What to Do If Your Rear Defroster Doesn't Work

Visual and Equipment Troubleshooting

The idea of running electric current through a circuit to generate heat and clear up your car's rear window arose a few decades after automobiles took to the roads. These days, with the touch of a button, those lines on your back window heat up to melt away fog and frost. But sometimes, in the midst of winter, they don't work. The good news is that you might be able to troubleshoot the problem and fix it yourself without heading to the service department.

Here's how to proceed:

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Visual Inspection

Rear defroster tab disconnected
A broken or disconnected tab can keep your rear defroster from working. Check your connections. photo by Matt Wright, 2012

Those little lines on the rear window are made of conductive paint applied directly to the glass, making the defroster compact and durable. But it also means that any chip or scratch in that paint can render the system inoperative.

Sometimes a break in the painted circuit or another problem can be spotted with a visual inspection. First check the connection tabs toward the left and right sides of the painted grid. Sometimes these connections are soldered in place. If a solder connection has failed, you might see a loose, dangling wire that obviously should be connected to the grid. It can be repaired with a special kit containing soldering paste (an epoxy glue full of metal so it will conduct electricity) that you can find at an auto parts store. If the dangling wire has a connector on the end, chances are it has worked its way loose from the painted grid. Reconnect the wire and see if you're back up and running.

If that doesn't resolve the situation, look at the small lines of the grid itself. A break in the grid pattern could have been caused by an impact that left a scratch or missing section in the paint. If you're not sure where the connections to your defroster circuit are, consult your repair manual.

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Test Equipment Inspection

Rear defrost test lamp
A test lamp like this one will help you find a break in your defroster circuit. photo by Matt Wright, 2012

If you found nothing wrong visually, you can buy an inexpensive tester designed for rear defrost systems. Its test lamp will illuminate with the slightest bit of power running through it, which is useful in troubleshooting the defroster lines.

Overall Power Test: First determine whether your defroster is getting any power. Old wiring or a blown fuse can cause a disconnect. You can test this with the defroster test lamp or an automotive circuit tester. Make sure the rear defroster switch and the ignition key are turned on. Disconnect both wires from either side of the defroster grid and touch one end of your tester to each wire. If the light comes on, you have power. If it doesn't, check for a bad fuse and replace it if necessary.

Grid Power Test: Check whether power is getting to your defroster grid. Disconnect the wire closest to the passenger side of the car and attach the test lamp clip to the wire (not the little metal tab attached to the glass). Next, touch the other test lamp wire to the tab on the other side of the circuit (don't disconnect the wire on this one). If it lights up, the grid is getting power.

Grid Breakage Test: If power is getting to the grid, your defroster problem probably is due to a break in the painted grid. This is especially likely if your defroster works on part of your rear window. Attach the clip end of the test lamp to the driver's side metal tab or some exposed metal part of that wire. Next, begin touching the other wire of your test lamp to the painted circuit. Frost Fighter, a well-known defroster repair kit, suggests wrapping a little aluminum foil around the end of the wire to be sure you don't scratch the painted circuit. Touch the grid every three inches or so to find the break. It's normal for the light to be brighter in some spots, as long as it lights up. If you find a break in the circuit, move the test wire back and forth until you know exactly where it stops working. If you find a break, you can repair it.