I have been getting a P0420(O2 bank 1) and P0300(cylinder misfire) on my 02 Trailblazer. I even once got a P0303 for #3 Cylinder misfire. I have replaced both O2 sensors. Is it possible that the one on the manifold is bad out of the package. It is a Bosch but I'm sure it has happened before. I just put new spark plugs in it yesterday and you can still hear a little skip in the exhaust note.
Also I have a slight exhaust leak in the donut where the manifold bolts to the converter pipe. Would this have anything to do with it(O2 sensor). Are they (Trailblazer's) that sensitive?
Need some advice. I don't want to pay out my rear to take it to a dealer.:screwy:

I have been getting a P0420(O2 bank 1) and P0300(cylinder misfire) on my 02 Trailblazer. I even once got a P0303 for #3 Cylinder misfire. I have replaced both O2 sensors. Is it possible that the one on the manifold is bad out of the package. It is a Bosch but I'm sure it has happened before. I just put new spark plugs in it yesterday and you can still hear a little skip in the exhaust note.
Also I have a slight exhaust leak in the donut where the manifold bolts to the converter pipe. Would this have anything to do with it(O2 sensor). Are they (Trailblazer's) that sensitive?
Need some advice. I don't want to pay out my rear to take it to a dealer.:screwy:

I think that you're barking up the wrong tree with the sensor replacements. If there's a cylinder misfire, it's not going to be caused by the oxygen sensor. If it's a common #3 misfire, I'd look at the plug, coil, injector or even things such as valve leaks or a blown head gasket (not likely). This misfire will screw with the oxygen sensor, causing the PCM to set a DTC and lighting the MIL.

I think that you're barking up the wrong tree with the sensor replacements. If there's a cylinder misfire, it's not going to be caused by the oxygen sensor. If it's a common #3 misfire, I'd look at the plug, coil, injector or even things such as valve leaks or a blown head gasket (not likely). This misfire will screw with the oxygen sensor, causing the PCM to set a DTC and lighting the MIL.
Agreed, the misfire is setting the 420 code as the unburned oxygen in the exhaust stream fools the post cat sensor into thinking the cat efficiency is down, usually 420 is not set with a hard 0300 failure, but as silicone212 says, focus on the misfire issue.

bmr- how many miles are on your 2002? Misfire codes really are not that common on the 4.2, especially the MAF-less 2002-2005 4.2's. Too bad you wasted the resources on a new fore and aft O2 sensor. That was a waste, especially the after cat replacement. A funky MAP or IAT sensor *could* throw your fuel trim off enough to cause a misfire, but you would likely get more codes associated with the MAP or IAT if they are bad. Have you changed spark plugs? Maybe a broken or loose coil connection? Also look/listen for a vac leak. Worst case you have a valve seating or timing issue causing a misfire. A compression test would pinpoint that.

Joel

Misfire codes really are not that common on the 4.2, ...
Joel

I didn't really mean common in that they occur a lot, but common as in the same cylinder continues to misfire vs. a random misfire that could affect any cylinder. I suppose I should have used different terminology.

Yes the plugs have been replaced. Suppose I will have to look at the injectors or vacuum leak then and see what I get. Maybe my Predator will tell me balance rates on the live data screen.

If you have a noticeable misfire, pull the plugs and look for a signs on the plugs, discoloration or carbon tracking. Also focus on # 3 cyl. Confirm this is not a mechanical condition with either a leak down test or a cyl balance test on a highline scanner or a compression test.

The rear 02 sensor is what monitors the CAT convertor for efficiency. The P0420 DTC is being set because the PCM has detected a CAT convertor with an unacceptable amount of oxygen storage capabilities. In my experiences it is typically a flaky CAT convertor. However, Bosch aftermarket 02 sensors may give you anamolies too. You should use AC Delco 02 sensors only.

The misfires are what damages a CAT convertor. Also, a clogged CAT may cause the misfires. I would recommend getting the CAT convertor tested for efficiency.

I have heard of atleast 2 reports of failed catalytic converters on 2002 4.2's. Symptoms were loss of power and eventual P0420 = Catalyst System Efficiency Below Threshold Bank 1 (bank makes no difference on the I6 as we have only bank 1). Like bnaylor says, a misfire will cook a cat in no time due to the heavy fuel loading, but I guess it could be possible the a 'plugged' cat could be causing the misfire?!:uhoh: Maybe an exhaust shop could shoot your cat with a temperature gun or diagnose it further for you.

Joel

I have been getting a P0420(O2 bank 1) and P0300(cylinder misfire) on my 02 Trailblazer. I even once got a P0303 for #3 Cylinder misfire. I have replaced both O2 sensors. Is it possible that the one on the manifold is bad out of the package. It is a Bosch but I'm sure it has happened before. I just put new spark plugs in it yesterday and you can still hear a little skip in the exhaust note.
Also I have a slight exhaust leak in the donut where the manifold bolts to the converter pipe. Would this have anything to do with it(O2 sensor). Are they (Trailblazer's) that sensitive?
Need some advice. I don't want to pay out my rear to take it to a dealer.:screwy:


As said a missfire needs fixed first thing.
And also any exhaust leaks.
What kind of plugs are you feeding it?
If the cat is bad they are under the 8 year 80K fed emission waranty.
After you fix the missfire clear codes and drive it and see what it does.
Good luck MT

Her is a little info on a code 420 if it comes back.

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

DTC P0420, P0421, P0430 & P0431: Check Possible Cause Of Misfire DTC P0420 and P0421 indicates bank one catalyst system efficiency is minimum requirement. DTC P0430 and P0430 indicates 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(s) is not present, go to next step. If misfire code(s) 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. Ć 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 engineand 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.

I used Bosch platinum 2 spark plugs because that was all Advanced Auto had available. I have heard of a bad converter causing problems too and I do have to fix the exhaust leak where the manifold bolts to the cat pipe. These things are that touchy where a small exhaust leak would cause a p0420 code?

As far as the truck it has 69000 miles on it. When I pulled the plugs the color looked fine but they had a large gap like they needed replacing. I haven't gotten a code yet after replacing the plugs but we will just have to wait and see. But it does sound like it has a skip to it still.

Is there any way to have a coil pack tested? Because the plug boots look fine so I kind of eliminated that. I can check the fuel pressure but what am I looking for as far as pressure?

Does it misfire under load at say 55 mph pulling a grade or have a defintive power loss if not and no code, the miss sounds like it is gone. An exhaust leak can skew the o2 sensor readings, so fix that, clear the code and see what happens.

I used Bosch platinum 2 spark plugs because that was all Advanced Auto had available. I have heard of a bad converter causing problems too and I do have to fix the exhaust leak where the manifold bolts to the cat pipe. These things are that touchy where a small exhaust leak would cause a p0420 code?

As far as the truck it has 69000 miles on it. When I pulled the plugs the color looked fine but they had a large gap like they needed replacing. I haven't gotten a code yet after replacing the plugs but we will just have to wait and see. But it does sound like it has a skip to it still.

Is there any way to have a coil pack tested? Because the plug boots look fine so I kind of eliminated that. I can check the fuel pressure but what am I looking for as far as pressure?


You might as well pitch them Bosch plugs in the trash can.:grinyes: :rofl: :lol:
Use the AC delco plugs the good book calls for.
And as said fix that exhaust leak.
I always use a spark tester to check coils or plug wires.
Fuel pressure would depend on what engine and engine code.
MT

Yes, for what ever reason, bosch plugs drove the (delphi) ignition sytem nuts on 2000+ isuzu rodeos and troopers. They would set the random/multiple misfire code (P0300). I *think* the hi end AC Delco plugs are denso. Denso is as good as it gets for spark plugs. NGK is next in line IMO.

Joel