2000 Chevy Astro trouble code P0420 failed emissions test

Hello everyone,

I have a 2000 Astro van (cargo) that has a trouble code of P0420. It failed the emissions test badly. It passed on all curb idle readings but failed on ASM2525. It's almost due for a tune up so I will do that because it needs one either way. I found some info on P0420 and found the following:

A code P0420 may mean that one or more of the following has happened:
An oxygen sensor is not reading (functioning) properly
The engine coolant temperature sensor is not working properly
Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
Retarded spark timing
The oxygen sensors in front and behind the converter are reporting too similar of readings

I just don't know where else to start, I will check the exhaust manifold for cracks. Any sound advice would be appreciated. O2 sensors are old as well.
This is a very high mileage vehicle with 260,000 miles on it. It does not burn oil or anything and runs well.
Thanks in advance.
Steve

HC ppm Limit 50 Reading 86
CO% Limit 0.28 Reading 0.61
NO ppm Limit 548 Reading 1037 at 1305 RPM

Start by verifying a good ignition system. Even though there may be no misfire codes, ignition could be marginal. The acceptable idle readings and poor loaded readings might indicate a weak ignition. Worn, wide spark gaps can suffer spark blow out and/or poor flame propagation and resultant poor combustion.

Verify the exhaust system, mainly to assure there are no air leaks. Include the EGR in that inspection.

Poor compression and/or leaky valves can cause generally poor combustion and the higher readings.

Of course, a clean inlet air filter is important. Make sure the MAF inlet screen is not contaminated with debris or heavy accumulations of dirt. This can skew the MAF reading and cause a leaner condition (high NOx readings).

Make sure the EGR is functioning.

Test the main O² sensor to see that it can make a minimum of 0.95V with a hot bench test (propane flame). Perform the same test of the rear O². A weak front sensor can skew the mixture rich and result in higher HC and CO readings.

Well, I started off by changing the spark plugs, cap, rotor, wires, PCV, and air filter. I used AC Delco parts other then the air filter (Wix). The plugs did not look too bad. I will carry on with the advice you have given, and post the progress and results. Thank You.

I did not retest the vehicle until there was further diagnosis because I felt it would not pass after performing just the tune-up. I ended up inspecting the O2 readings pre and post cat, and the readings were the same. I put on an aftermarket catalytic converter, and it passed with flying colors.
Thanks
Steve