Hey Glennster and other code readers

After 100 plus miles after putting the cap and rotor in, I can't keep the check engine light off. Came on after 30-50 miles three times. P0430. The last time P0300 came on too.

Don't you check a o2 sensor by reading voltage? I don't know how to do it.

It seems to me that based on everything you guys told me yesterday plus what I have learned from the internet, the probable cause is going to be at least one of the cats. They are both welded onto the Y-pipe as one unit $400.00 plus labor plus 02 sensors.

I cannot print out how I feel about this, lest Miss Kim would remove me.

Thanks for your input.

the P0300 is a misfire code the P0430 is catalytic converter efficiency below spec bank 2. It is likely a failed cat, but if you have something going on upstream it could be preventing the cat from working properly and setting the code. It is possible the cat will work properly once the other issue is taken care of. I didn't see a previous post if there was one, so not sure what vehicle or problem you are having. A good scan tool with access to PID data makes it a lot easier to diagnose

05 Sierra with a 4.3 six. Original codes were 300,420,430, 138,158, and 171

When you were driving it with a misfire, you crapped up the "cat(s)" with unburned gasoline.

I think there is a CHANCE of it/them cleaning up after you drive it hot and hard for a while.

I'm a Ford guy so not going to be much help to you, the 171 is a lean code, not sure what the 138, 158 are .

Guilty as charged. No one to blame but me.

My old 95 GMC would give me a code that said check O2 sensor. I had 3 sensors. It could also be a vacuum leak. I discovered the mechanic that last worked on my distrubutor, forgot to put a hose back on the breather. The hose ran from valve cover to breather. That was the last time I use that mechanic.

So look to see if you have a hose missing. My code wouldn't come up all the time.
George

You mention changing the cap and rotor. If you can not afford spark plug wires, spark plugs, and a $50 scanner instead of a $25 scanner. I do not know what to say.
PO420 and PO430 codes read when the cat. converter is 4-6 percent plugged. I have put a gallon of lacquer thinner in a 1/2 tank of gas and run it at 55mph for 50 to 100 miles. This usually works if it is just the cat. converter.
Sometimes helps O2 sensors good but not perfect.
If you can not get the engine running smooth and do not want to work on it. Go to a garage; it might cost you a lot more than $400. If you do need O2 sensors, I would stand right there, and save the empty boxes. Parts quality varies greatly even at a dealership. I still like Bosch O2 sensors. A lot cheaper and made in Japan.
You asked about testing O2 sensors. You do not use a $9.99 meter from Walmart. It is the quickest way to screw them up. I like a Simpson 260 meter(high impedence meter). When I get down to one meter, I buy another on ebay. Voltage is easiest way but ohm function can also be used.
A little better scanner will give you IM information. Right now it looks like you are just erasing codes and not checking which sensor monitors are working. In New York you need 6 out of 7 monitors on. It is not hard to get 7 on. I am pretty sure EVAP monitor will say small leak or large leak. I usually change the vacuum hoses in the back. Gm uses a cheap hose good for ten years max. Cost for hose less than $5.00.
If you need help, do not be afraid to ask for help on this forum. Your problem is not all that hard to solve. There is all you need to know on the internet. There is
a lot good stuff and stuff that looks like a crackhead posted it(ignore that).
Speaking of something a little wacky. One of my friend's mixes up dish detergent and water and puts the converter in it shakes it around. Lets it sit for 12 to 24 hours. Shakes it some more and rinses it out with a garden hose. Lets it dry out and most of time they work fine. I tried it a few times with laundry detergent and the converters are still working 3 to 5 years later.
One last thing you would not mind posting how often you change oil? If it has not been changed to of still be able to somethings The old saying is no pain no gain. On this I would at as more of an enducational experience.
If you can get PO300 and PO420 cleared up, There is a good chance the other codes will go away. One code can have a cascading effect and trigger other codes.

here is a link to testing the o2 sensor with a scanner. you should have 4 wire sensors that are heated. i would go under the truck and one at a time unplug the sensor, check the harness for breakage and check the plug connector for corrosion. plug the sensor back and and test run. see if the code clears. i still suspect a bad sensor as opposed to a cat at this point.
poke here

That"s a good point... the unburned gas from the misfire may have fouled the sensor.

If you are throwing a PO300 code how would a bad
O2 sensor change that.

0300 is a random misfire code. 0430 is a cat efficiency code. the system is basically cycling the fuel delivery from rich to lean to try and compensate for the 430 codes. the problem with codes, they dont always identify exactly what component has failed, but what readings are out of spec. diagnostic software like alldata has testing proceedures for the codes.

Glenn,
I wonder if the engine has been accelerated when sitting to see if it is misfiring. PO300 as you know is multiple misfire. The OP did not say anything about plugs or wires. The OP said he as he was getting PO420 and PO430. As you know the codes are just a guide. Like the old saying do the simple stuff first. Maybe he has not maintained the vehicle. It could be as simple as changing the air filter and oil. Could be at least a dozen things to check before you start changing parts. On the other side Bosch O2 sensors are fairly reasonable. I think talking about fuel trim at this point might confuse the OP.