2000 SLE. I've searched and read a few posts about this. Turn the key, sometimes it starts sometimes it won't. When it won't, headlights, dash lights, guages, ect... all come on. Turn the key, nothing except I can hear relay near battery click. After reading...pulled the back seat, cleaned battery connections and check cables for corrosion. Clean with no corrosion found. Similar intermittant starting issue with another vehicle in the past was the ignition switch. Got a great price on OEM ignition switch and changed it. Seemed to work great for a week...happens again. I go out today with voltmeter in hand to see if I'm getting voltage to the starter when key turns. Car starts. I can't believe I was hoping for car not to start to help diagnose problem...ironic. I did not hear click from starter previously but was by myself and am not sure. I'm hoping it'* the starter. If not??? Neutral switch, where and how to check? Ignition module(expensive)? I hate to get stranded but it'* hard to diagnose when it'* working. Suggestions and advise will be much appriciated.

when it won't start does the SECURITY light on the dash stay on solid?

No, I'm fairly sure the security light goes out after a moment. Each time the key is turned back on it will come back on but then off again. I'll watch closely when it does it again. I didn't mention, keys have PK3 stamped on them which I understand are chipped keys. When problem started, neither key would work.

The clicking is the relay. Things that can cause that are dead or bad battery, corroded battery cables, corroded connection at the ground(block), Corroded connection at the starter, bad starter. You said you cleaned the cables. Both ends? Have the battery load tested. Wiggle the battery posts, I've seen internal failures. Have the starter tested.

Danthurs has many good suggestions. Try those first. It is unlikely a neutral switch if your cluster shows the vehicle in PARK. The shifter does have a mechanical linkage, but failure to start based on not being in park or neutral will be computer controlled. The cluster will show the gear that the PCM believes the car is in. This is because the gear indicator on the cluster changes based on a class 2 data bus message. Unless you have a concurrent cluster failure, which really seems unlikely, it should show the gear the PCM believes the car to be in.

Update: I pulled the starter. Took it and battery to shop and both tested good. Extensively cleaned connections on both ends. Wires on both sides show no corrosion. Before pulling starter, I had 12.75 volts everywhere. Car started to get on rack, 10 minutes later would not start, voltages remain constant. Could a relay be intermittant? Could the starter soloniod be intermittant? My experience with those has been they either work or they don't. What else is in between? Reistalled battery and starter...all is working well, for now, but afraid to leave the house.

Refresh my memory. When you say it wont start, does it turn over? Or does it crank and simply not start?

When it doesn't start it does nothing. Well, lights and dash cluster come on but when key is turned...nothing. When the back seat was off, I could hear a relay click. No noise from the starter. Lights do not dim. I would go double check all symptoms but right now it'* working.

Could be a bad solenoid, or possibly a bad relay. Did the lights dim when it didn't turn over. Sorry if I'm asking the same question again.

No, lights do not dim. Similar problem with an older vehicle was ignition switch. Not this time. Changed that right away. Even though starter checked good, I'm tempted to swap it out and replace relays. Just doesn't make sense...