I'm trying to fix the P0420 DTC, but it just won't go away....

99 Astro 4.3 (W) AT,AC(f&R). Fresh tune-up using all ACDelco parts, Oil change, etc... Van runs great until the code sets.

Following 'Old_Master's' TWC flow chart (hope this makes sense):

Step 1 - Ran the OBD test and passed (after first clearing the P0420 code)

Step 2 - No other codes: Go to step 3

Step 3 - Check exhaust for leaks, and found bolt missing from the muffler to cat flange: Problem found - Go to step five per the doc (didn't repair yet)

Step 5 - Entered the DTC P0420 in the mt2500 scan tool, operated the vehicle until the scan test ran, and DTC P0420 was once again set: Didn't pass - Go to step 6

Step 6 - (this is where i'm lost) Important: Refer to test description before replacing the catalytic converter.

What is the 'test description'? Didn't i just do that, or is this telling me the cat is bad?

I hope someone can shed light on this, cause i think its the cat and i'm about to shell out the $400+ bucks to replace it... Just simply tired of testing this stupid P0420 code.

Thanks - This forum rocks!

An exhaust leak can set a P0420.

Step 3 says if you find a leak, "repair as necessary". Did you repair the exhaust leak?


After the leak is repaired, proceed to step 5. Step 5 has you re-test to see if the P0420 sets again after you repair the exhaust leak. If it sets again, go to step 6. The "test description" is there so you don't ruin the new converter, or replace the old one unnecessarily. (Just a double check due to the cost of the converter.)

ok... If the leak can trigger the code then i'll definately get it fixed and run the test again to see if that changes anything... I'll post back when done.

This leak is between the muffler and cat at the three bolt flange... It's missing a bolt, but if i remember that was like that for awhile. My van only has the 3 HO2S.

Thanks

It's doubtful, but not impossible for a leak downstream of the converter, to set a P0420. Cross your fingers. How many miles on the oxygen sensors? The ECM relys on quick oxygen sensor reaction time to keep the engine running efficiently. The faster the reaction time, the closer the ECM can keep fuel mixture to the desired 14.7:1 ratio. O2 sensor reaction time to detect changes from lean to rich and from rich to lean slows during its life. After 100,000 miles they have slowed enough to affect fuel mileage and could very well set a P0420 also. This is why it's recommended to replace them at 100,000 miles.

It's doubtful, but not impossible for a leak downstream of the converter, to set a P0420. Cross your fingers. How many miles on the oxygen sensors? The ECM relys on quick oxygen sensor reaction time to keep the engine running efficiently. The faster the reaction time, the closer the ECM can keep fuel mixture to the desired 14.7:1 ratio. O2 sensor reaction time to detect changes from lean to rich and from rich to lean slows during its life. After 100,000 miles they have slowed enough to affect fuel mileage and could very well set a P0420 also. This is why it's recommended to replace them at 100,000 miles.

I had the van for 2 years now and never changed the HO2S.. Don't know if they are original or not, but they did look old though.... Which reminds me - I also cleared the code, and swapped the HO2S to each others side and the P0420 still set. Even after i drove for it for awhile, but i would of expected the B2S1 (P0430?) to set when it failed the efficiency test, which is kinda why i didn't think it could be the HO2S.... Either way, i'll fix the leak first then test again... Besides, if it's not the leak, buying the HO2S would be cheaper then the cat... LOL

OK... I fixed the exhaust leak, and ran the OBD test... It failed!!!

I captured the following with the mt2500 scan tool

freeze frame / failure record data

P0420 catalyst efficiency low bank 1
engine load (%)_____3 Coolant (*C)_____87
st trim-1(%)_______1 st trim__________129
lt trim-1(%)_______-5 lt trim__________122
st trim-2(%)_______2 st trim-2________130
lt trim-2(%)_______-5 lt trim-2________122
map(kPa)_________39 rpm____________519
veh speed(kph)____0 maf(gm/sec)_____4.77
tps(%)___________0

I don't know what the real values should be (don't have the master gm book), so any help reading these and finding out what the 99 astro values should be is appreciated.... Thanks

i have a 97 awd and i'm having the same issue but on mine at times i hear a rattle coming from inside the cat i've already change 2 of the 02 sensors mine only come on after hard acceleration

After the engine is at operating temp, hold the RPM about 1,500 and watch Oxygen sensor bank 1 sensor 2 voltage, it should be steady around .3 or .4 volts. Post what you find.

I only have 3 O2 sensors - B1S1, B2S1 and B1S3(post). Not sure i seen a B1S2 reading on the scan tool... Will check!

old_master, I could be doing this wrong, but i went into the obd-II trouble shooter for the voltages and all 3 sensors were fluctuating @1500 rpms, from .085 up to .875... None of them were steady, so again i'm not sure if that is the correct way to verify the voltages, but it's the only way i found on the scan tool.

Sorry, my bad, I should have clarified, the sensor to test is the post converter sensor, in your case bank 1 sensor 3.

The post converter sensor should show a low, steady voltage, indicating the converter is doing it's job. The front sensors should swing up and down as yours are, that indicates changes in fuel mixture. The downstream sensor should remain steady around .3 or .4 volts. Sounds like your converter needs replacement.

I was afraid you were going to say that... I'll start looking for a cat, but this testing also leads me to believe that the HO2S are very old and should be replaced as well. I'll post back soon.
Thanks for the support old-master.

I think I would replace all 3 sensors, seeing that they're due, and then run a drive cycle and see if the P0420 sets again. Cross your fingers, close one eye, hold your tongue just right, and you might be pleasantly surprised. There's a fine line for setting the P0420 DTC. Please post your results for others to see.

OK, good news old_master - i had the entire exhaust system replaced with walker (cat, muff & tail pipe), and ran the OBD test and it passed. YAHOO! I shouldn't speak to soon, but after checking the readings with the mt2500 all 3 HO2S seemed to be very stable, not jumping all over as they were before, so i hope they stay that way. The van also seems to have a smoother excelleration. Also, i didn't replace the HO2S yet, but will soon, as i should see better gas milage. Right?? Currently getting 14.7 mpg per the overhead readout.

Side note on the exhaust - I prefer to have a super quite muffler on the Astro for many reasons. The new Walker muffler i had installed with the cat has a low tone to it (seems cheap), so i'm planning to re-install my old muffler in a couple days. I think it will once again be super quite like it was before the change... Just felt that an entire new exhaust would be super quite too! NOT!

Thanks old-master for keeping me going on the Astro.... It paid off!

Guess what? My 99 blazer ZR2 seems to have a P0452 and P0453 code now and the muffler seems to be going out... Crap... Here i go again. Once i dig into that i'll jump back in with a new post in the BLAZER section.

2000 Astro 160K Similar problems came up over the winter - P0300 & P0422 (consistently now, after I clear them with a reader), 15+ mpg & poor acceleration / missing in the higher end of each gear, plus won't start in wet or cold - 3 blizzards in the last 3 weeks and hard starting after each one.

I've had this van since Dec 07, replaced the upper manifold gaskets when I saw the leaks, all 3 O2 sensors, plugs, wires, distributor cap and rotor and water pump while I was in there last spring/summer plus both heater hoses when one blew during the first blizzard. I'm thinking cat, since it's probably the original. does it matter if I cut a new one in, or do I really need the stock version?

Thanks - harryevan

.....I'm thinking cat, since it's probably the original. does it matter if I cut a new one in, or do I really need the stock version?

Thanks - harryevan

Always diagnose and repair DTC's in the order they are retrieved. After the repair, clear the history file and perform a drive cycle to make sure the DTC is repaired and that no new ones pop up.

P0300, under certain circumstances, can also set a P0442, along with a whole list of other DTC's. When the ECM runs the monitor for the converter, the ECM watches for specific changes in oxygen sensor voltage. If there is a random missfire going on, the results are skewed and the ECM might set any number of DTC's.