Q: Why does my brake light fuse keep blowing?

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the car started fine but would not shift out of park. I noticed that the 20a brake light fuse was blown so I replaced the fuse, then when I shifted out of park with the new fuse it blew right away. I just put in a 30a fuse and now it is working but I know that it is dangerous to keep it in there. I need to know of what is possibly wrong before i go to the mechanic.

My car has 105000 miles.
My car has an automatic transmission.

The simplest possibility is the bulbs (brake light bulbs) on the circuit have corroded and the contacts have melted such that they touch creating a short. Another bulb related fault occurs if the filaments in a dual filament bulb short together. If the problem is not found in the bulbs or contacts, basically any wire, ground, or component in the fused circuit is a possible cause. Failures can often be simple insofar as cause so of course check the bulbs and terminals carefully first. If not successful, YourMechanic can fix this for you, on a mobile basis, at significant savings over taking the car to a shop. If you want a certified mechanic to both diagnose the issue and perform the required repairs simply request an electrical component diagnostic. Obviously, it is not recommended to use a higher capacity fuse, even on a momentary basis as you can ruin other parts of the circuit and components.

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The brake light switch is powered by the fuse and then powers the rear brake lights and third brake light also. The power also goes into the body control module to signal to unlock the shifter interlock. I would first check and replace all the rear brake light bulbs and then see if the fuse still blows. If not then problem was in one of the bulbs. If it still blows fuse then replace the shift interlock solenoid. You can disconnect it first to make sure it is the cause of the short and then replace solenoid.

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