2002 buick century giving code P0420

I have a 2002 century 3.1 L That is giving me code P0420.I had to replace the lower intake gasket before the code ever showed up.I also replaced the cat. converter because it was plugged do to the coolant leaking into the intake for sometime.After the cat. was replaced this code showed up.I replaced the ignition module and both O2's.I've only gotten it to go out long enough to pass emissions then it came back on.The car is on its third after market converter.any help welcome thanks.

Is this new cat. an OEM part or aftermarket. I've read many posts where people have tried an aftermarket cat. on newer cars, just to have this p0420 show up again. I would try a GM cat.

This is an after market cat.I should of also mentioned that the engine is running rich.

your running rich is probably related to the cats poor efficiency.

OK,GM CAT is $500.00 w/core.That is a pretty pricey guess.Has anyone else had this problem?Could it be something else?I just don't know about an aftermarket Cat. failing to do the job if it has been built to GM specs. :banghead:

OK,GM CAT is $500.00 w/core.That is a pretty pricey guess.Has anyone else had this problem?Could it be something else?I just don't know about an aftermarket Cat. failing to do the job if it has been built to GM specs. :banghead:

Engine running rich or miss fire bad tune up is one main causes of code 420/430.

DTC P0420, P0421, and P0430 & P0431: Check Possible Cause Of Misfire DTC P0420 and P0421 indicate bank one catalyst system efficiency is minimum requirement. DTC P0430 and P0430 indicate bank 2-catalyst system efficiency is minimum requirement. Possible causes are as follows: Use of leaded fuel. Oil contamination. Cylinder misfire. Fuel pressure too high. HO2S sensor improperly connected. Damaged exhaust system component. Faulty ECT sensor. Faulty HO2S. Ensure ignition timing is correct. Retrieve all Continuous Memory DTCs. If misfire code is not present, go to next step. If misfire code is present, isolate cylinder and repair as necessary. Check HO2S Monitor DTCs If DTCs P0136, P0138, P0140, P0141, P0156, P0158, P0160, or P0161 were present in step 1), service as necessary before continuing. If none of these codes are present in step 1), go to next step. Check ECT Sensor DTCs If DTCs P0117, P0118, P0125 or P1117 were present in step 1), service as necessary before continuing. If none of these codes are present in step 1), go to next step. If any codes except P0420, P0421, P0430 and/or P0430 were present in step 1), service as necessary before continuing. If no codes except P0420 and/or P0430 were present in step 1), go to next step. Check Rear HO2S Wiring Harness Turn ignition off. Ensure HO2S wiring harness is correctly routed and connectors are tight. Repair or replace as necessary. If wiring harness and connectors are okay, go to next step. Check Fuel Pressure Turn ignition off. Release fuel pressure. Install fuel pressure gauge. Start engine and allow to idle. Note fuel pressure gauge reading. Increase engine speed to 2500 RPM and maintain for one minute. For fuel pressure specifications, see FUEL PRESSURE SPECIFICATIONS article. If fuel pressure is as specified, go to next step. If fuel pressure is not as specified, go to CIRCUIT TEST HC. Check For Exhaust System Leaks If exhaust system leaks, it may cause catalyst monitor efficiency test to fail. Inspect exhaust system for cracks, loose connections or punctures. Repair or replace as necessary. If exhaust system is okay, go to next step. Check For Exhaust System Restrictions Inspect exhaust system for collapsed areas, dents or excessive bending. Repair or replace as necessary. If exhaust system is okay, go to next step. Check Manifold Vacuum Install tachometer. Connect vacuum gauge to intake manifold vacuum source. Start engine and raise engine speed to 2000 RPM. Manifold vacuum should rise to more than 16 in. Hg. If manifold vacuum is okay, go to next step. If manifold vacuum is low, go to step 11). Leave tachometer and vacuum gauge connected. Start engine and raise engine speed to 2000 RPM. On a non- restricted system, manifold vacuum should quickly rise to normal range as increased RPM is maintained. On a restricted system, manifold vacuum will slowly rise to normal range as increased RPM is maintained. If manifold vacuum is okay, no indication of exhaust leak or restriction has been detected and testing is complete. If manifold vacuum is low or slow to respond, go to next step. Leave tachometer and vacuum gauge connected. Remove exhaust pipe from exhaust manifold. Start engine and raise engine speed to 2000 RPM. If manifold vacuum is now okay, fault is downstream from exhaust manifold. Reconnect exhaust pipe to exhaust manifold and go to next step. If manifold vacuum is still low or slow to respond, fault is in exhaust manifold or intake manifold gasket. Repair or replace as necessary and repeat QUICK TEST. Leave tachometer and vacuum gauge connected. Disconnect muffler/tailpipe assembly from rear of catalytic converter. Start engine and raise engine speed to 2000 RPM. If manifold vacuum is now okay, fault is in muffler/tailpipe assembly. Repair or replace as necessary and test-drive vehicle to verify elimination of symptom. If manifold vacuum is still not okay, fault is in catalytic converter. Repair or replace as necessary. Check tailpipe/muffler assembly for debris from catalytic converter. Test drive vehicle to verify elimination of symptom.

dreamracerx, what did you ultimately do to solve your problem? i have 2002 century, cat converter slowly failed (melted down) during that process changed the plugs, wires, filters, flush cooling (elephant snot issue) and finally changed cat converter 3 mos ago, runs great now but just got the P0420 code. I need the emissions test & you cannot pass w/ the light on here, i am ready to trade this buick in-

dreamracerx, what did you ultimately do to solve your problem? i have 2002 century, cat converter slowly failed (melted down) during that process changed the plugs, wires, filters, flush cooling (elephant snot issue) and finally changed cat converter 3 mos ago, runs great now but just got the P0420 code. I need the emissions test & you cannot pass w/ the light on here, i am ready to trade this buick in-

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