Car Evaporator Coil Leak: How to Diagnose It

Evaporator Coil Testing: A Complete Guide

Testing the evaporator coil in your car for a refrigerant leak can be a challenging task, as the evaporator is typically mounted inside the heater box inside the dash, making visual inspection difficult. Diagnosing an evaporator coil leak requires the right tools, as improper testing can lead to costly mistakes and labor-intensive repairs. While it’s crucial to ensure the diagnosis is accurate, there are several methods that can help you test the evaporator coil for leaks, ensuring the efficiency of your car’s air conditioning system.

Four Ways to Check Your Car’s Evaporator Coil for Leaks

Each method varies in complexity and the tools required, but they all share the goal of locating refrigerant leaks that affect the performance of your AC system. Below are the most commonly used techniques for testing evaporator coil leaks.

1. Electronic Leak Detector— One of the most effective ways

Leak detector/sniffer

to test for a car AC leak is by using an electronic leak detector. To perform this test, your AC system needs to have a minimum of 50 PSI of pressure. This method is particularly useful because it can help detect even small refrigerant leaks around the evaporator case or its connections.

You can buy a decent AC leak detector (also called a “sniffer”) for around $50 and as much as $400. Make sure the detector is rated for the older R-134a and the newer R-1234yf refrigerants.

Remove the heater box drain tube and allow time for any collected water to drain out. Then, place the probe near the condensate drain. WARNING: It’s essential to ensure that no water from the drain tube comes into contact with the probe, as this can short-circuit the leak detector. Or, remove the blower motor resistor block and insert the probe into that space.

For the most accurate results, turn the AC blower to high for at least 15 seconds, then shut it off. This allows the refrigerant to build up in the evaporator case. Insert the leak detector probe into the blower resistor opening (if located near the evaporator), the condensate drain, or another suitable access point. Use a heater or vent duct only as a last resort. This setup should allow you to perform an effective refrigerant leak test.

Move the leak detector probe slowly—no more than 1 to 2 inches per second—and keep it within 1/4 inch of the surface. If the detector signals a possible leak, confirm it by blowing compressed air around the suspected area and testing again.

Don’t operate the blower while performing the test because airflow can cause a false reading.

The leak detector shown to the right is a SENSYX Rechargeable HVAC Refrigerant Leak Detector from Amazon.com for $50. It has a 4.8 review rating.

2. Fluorescent Dye Test— Another common method for testing the evaporator coil for leaks is using fluorescent dye. To perform this test, your AC system must be fully charged with both refrigerant and oil. Since oil naturally leaks out with refrigerant, adding fluorescent dye into the system will cause any leaks to become more visible. Typically, you would inject a small amount (about 1/4 ounce) of dye into the system.

If the system is empty, you can pour the dye directly into the

Dye Injector

low (suction) side of the system. However if it still has some refrigerant in the system, you’ll need a dye injector to force it into through the low side port. You can buy an injector for $16 at Amazon and a small bottle of dye for around $8 (Amazon)

Keep in mind that if the dye is injected recently, it may take several days for the oil and dye to travel through the system and exit at the drain. Also, be aware that some vehicles may have a thermostatic expansion valve (TXV) mounted inside the evaporator case. If you detect a leak in this area, it could be coming from the evaporator itself, the TXV, or the connection between the two components. A visual inspection after opening the case will help identify the exact location of the leak.

Once the dye is introduced into the system, you can shine an ultraviolet (UV) light on the evaporator drain tube. The dye will glow under the UV light, making the source of the leak more obvious. For a more efficient inspection, you can wipe the drain tube with a clean rag, preferably white, and hold the rag up to the UV light. The dye that has leaked out will be more visible on the rag.

Keep in mind that if the dye is injected recently, it may take several days for the oil and dye to travel through the system and exit at the drain. Also, be aware that some vehicles may have a thermostatic expansion valve (TXV) mounted inside the evaporator case. If you detect a leak in this area, it could be coming from the evaporator itself, the TXV, or the connection between the two components. A visual inspection after opening the case will help identify the exact location of the leak.

3. Vacuum Test— The vacuum test is another reliable method for testing an evaporator coil for leaks. To perform this test, you must first discharge the system and disconnect the accumulator and refrigerant lines. Then, using flush adapters on the evaporator fittings, apply a vacuum of 28 to 30 inches of mercury (Hg). The vacuum should hold for at least 30 minutes.

If the vacuum holds steady throughout the test, that indicates there are no leaks in the evaporator coil. However, if the vacuum level drops, it means there is a leak somewhere in the evaporator or its connections. It’s important to verify that the vacuum gauge and adapters are not leaking, as this could skew the results.

This test is also useful for testing new evaporators before installation, as it can ensure the part is free of any defects before it is put into service.

Notice the oil stain in the corner of the evaporator coil

4. Visual Inspection—I’ll be honest with you on this; it’s
really hard to visually inspect your car’s AC evaporator coil because it’s buried so deep inside the heater box under the dash. However, if the leak is large, you can sometimes see an oil stain on the coil using an inexpensive borescope. If you suspect a leak after performing the electronic leak detector or fluorescent dye test, the next step is to snake the borescope inside the heater box from either the drain tube or down one of the vents. If the blower motor resistor is mounted on the heater box, try removing it to insert the camera. You can buy a cheap endoscope for about $25 from Amazon.

Inexpensive endoscope camera to use with your cell phone

Why Test the Evaporator Coil?

The evaporator coil is a vital component in your car’s air conditioning system. It absorbs heat from the cabin and cools the air. When the evaporator coil develops a leak, it can result in refrigerant loss, which can cause the AC system to malfunction, leading to poor cooling

Evaporator coil

performance or complete system failure. A leak in the evaporator can be particularly difficult to detect because it’s often hidden inside a sealed case. That’s why it’s important to know how to test the evaporator coil for leaks and what methods can be used to accurately diagnose the issue.

If you detect a leak in your evaporator coil, you’ll most likely have to remove the dash and heater box to replace it. It takes a skilled technician almost 8 hours to do the job. However, it’s DIY-able if you have a shop manual to show you the step-by-step procedure. You’ll have to discharge the AC system before removing the heater box, and that’ll have to be done by a shop. Even with that expense, you’ll still save a fortune. A replacement evaporator coil usually costs less than $100.

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