Went to start the car this morning and it immediately began to misfire. DTC scan revealed P0300. Usually when I cook a spark plug wire it tells me which cylinder is acting up, but "random misfires on all cylinders" indicates something else is going on.

Primary ignition coils tested at
6/3 = 1.4 ohms
2/5 = 1.4 ohms
4/1 = 1.4 ohms

Secondaries measured
6/3 = 6.34 kohms
2/5 = 6.37 kohms
4/1 = 6.41 kohms

They're all consistent, but are those within spec? From what I have read elsewhere on this forum the primaries should be about half of that. I did a quick test for vacuum leaks and found none. Haven't tested the plugs or wires yet, they are about 4 years old.

The exhaust smells a little funny so I am wondering if the cat is clogged? Is there a way to determine this? The car has 174k on it and I believe it is the original exhaust (I have owned her since 90k). Car ran great when I came home last night. Prior to today, everyone once in a while the first morning start will idle a little rough, but if I blip the throttle everything smoothed out. She is probably overdue for an O2 sensor, but I don't think that is the cause here.

Any suggestions are appreciated,
-Shawn

if your coils are original they would be suspect to me. the way you describe the starting is usually an ign fault. you said 4yrs on the plugs and wires how many miles? it might be time for those as well.

Those test numbers look a bit high to me. Look here and you will see where your numbers should be. https://www.gmforum.com/trouble-shooting-test-procedures-141/testing-coils-spark-plug-wires-278818/

Just picked up plugs and coils. Going to do plugs and wires tonight and see if that helps. This set of plugs has 40k on them. If it doesn't, then I'll put the new coils in. I just tested the new coils (Duralast) and the primaries were 1.4 ohms and the secondaries were 8.74 kOhms (higher than my old ones). I'm not sure a DC resistance test is definitive since it doesn't measure the reactance, but I guess it would catch the more egregious failures.

I just replaced 5 of the plugs and wires (didn't have a 6th wire last night) and no change. I swapped the coils for 3 new ones and no change. I put the old coils back in with some dielectric grease on the spade terminals and the ICM connector. Any ideas on what to look for next? Should I throw a new O2 sensor at it? It seems like if the O2 sensor were bad I would get a code for rich or lean fuel mixture; makes me wonder if the ICM is the problem (ouch $). Bad ground somewhere? I jiggled wires on the ICM to see if there is a bad lead...

After the last 2 days of limping around without any power, she is going to feel like a Ferrari when I get this sorted out! I'll keep going through the Helms to look for answers.

Have you checked your fuel pressure?

Just checked it:
47 psi with ignition on and engine off
49 psi with engine running

What brand of plugs and wires did you use?

I used AC-Delco 41-101 plugs and duralast wires

Err... actually the wires were from carquest... I have a box of duralast sitting here.