Four ways to check the bulb in a car

Working lights in your car are essential for safe driving. If one of the lights is not working, most of the time we see this happens because of a burnt bulb. However, there are cases when the bulb is good and the light stops working for different reasons.
How can you check if the bulb is good? There are four ways to test the bulb:

1. Does tapping lightly on the light assembly make it work?
Often when one of the lights is not working, mechanics try tapping the light assembly lightly. If it starts working after that, in 99 percent of the time it's the bulb.
This happens because when the filament inside of the bulb is separated in one spot, a small shock can cause to loose filament to reconnect and start working again. In this case, also check the bulb socket/connector to make sure it's not loose or corroded. If the socket and connectors are good, replace the bulb.

2. Look closely at the filament inside the bulb:
Often, it's easy to see when the filament inside the bulb is burned out, like in this photo. Look closely, the filament might be connected to one post, but burnt out at another post.
If it's a double-filament bulb, look at both filaments as one can be OK, but the other can be bad. See more examples:

If the filament looks intact, but the bulb won't work, the next way to check the bulb is to measure its resistance.

3. Check the bulb resistance with a multimeter:
Set the multimeter to Ω (Ohms) to the lower settings. Connect the black proble to the COM terminal and the red probe to the terminal that shows a sign Ω for Ohms. Measure the resistance between the two connectors of the bulb. If the bulb is good, the resistance should be close to zero Ohms. This bulb in the photo is good, as we measured its resistance at 0.0 Ohms.
If the multimeter shows OL (Open Loop), it means that there is no continuity indicating that the bulb is bad. If it's a double-filament bulb, check the resistance of each filament.

4. Try connecting the bulb to the battery, see if it lights up
Try connecting the bulb to a 9V or 12V power source. If you don't have wires to connect the car battery, even the regular PP3 9V battery like the one in the photo will work. If the bulb lights up, it's good.
More powerful bulbs get really hot when powered, so it's a good idea to wear gloves.

Check the bulb socket/connector
Whenever checking or replacing the bulb, always check the bulb socket or connector for looseness, corrosion or signs of melting. Often the bulb doesn't work because the socket is damaged or has a bad connection; see the photo.
In this case, the bulb socket will also need to be replaced. If your local parts store doesn't have it, you can order it from a dealer. A bulb socket costs from $5 to $15. In some cars, the socket comes as part of the taillight or headlight wiring harness and may cost from $15 to $65.