22x9 wheels[painted inserts]
245/30/ZR22 Lexani Tires
Avic D-3
(4)12"Premiers
K&N CAI
slp3.5pulley(not on now)
green hid lows/fogs
KelleyGreen Inlays

You would get a check engine light if you just cut the cat out. I would buy the Down pipe without the cat and the u-bend like this one https://www.zzperformance.com/grand_pri ... &catid=110. and buy the O2 simulator from here https://www.zzperformance.com/grand_pri ... &catid=107. To pass emissions just put a convertor heat shield were the cat would go. I pass emissions every time. I pass the visual and the code scanning.

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99 Gtp XP cam, Map intercooler, PRJ fuel rail, Pro magnum 1.6 rockers, Titanium retainer, Wizaird air box. 4 inch Intake pipe, SLP headers, Cat delete, Paceset 3inch Exhaust, GMPP front rear sway bars and rear trailing arms,
HID lights, zzp 3.4 10 PSI. 13.269 @ 106.939 mph.

you would but you can delete the code from the pcm.

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'05 WRX STi - COBB Stage 2|Invidia DP|3" Borla XR-1|Coilovers|Camber Plates|Swaybars|Endlinks|Bushings|Braces|Anti Lift Kit|DBA4000s|SS lines|
Sold: 06 Cobalt SS/SC, 99 GTP 245WHP/326 Ft-lbs, 06 R6

Yup, I'm deleting P0420, all EGR and that crap too like on my Formula.

Had a bitch of a time getting my Formula to be happy with some of that emissions equipment- delete the code and then who cares if the shit fails.

If anyone else gets that code and they do not want to get the code removed, a TEMPORARY way to fix it is to pull the boot off of two spark plugs, when engine is warm, ground them, then run engine at 2000 for 2 MINUTES, do this with the doors open to keep interior cool or else you will be roasting you carpet. If possible, have someone keep an eye on the cat, if it starts to get red, stop immediatly.

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2000 Pontiac Grand Prix GT

To control emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides of Nitrogen (NOx), a three-way catalytic converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it to nitrogen. The PCM has the ability to monitor this process using the HO2S 1 and the HO2S 2 heated oxygen sensors. The HO2S 1 sensor produces an output signal which indicates the amount of oxygen present in the exhaust gas entering the three-way catalytic converter. The HO2S 2 sensor produces an output signal which indicates the oxygen storage capacity of the catalyst; this in turn indicates the catalyst's ability to convert exhaust gases efficiently. If the catalyst is operating efficiently, the HO2S 1 signal will be far more active than that produced by the HO2S 2 sensor. If the PCM detects a level of HO2S 2 activity that indicates the catalyst is no longer operating efficiently, DTC P0420 will be set.

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Thanks
Scott McKinney
http://www.motortopia.com/cars/16218" onclick="window.open(this.href);return false;