What might be the problem if a 1997 Isuzu Trooper's low beams are out but the high beams still work?

Answer

Hi, there. This is exactly what happened to my '99 Trooper. Believe it or not, both the low beam filaments went out at the same time. I replaced on bulb and, bingo, low beams worked. Then, I replaced the other bulb and got the same result. Good luck. Hope this works for you.

-------- Hello, this is Lee adding a bit more to the answer.

Burned out bulbs suggest "high vehicle voltage levels".. High voltage levels are caused by a battery developing high internal resistance or a high resistance in the battery ground cable.

Automobiles typically use an alternator that charges the battery at a constant current. If the battery or the wire connecting the battery's negative terminal to the engine block develops a high resistance, then the voltage of the entire vehicle climbs a half a volt or maybe even a whole volt above normal. It turns out high intensity headlight bulbs quickly start blowing out when they are fed a little too much voltage.

So here is a checklist.

1. Check the voltage drop for the battery to engine block ground connection. Check the battery connections. Remake as indicated.
   
2. Does the battery need water? Not a very common problem these days, but small battery vehicles like motorcycles will develop a cell with low water.
   
3. Is the vehicle battery at it's age limit? Aging car batteries sometimes develop higher internal resistance.