With 2 codes surfacing on the Avalanche that at the time of the change has over 241,000 miles on it, various items have never been changed.
One being the spark plugs!
Although the drivers side plugs were easier to reach, it isn't smart to just pull on the spark plug wires, not unless you want to run the risk of damaging a wire that already is stressed from age and engine heat.
Isolated by a metal sleeve is the rubber boot over the plugs.
I chose to use a 1/8 thick open end wrench to slip over the metal boot. Measured at 0.9 O.D. in the sleeve groove, a seldom-if-ever open end 7/8 wrench was enlarged slightly using a 4 inch grinder.
It allowed slipping over the boot to put less stress on pulling off the wire/boot combo.