Pontiac Montana was created in 1999.
In exhaust pipe before catalytic converter unscrew to replace
Replace the fan blower resister
The question you ask is too involved for a proper answer. You need to go to a car parts store, and get a repair manual for your car. They cost about $16.00 Or, go to a Public Library.
Cam sensor location 02 pontiac montana 3.4
There is a resistor pack operated by the fan switch. It has failed. You need to replace it.
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For a Pontiac Montana the battery is a group 78. Follow this link for dimensions.http://www.rtpnet.org/teaa/bcigroup.html
it would be best because you could tear the old ones while removing them.
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Begin by removing the positive cable from your battery. Remove the cables from the front of the starter. Remove the starter retaining bolts. Reverse the process to install your new Pontiac Montana battery.
Pay a mechanic to replace it
You can find the oil pressure sensor for your 1999 Pontiac Montana on the front of the engine. The low pressure oil sensor will be just below the left cylinder head.
Drain the water from the radiator. Remove the water hoses. Remove the retaining bolts. Reverse the process to install the new radiator.
On the firewall, right next to the Power brake booster. If you follow the metal tubes around the engine, the hoses are connected to the steel tubes.
One thing that would cause your 1999 Pontiac Montana to stall at idle is the idle being set too low. You should raise it to see if that fixes the problem.
Take it to Mechanic!
In order to change the brake light, check your manual for directions. You will need to remove the cover and replace the burnt out bulb.