You jump in your car on a cold morning, turn on the ignition, and hit the rear defroster button. When, after a couple of minutes, you glance up expecting to see those warming gems at work in the back window, you see nothing.
What’s wrong when your rear window defroster is not working? Not just irritating, it’s potentially dangerous as it restricts rearward visibility. Let’s find out exactly what’s wrong and how costly the fix may be before calling the Repair Shop.
What Does a Rear Window Defroster Do?
Those horizontal lines you see across your rear window are actually thin heating elements embedded in there. When you press the defroster button, electricity flows through them, heating them up enough to clear away any frost, ice, or fog on your window.
In all honesty, the process is pretty simple. Power from your car’s battery goes through a relay and a switch to power these lines. Press the defroster button, and after about a minute or two, you should see your window starting to clear.
Under normal circumstances, the entire window should be clear in 5-15 minutes, depending on the outside temperature and the thickness of frost or fog.
Common Rear Defroster Problems
Typically, a problem with the rear defroster is either electrical or physical. Here’s how to figure out which one happened to yours.
#1. Damaged grid lines.
The lines on your rear window aren’t as tough as they look, and are often why a rear defroster breaks down. Most often, they wind up broken because of using metal scrapers or razor blades to clean them off the inside of the window (think stickers or decals) an object striking it.
Tossing things in the back that hit the glass is another way to hurt them (say you decide to pop your son’s bike into the back of your SUV: if the bike pedal’s touching the glass, every bump in the road will rub it against the lines until part of that line is “erased.”).
Or even improper cleaning supplies could damage them (rubbing too hard, the wrong chemistry, etc.)—in fact, a single break in the line can stop the passage of electricity, as accounted for by those strips that get warm, and those that don’t.
#2. Electrical System Problems.
Your defroster relies on a few electrical components. Two of them are typically faulty:
- Fuses: Since your defroster draws significant power, it has its own fuse protecting it from damage if it shorts out. When this fuse blows, there is no way for you to know but that your entire system has suddenly stopped functioning. Luckily, fuse problems are usually the simplest and cheapest to remedy.
- Relays: Think of your relay as a heavy-duty switch that closes, allowing electrical current to flow freely and providing the necessary heavy load your defroster draws. Clicking when you attempt to engage the defroster is a telltale sign that a relay has gone bad and very little else happens.
#3. Connection Problems.
Metal strips (bus bars) close to the edges of the rear window connect the defroster to the electrical system of the vehicle; at the point of connection, the bus bars are subject to repeated heat stress from temperature change and vibration due to operation of the vehicle.
A poor connection here frequently causes poor heating of the glass; in fact, you may notice that the defroster performs differently when the trunk is opened or closed, or you may even be able to see that the point of connection is oxidized where the bus bars are in contact with the glass.
The harness in your trunk or rear door that is connected to the electrical system may also cause trouble due to opening and closing the door.
#4. Circuit Issues.
Sometimes the problem is with the overall circuit. Issues such as poor ground connections, broken wiring, or bad high-resistance points in the circuit, in general, may interfere with the operation of the defroster. These faults typically result in weak overall heating or in unreliable operation.
You may suspect that on some days your system seems to do a little better job than on others, or that its heating performance is affected by engine speed.
Electrical problems like this can be hard to find, as the fault could be just about anywhere along the path of the electrical system. You should let a professional hunt them down.
#5. Control Switch Issues.
Like any other button in your car that gets pushed a lot, press it enough and the contacts inside will begin to wear or corrode. It may just feel different—mush rather than snap.
Sometimes the indicator light will stay in working order even though the defroster isn’t operating. That would indicate a switch failure. Some vehicles have the switch as part of a larger button array, complicating diagnosis and repair.
#6. Timer Module Issues.
Most vehicles nowadays incorporate a timer module that only allows the defroster to run for a predetermined period of time (usually 10-15 minutes) before cutting it off to prevent draining the battery. When this module is faulty, it can cause a variety of annoying problems.
The system may turn off too soon, refuse to turn on at all, or simply continue to run all the time. Some drivers may notice the defroster running for a different period of time each day: only the day before yesterday it worked for its full time, but the next day it ran only a couple of minutes before shutting off.
This complicated component requires special diagnostic equipment for testing.
DIY Diagnosis Steps
Before you call the repair shop, try these simple checks. The good news is that, most of the time, repair is possible by the DIY enthusiast and often is quite inexpensive.
Basic Visual Check:
- Clean your rear window thoroughly
- Run your fingernail gently over the lines to feel for breaks
- Look for discoloration or bubbling in the grid lines
- Check the edges where the power connects to the window
Electrical Testing:
- Find and check the defroster fuse in your fuse box
- Listen for a click from the relay when you hit the button
- Make sure the switch feels and works normally
- Look for any obvious wiring damage
If you’re comfortable using a multimeter, you can test:
- Voltage at the fuse
- Continuity across grid lines
- Power at connection points
- Relay operation
Repair and Replacement Costs
Here’s a rough idea of what you might pay for various types of defroster repairs. Actual cost of professional repair varies widely depending on your vehicle, who does the work (i.e., hourly labor rate), and the quality of replacement parts used.
Basic Electrical Fixes:
- Fuse replacement: $5-10 (parts only)
- Relay replacement: $20-40 (parts only)
- Switch replacement: $75-200 (parts and labor)
Grid Line Repairs:
- DIY repair kit: $15-60
- Professional line repair: $50-100 per line
- Complete window replacement: $300-800
Wiring Issues:
- Basic wire repair: $100-300+
- Full harness replacement: $500-800
- Diagnostic fee: $75-100
Maintenance Tips
Tip #1 – Only use a microfiber towel or soft cotton cloth on the interior of your rear window. Tough materials can nick grid lines, creating tiny breaks you won’t notice right away. Stick to automotive glass cleaners like Stoner Invisible Glass or Sprayway, and avoid anything too harsh that could damage the conductive stuff.
Tip #2 – Don’t let just anything touch your rear window, particularly in an SUV, minivan, or station wagon. Never put skis, lumber, tools, or pretty much anything else against the window—light contact repeatedly knocks the grid lines out. Use an appropriate cargo area barrier or tie-down product to keep stuff from contacting the back window.
Tip #3 – Repairing small grid line breaks immediately pays off. The little break isn’t going to get better and will only get worse. DIY repair kits work best if you nip the problem in the bud; don’t wait until half your window has gone out.
Tip #4 – Treat your trunk seals well. Poor seals allow water to reach electrical connections at the edges of your window. Moisture leads to corrosion, then problems connecting properly, which gradually get worse. Be sure to replace cracked or broken seals as soon as possible.
Tip #5 – Be extra careful of window tint. If you’re having your rear window tinted, the best type of shop will understand how to work around the defroster lines. When it comes to removing old tint, avoid metal scrapers, harsh chemicals, or anything that could damage the grid lines.
Tip #6 – Think about your window every fall. Run your finger lightly over the grid lines each fall before you face cold weather. You might feel a rough spot going bad that’s headed toward no longer working at all. That first sub-zero day might not be pleasant if you find your defroster is dead.
Tip #7 – Don’t skip cleaning along the edges of the window. If it’s possible that dirt could have gotten on the metal runs that make up your defroster, clean that area out really well, but take care doing it. Corners of a swab dipped in glass cleaner or even diluted isopropyl alcohol work nicely for this area.
Tip #8 – Safely remove stickers from your rear window. Never use a razor blade, and probably not a plastic scraper either, but treat the area delicately anyway. A little automotive-feel-good safer adhesive remover is called for, and again, be careful of the grid lines.