* Neutral Safety Switch: This switch prevents the engine from starting unless the transmission is in Park or Neutral. If it's faulty, it may be sending the wrong signal to the computer, preventing the starter from engaging.
* Ignition Switch: A failing ignition switch can intermittently stop sending power to the starter motor, even if other electrical systems work.
* Starter Motor: While the battery and electrics work, the starter motor itself could be failing. It might be getting power, but not functioning properly to crank the engine.
* Crankshaft Position Sensor (CKP): This sensor tells the computer where the crankshaft is positioned. A faulty CKP sensor would prevent the engine from starting as the computer wouldn't know where to send the spark and fuel.
* Cam Position Sensor (CMP): Similar to the CKP sensor, this tells the computer the position of the camshaft. A faulty CMP can also prevent the engine from starting.
* Fuel Pump Relay or Fuel Pump: Though you say the electrics are working, the fuel pump might not be receiving power, or might itself be failing. Try listening near the fuel tank for the whine of the fuel pump when you turn the key to the "on" position (but don't crank it repeatedly).
* Security System: Some anti-theft systems can disable the starter if they detect a problem. Check your owner's manual to see if there are any security system indicators.
* Wiring Issues: A loose connection, damaged wire, or corroded terminal somewhere in the starting circuit can cause intermittent problems. This requires a thorough visual inspection.
Troubleshooting Steps (Do these in order, starting with the simplest):
1. Check the obvious: Make absolutely sure the transmission is in Park or Neutral.
2. Listen for the fuel pump: Turn the key to "on" (not the start position) and listen near the fuel tank. You should hear a brief hum.
3. Have someone try to start it while you check for clicking sounds: A rapid clicking sound from the starter relay or starter itself suggests a low battery (despite your statement that the electrics work – a weak battery might power lights but not the high-current demand of the starter), a bad starter, or a bad battery cable connection. A single click could point to a relay or wiring issue.
4. Check all fuses related to the ignition and starting system.
5. Visual inspection of wiring: Look for any obvious damage or corrosion around the battery terminals, starter motor, and ignition switch.
If you're not comfortable doing these checks yourself, take your Avalanche to a qualified mechanic. Trying to diagnose these problems without experience can be dangerous and lead to further damage. The fact that it died while driving makes this even more serious; it could point to a more significant underlying issue.
