How did it run before the maintenance? did you perform maintenance to fix a problem?
Valve stem seals are not the culprit for a no start.
I have a checklist that I use when I'm starting a SHO after heavy maintenance.
1. Plug wires Right side of coil pack is cylinders 1,2,3 (in that order), left side of coil pack are 5,6,4 top to bottom, in that order.
2. Since I'm back there, I check that there are no wires or hoses pinched between the intake and the heads/valve cover.
3. Check that the IAB is plugged in and that ITS wireing is free. (this is a big culprit for me as that wire loves to get pinched, somewhere).
4. From here, I check that the secondaries are plugged in and the vacuum hoses for that are free from kinks.
5. Next, I move to the brake booster vacuum line and the intake grounding strap. Check the other side of the intake ground strap and make sure it's firmly secured to the body.
6. I make sure at least one of the intake end tanks is BOLTED (this is the DIS ground path...another big no-start culprit). If there is any question about the DIS ground, get a digital volt meter set to "continuity tone" and make sure it sounds off when you touch the BOTTOM DIS BOLTS with one terminal and a spot of bare body metal with the other termial. If it doesn't tone, your DIS is NOT grounded.
6.5 check that the fuel lines are not crimped or twisted.
7. Check all the body wireing on the accessory side of the engine bay. This would make me touch the CAM sensor, the SPOUT connector, the ground wires by the washer reservoir, the battery terminals, and anything else I could see.
8. Double check that the fuel pressure regulator vacuum line is connected and the CPS is connected, along with the little single wire that's on the same harness (this is kinda like a wiggle test, just make sure you don't see anything funky and the wireing is not pinched...there's very little room for the harness right there)
9. Move to the coil pack. Check that the condenser capacitor is bolted to the coil and plugged into the harness.
10. Disconnect TPS and take a hard look at the PCV hose.
If it still didnt start, I'd use the paper clip trick to make sure the computer was happy. The fan should come on and you should be able to pull codes (sounds like you've confirmed that)
If that all checks out then you might have a mechanical problem but I'd first swap out the coil pack and DIS to rule those out. Both can cause intermittant starting.
Mechanical: remove the crank pully and lower timing cover. Set engine to Cylinder #1 TDC by rotating it until the crank sprocket timing mark is pointing at the little bump inside the oil pump housing (at abouit 5:00). Remove top timing cover and ensure the sproket marks are lined up with 12:00.
If it still won't fire, reseat the intake. Chances of a large vacuum leak have been increasing every step along the way. To help ensure the intake is not leaking, loosen all the runner hose clamps a couple turns so the intake can "settle" on the heads. Torque the intake to the heads and retighten the hose clamps.
If it still won't fire, possible culprits are CCRM and fuel pressure.
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