01 Pontiac Sunfire P0420

keith

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#1
This lady brought me this car and said her brother put the theromostat in backward and it needed a head gasket, GM 2. 2. I did the job and even had the head rebuilt. About 10 days later she had the car towed back to my shop said it would not run. I check it and one of the coil packs was not working so I replaced it and cleared the codes. 2 days later she says the check engine light is on again this time it is a P0420 (Catalyst system efficiency Low), I cleared the code again just to make sure and it came back the next day. So told her the misfire probably dumped fuel into the cat and ruined it. So I replace the cat and the next day same code came back. Then I looked at the oxygen sensors. Both are working and seem to be working fine. But I replaced the down stream one just in case. I drove the car a whole day no codes. Next day code came back. I ran the code through alldata and all it says is, "bad cat" or "exhaust system leak" I can find no leak. Anyone have any ideas.
 
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#2
Engine running rich or not right or missfire is the main cause of cat codes.
And sometimes 02 sensors.


http://www.troublecodes.net/articles/catfailure/

A little more info. :)

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.
 

keith

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#3
thanks for the info that you were able to provide. I had a couple more questions if anyone is able to help. The car is not missing at all it is running great. I am not sure but it seems if the fuel pressure were to high it would send a rich code not a cat code and the O2 sensors are working but again I don't think they are the problem because i replace one and both are moving alot. Likewise O2 sensors should set that code. I just took that car for a long drive. For most of the trip the Long Term fuel trim was around -5. Does that mean it is getting to must gas and the computer is trying to take gas away? However the fuel trim was moving and at times was at zero. Both times the code set for me it happened the same way. I would drive on the highway 60 mph and everything was great even 20 miles or more. But when I came off the highway and say stopped at a stop sign the code set. I dont know if that will help any one understand what could be causing this problem. By the way what is the O2 sensor middle number is it 456 or something close to that. And on my snap on scanner I have a reading called "FT cell" does anyone know what that is?

thanks
 
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#4
The catalyst monitor tests on these vehicles run at idle, so the code will never set while driving or on the highway. Driving it the way you did enabled all the criteria for the drive cycle to test the cat. A check of the FRONT O2 sensor with a labscope is a must.
If you used an afermarket cat on a 2000- GM car it will almost certainly cause a P0420 to set. I tried this last year and got burned, even after the company guaranteed me their cat was good. An OE cat fixed it. Not enough oxygen storage capacity on the smaller aftermarket cat. I hope you have the original cat around and haven"t dumped or sold it. The GM OE cats will charge for a core,probably 70+$.
 
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#5
FT is fuel trim.
FT cell is the cell or parameter that it is operating in.
I think the cell is determined by MAF sensor.
Which may be a good thing to check.
What is the baro readings or do they match your altitude?

O2 sensor middle voltage is usually .5 volts
 

Gus

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#6
Don't know if your scan tool has the ability to plot a waveform.......

If it does, you plot the data from the pre and post O2 sensors.....at idle, should see a changing sine wave from the pre.....the post should change very slowly and almost be flat rich or flat lean, depending upon if the car is storing oxygen.....if the post mirror images the pre, bad cat...

If it doesn't plot, just observe data....pre O2 should be constantly changing, between 0.100 to 0.800 volts DC.....post should change very slowly.....if it changes just like the pre bad cat.....

If a headgasket went, the coolant could contaminate not only the cat, but also one or both O2 sensors....

Could also have an intake or exhaust leak that could skew the O2 sensors...
 

keith

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#7
thanks again guys i really appricate the help. I just got a snap on solus, I was playing with it and found out it does plot wave forms. Before I found that out I replace the front O2 sensor. So now I am waiting to see if the code will set again. I did replace the cat with an univeral aftermarket one so I am hoping that wont burn me. I have used universal ones before and they worked, but who knows? The first O2 sensor was moving good as far as I could tell and the down stream was moving at about 1/3 of the frequency of the up-stream one. However the waves seemed to be moving up and down pretty far. Anyway, the map was at about 10 and the baro 29, I think both those are ok. I read and carefully studied the article by Rick Seagle and found it very helpful so thanks for that. So would I be understanding O2 sensors correctly to say that the front O2 sensor acts with the PCM to control fuel dilivery. While the downstream O2 sensor is used by the computer not to control fuel but only to see if the cat is working properly. I will let you know if the code comes back. I have both Alldata and michell on demand, If I search them will they tell me if a update flash is avaiable to loosen the parameter for the catalyst system. thanks
 
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#8
I am not aware of any reflashes to loosen parameters on GM. Ford has some, but for fuel control only. I really doubt the aftermarket cat will wrk. Since appr. 2000- GM's catalyst monitors are quite strict. If you see movement in rear O2,that's not good. The OE cat will have the rear O2 nearly pegged flat unless revving the engine. Good luck and keep us posted.
 

keith

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#9
put on the right cat and it fixed the problem. Lesson learn just because a cat looks the same on the outside does not mean it is the same on the outside. Thanks everyone for the help.
 

Gus

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#10
I did replace the cat with an univeral aftermarket one so I am hoping that wont burn me. I have used universal ones before and they worked, but who knows? The first O2 sensor was moving good as far as I could tell and the down stream was moving at about 1/3 of the frequency of the up-stream one. However the waves seemed to be moving up and down pretty far.

That is a clear indication of the cat not working.....if you plot both at the same time, that second one should not be moving up and down....glad you got it fixed, and posted back about it....