Holden Commodore 1996 Problems

Check the air/fuel ratio to see if it’s running rich. I would guess it thinks it’s running cold, but as you’ve changed the oxygen sensors and the temperature sensor I would focus on the ECU.

The engine is overheating, that's the gauge is telling you. You need to have a mechanic check it; any number of things could be causing it, from a blocked radiator to a blown head gasket. It's nothing to do with the grille.

I would expect your car to return around 11.0 L/100 km, so yours is very poor as you say. You would have to allow for slightly higher fuel consumption due to wear on the engine, loss of compression etc., but I would still expect that you should get around 12 L/100 km. I would be checking the basics, like spark plugs, spark plug leads, air filters, fuel-injectors, and throttle body to make sure they are all in tip-top shape; then look at sensors like the oxygen sensor to make sure they are conveying the correct signal to the engine computer.

It would seem either the ignition switch or the neutral/park safety switch is faulty. I would guess it's the former. A mechanic or an auto electrician should be able to fix it for you.

It depends on the gradient, the idle speed etc., but generally an auto will sit on a slight gradient without rolling back. The idle speed could be a little low. But I wouldn't be concerned about it if it does roll back, it shouldn't have any effect on take-off when you're out driving and have to stop on a hill.

THOSE early Commodores often have trouble with a crank sensor, so that may be worth checking.

IT COULD be any of several things. You really need to have it checked by a dealer. It could be a temperature sensor, the crank-angle sensor, a direct-fire ignition pack, or several other components.