Troubleshooting Your Auto-Dimming Rear View Mirror

a rearview mirror in a car

An auto-dimming rear view mirror is a great convenience when there is too much light on the mirror. This is not only good for you as the driver, but also for people who are driving behind you. The mirror will dim to a level where the light does not reflect back in your eyes or the eyes of the person following you. This auto dimming feature works much like the small lever that could manually be used to change the angle of the mirror to dim the light.

After the rear view mirror has been in use for several years, things can start going wrong. Much like the other parts of the vehicle, they begin to fail with age. There are also times when the problems are not caused by age, but by faulty parts. If you are having problems with the auto-dimming feature of your rear view mirror, here are some tips to help you troubleshoot this yourself.

Nothing Works on Mirror

Most of the auto dimming rear view mirrors also have other electronic displays within the mirror itself. They can display the time, the temperature, and direction of travel. If nothing is working when you turn on the key, there is a problem with either the wiring or the actual mirror itself. Check the wire connections at the rear view mirror and wiring harness. Also check any fuses associated with the lighting.

Auto Dimming Feature Not Working but Other Displays Are

One of the common problems with the auto dimming rear view mirror is that they just will not dim. If the other displays are working, but the dimming function is not, you should check the sensors. Look at the back of the mirror and locate the sensors aligned there. Clean them off so that there is nothing blocking the sensors from the light. Shine a flashlight onto the sensors and see if the mirror dims. If not, then you may have a faulty sensor.

Dimming Feature Works Sometimes

There may be times when the mirror works like it should, and then others where it does not. The culprit could be a short in the wiring harness or a loose connection. Use a voltmeter on the wiring to see if you can find any fluctuations in the current.

Check Rear Window

If the dimmer does not work like it should when there is light shining on it from the rear of the vehicle, it could be something that is not related to the rear view mirror at all. Many vehicles today are sold with a factory tinting on the windows. If someone adds even more aftermarket tinting to the rear window, this will have a direct impact on the function of the auto dimming feature in the mirror. To check this, shine a light on the mirror from inside the car rather than outside.

Faulty Mirror Trigger

Besides the sensors on the outside of the rear view mirror, there is also a trigger on the inside that tells the mirror to dim. When this fails, the mirror will not dim like it should. Test this with a voltmeter to see if it is working.

Auto-Dimming Rear View Mirror FAQ

Where is the sensor on an auto dimming mirror?

Auto-dimming rear-view mirrors typically have multiple sensors. Often, there are at least two sensors, one on the backside of the mirror and one on the front.

One of the sensors detects ambient light and dims the mirror based on how dark or bright it is. The front sensor detects glare from vehicles driving behind you.

The sensors work together to determine how dark or bright the mirror should be.

Why does my rear view mirror keep going dark?

Rear view mirrors that dim automatically adjust according to the light in your vehicle and the lights coming from vehicles driving around you. The mirror will darken as needed to reduce glare so that you can still see clearly in spite of all the bright lights.

Why do auto dimming mirrors fail?

Auto dimming mirrors will stop working if the sensors are turned off. Make sure the sensors are getting power and reset them if possible to see if they start working again.

Sensors may also be grimy or dirty due to simple buildup over time. Wipe off the sensors with a clean, damp cloth.

Use a soft cloth and work gently so as not to scratch or damage the sensors, which are sensitive. Also carefully wipe around the sensors to make sure there is no dust, hair, or other debris possibly blocking the sensors and preventing them from working properly.

Is auto-dimming rearview mirror worth it?

Rear view mirrors that dim automatically are not just a feature to show off, they make it safer for you while driving at night. It reduces the glare of the mirror that could otherwise distract you or interfere with your vision.

People who are photosensitive or have difficulty driving at night may especially appreciate the features of an auto dimming rearview mirror but arguably, all drivers can benefit from any extra safety features.

What causes a mirror to discolor?

Mirrors that turn black at the edges or otherwise become discolored are likely being affected by mirror rot. This is a phenomenon that occurs when moisture gets trapped between the layer of glass and the layer of metal that creates a reflective mirror.

Mirror rot can often be removed. Try polishing the mirror with white vinegar to see if this removes the discolored spots.