Replace it if necessary.

Inner Tube Inspection
Spring Tension InspectionTire Tread Wear Inspection
As the tire tread wears down, the tire becomes more susceptible
to puncture and failure. An accepted estimate is
that 90% of all tire failures occur during the last 10% of tread
life (90% worn). So it is false economy and unsafe to use
the tires until they are bald.
Measure the tread depth a ...
Spark Plug Condition Inspection
Remove the spark plugs (see Spark Plug Replacement in
the Periodic Maintenance chapter).
Visually inspect the spark plugs.
If the spark plug center electrode [A] and/or side electrode
[B] are corroded or damaged, or if the insulator [C]
is cracked, replace the plug.
If the spark plug is ...
Rear Brake Pad Installation
Check that the pad spring is in place on the caliper.
Push the caliper piston in as far as it will go.
Install the piston side pad [A] first, and then another pad.
Fit the brake pad end into the recess [B] of the caliper
holder.
Install the pad pin and the plug.
Tighten:
Torque - ...