1998 GMC Sierra 2500HD won't pass emissions
 

The truck was bought used.
The mechanic says that all 3 Oxygen sensors need to be replaced.
He said the test indicates that there might be fuel in exhaust.
I indicated that I suspect there is occasional misfire (especially under load).
Is it any worth replacing all oxygen sensors? Should we focus on spark plugs/reason for the misfire?
The oxygen sensors alone will set me back some $500 including parts and labor.

If you trust the mechanic you should replace the sensors. If there is gas in the exhaust you may have a clogged cat. and that could cost you a lot more. The bad sensors may be the cause of the misfire under load. Replace the sensors and see how it runs.

sounds like the problem I had with my Tahoe... 5.3L ... same symptoms... replaced the Cat.

Do you have a service engine light? If 02,s are bad will get a light.
But the only ones that counts is the one before the cat. Others read what emission are.
If they are reading fuel is rich they are working. If scan say voltage low or high on 02 then might be bad. If you have a rich condition clean the maf sensor .
Check the coolant temp sensor located beside the thermostat. And another thing if running rich will get a flashing service light ..means do not run till you figure why it is running rich ..burn the catalysis converter up.

Well either your o2 sensors are reporting incorrectly, thus making them bad and the engine running rich, there are many things that can cause your engine to run rich. The MAF sensor is one of them, your spark plugs could be starting to foul and not burning all of the fuel.

Your hydrocarbon levels could be high, which would pretty much indicate a rich condition, but it could also be a catalytic converter issue. The mechanic should be able to tell if the sensors are working properly ... not just replace them. He can use a scope and graph the voltage pattern to tell, he could read the PCM's datastream and look at the voltages being reported.

Yeah, I'd focus on the misfire, but the o2 sensors.

126K on the truck
replaced o2 sensors, spark plugs and wires

Truck appears to drive fine now. It doesn't have the occasional jitters anymore, however I do think it's somewhat weak in low RPMs.

However, given that my other car is a mustang, and I've never driven a truck, I may just have high expectations on the performance truck, even though it does have a 5.7 engine.

While I will probably follow the mechanic's advice and get truck plates to avoid dealing with emissions in the short term, I am still worried about the high HC, as all readings I have done say it's unburned fuel, which means I'll be likely wasting money every time I drive it (or idle it, as below indicates)

I do want to look at all options before I proceed with any further fixings.

Here are the readings:

HC ppm (limit 125)
first reading: 212
after o2 sensors replaced 157
last reading 185

HC @ 2500RPM (limit 125)
first: 8
second: 16
third: 16

so it appears it's not burning all fuel on idle only.

Catalytic Converter and Gas Cap Pressure tests passed. I am not sure what they test with the CAT thou.

BTW, mechanic says the computer is clean.
He mentioned that the cylinders could be worn out.. but I don't see how that wouldn't be an issue at higher RPMs.

Somewhat mentioned to clean the "maf sensor" where do i find that :)

Sounds like you have another issue causing the rich condition at idle.
I believe the HC readings from the first set are at idle?
The failed O2 sensors may just be a symptom--not the root cause.

Yes, first readings are at idle.

When would it run rich at idle, and not rich at speed?

I've been doing a lot of searching, but the issue could be pretty much anywhere from what was mentioned earlier, as well as low pressure in the cylinders. I guess I'll continue searching.

For now, I'll just try one of those "injection cleaners" like redline si-1

This sounds something serious, i think you should sort it out if mechanic is reliable.

It could possibly be a falsely reading MAF sensor - a MAF's job is to meter the air coming into your intake, and judge how much fuel it will need. If that sensor, I believe it's a hot wire type, doesn't warm up to the proper temperature as it should, it will think that the truck is sucking in more air than it really is. Which would cause a rich at idle condition. Your mechanic should be able to tell based on the readings he gets if your MAF is working or not.

All I know about CAT testing is that it should have about a 100 degree difference in inlet temperature to outlet temperature. Someone once told me about an exhaust back pressure gauge to check for catalytic converter blockage. I've never used one, but apparently it's a test than can be done.