MRCOOL air conditioner E1 error code: indoor and outdoor units not communicating

What the E1 code means

E1 on a MRCOOL mini-split means the indoor air handler and the outdoor unit have lost communication. Gen 4 and Gen 5 units show the same fault as EL 01. MRCOOL says it is usually a loose connection or a wiring fault in the cable between the two units.

The indoor and outdoor units send signals to each other over the cable that joins them. MRCOOL says that on a healthy system, the DC voltage between terminals 2 and 3 keeps switching between positive and negative. If the indoor board stops hearing from the outdoor board, it shows E1 or EL 01 and the outdoor unit stops.

On a new DIY install, E1 is almost always a wiring mistake: a loose screw, a wire on the wrong terminal or a splice. On a system that has run fine for months, look for storm damage, pests or a failed board.

Common causes

Listed from most to least common.

1. Loose wire at the terminal block on the indoor or outdoor unit
2. Communication wire spliced, damaged or wired to the wrong terminal
3. Breaker or disconnect for the outdoor unit off or tripped
4. Power surge, storm or brownout upsetting the control boards
5. Damage from pests chewing the wiring
6. Failed indoor or outdoor control board

How to fix the E1 code

Unplug the air conditioner or switch it off at the breaker before removing any panel or filter.

1. 1

   Step 1: Power cycle the system

   Turn off the breaker and the outdoor disconnect. MRCOOL suggests leaving it off for 15 minutes before turning it back on.

2. 2

   Step 2: Check the outdoor breaker

   If the outdoor unit is silent while the indoor unit has power, check that its breaker has not tripped and the disconnect is switched on.

3. 3

   Step 3: Look at the wiring you can see

   With the power off, check the cable between the units for cuts, crushed spots, chew marks or splices. MRCOOL says the communication wire should not be spliced.

4. 4

   Step 4: Check the terminal connections on a new install

   With the power off, make sure terminals 1, 2 and 3 match at both ends, each wire is tight and no bare copper is touching its neighbor.

5. 5

   Step 5: Note any recent storms or surges

   Tell the technician or MRCOOL support if the code appeared after a storm or outage. Take photos of both terminal blocks to speed up diagnosis.

You'll need: Screwdriver, flashlight, phone camera.

How to reset the MRCOOL air conditioner

1. Turn the mini-split off with the remote.
2. Switch off the breaker for the system and the outdoor disconnect switch. MRCOOL suggests leaving the power off for 15 minutes so the boards fully reset.
3. Restore power and turn the unit back on. The compressor normally waits about 3 minutes before starting, so give it time before judging.
4. If the same code comes back straight away, the fault is still there. Work through the fix steps above, or call a technician.

When to call a technician

If E1 returns after the power reset and the wiring looks right, a technician needs to test the voltage on terminals 2 and 3 with the power on. That test tells them whether the indoor or outdoor board has failed. Do not do live voltage tests yourself unless you are trained.

A typical repair for this code costs $150–$700 in the US, including parts and labor. Prices vary by area and model.

Models this applies to

• DIY Series ductless mini-splits (Gen 1 to 5), single-zone and multi-zone
• Advantage Series mini-splits
• Olympus Series mini-splits
• EZ-PRO mini-splits

Codes can differ slightly between models. Your owner's manual is the final word and is available from MRCOOL support.

Frequently asked questions