1999 Rodeo,, Isuzu, starter motor and solenoid

The key lock cylinder operates the ignition switch sometimes ignition switch is separate sorry I didn't find where ignition switch is located if the lock cylinder is plugged into it.
You should have a starter relay and starter relay fuse.
This diagram of starter circuit shows the clutch pedal switch in the circuit on an automatic should have the neutral safety switch in the circuit. This is for 95 Rodeo.
Stater Motor Circuit Diagram (1995-1997 2.6L Isuzu Rodeo) (easyautodiagnostics.com)

How do I remove the key ignition cylinder? ' Isuzu SUV Forum (planetisuzoo.com)
Isuzu Trooper Ignition Switch Replacement ' Cost & Service ' RepairSmith
Changing the Ignition Lock: Changing the Ignition Lock, Safley ... (2carpros.com)
Starter and solenoid
Isuzu Trooper Starter replace under 1 Hour - YouTube
Isuzu Trooper (1995 - 1996) - fuse box diagram - Auto Genius
Starting System & Wiring Diagram - YouTube

Crystal, this is what I found on the Rodeo starter fuse/relay.
Check the attached links,instruction and guides, Good luck
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1998 Trooper Relay Solenoid Starter 1998 Isuzu Trooper All of

If it is just clicking then the starter motor is not turning over and you will not be able to start the engine.

You need to check...
1. The starter motor relay. If it has failed you will get no power to the starter motor and you replace it with a new relay. Your owners manual shows you the location of the relay.

2. Check the power supply to the starter solenoid and then from the starter solenoid to the starter motor itself.

3. If these are getting the correct current then you may have either a failed starter motor solenoid or the motor itself. Do not just replace the starter motor and assume that will fix the problem (as some people do) without doing all the essential wiring and current checks. If there is a wiring failure or the starter unit is not receiving the correct power you need to address that issue.

The P0240 code I believe has nothing to do with the starting issue. It relates to the catalytic converters on the exhaust system.
Read about the issue from the link below or look up numerous other references.

http://ricksfreeautorepairadvice.com/p0420-after-catalytic-converter-replacement/

like all cars. youtube it.
1: REMOVE battery neg lug.
2: remove the wire to starter solenoid.
3: remove the bolts, holding it to bell housing.
bam its out.
reverse above to put back.

Check to see if you are using the proper fuse. If so, disconnect the starter solenoid and then try to start the vehicle. If the fuse does not blow, the starter solenoid (entire starter, actually) should be replaced.

The solenoid on the starter motor is bad. You can just replace the solenoid or the entire starter.

Your battery has enough amps and voltage to power up the accessories, but does not have enough amps to engage the starter motor. The click you here at the starter is the solenoid trying to allow the starter motor to turn. Either replace the battery and/or check the battery connections for corroded terminals.

The repair manual book says first disconnect the battery , then lift the car to have more space to work on it make sure that is secure if you liftit ,remove the nut to Starter ,also nut to solenoid cable ,or if you think it is easier take off starter first and then after remove nuts to starter and solenoid ,then you ready to install you new or rebuilt starter ,,good luck

The starter is located on the pass. side of the block at the bottom.

Its on the starter