Nest Thermostat Error E13 — Cooling System Not Responding Fix

What Does This Error Mean?

Nest error E13 means the Nest sent a cooling signal to your air conditioner but the temperature did not drop — the AC did not respond. Like E12 for heating, Nest monitors the actual room temperature after sending a cooling command. If the room does not cool within the expected window, E13 appears. A tripped breaker at the outdoor unit, a blown fuse, low refrigerant, or a wiring problem are the most common causes.

Affected Models

Nest Learning Thermostat (1st, 2nd, 3rd gen)
Nest Thermostat E
Google Nest Thermostat (2020)

Common Causes

The outdoor AC unit circuit breaker has tripped or the outdoor disconnect fuse has blown
The Y wire (cooling signal wire) between the Nest and the air handler is disconnected or broken
The outdoor unit has shut down due to low refrigerant charge tripping the low-pressure safety switch
The capacitor in the outdoor unit has failed, preventing the compressor and fan from starting
The air filter is severely clogged, restricting airflow and causing the indoor unit to freeze up and trip its safety shutoff

How to Fix It

Check the circuit breaker for the outdoor AC unit. Find the breaker labeled 'AC', 'Condenser', or 'Air Conditioner' in your main panel. If it is tripped (in the middle position), switch it fully off then back on. Also check the outdoor disconnect box near the unit for a blown fuse.

Outdoor units have their own dedicated high-amperage circuit (usually 30-60 amps). This breaker trips more commonly than indoor breakers, especially in hot weather.
Look at the outdoor unit. Is the fan spinning? Can you hear the compressor running? If neither the fan nor compressor is operating after the cooling signal is sent, the problem is at the outdoor unit — power, capacitor, or refrigerant pressure.

Stand back and listen from a safe distance. You should hear the fan spinning and feel air coming out the top of the unit within 30 seconds of a cooling call.
Check and replace your air filter. A severely clogged indoor filter causes the evaporator coil to ice over, which triggers a safety shutoff that prevents cooling. Replace the filter and let any ice thaw for 2-4 hours before testing again.

You can tell the coil is iced if the supply air vents blow very little air, or if you can see frost on the refrigerant lines near the air handler.
Check the Y wire connection at the Nest and at the air handler. The Y wire carries the cooling signal. It should be connected at the Y or Y1 terminal at both ends. Turn off the HVAC breaker before checking wiring.

On heat pump systems, the cooling signal goes through the Y wire to the outdoor unit. On split AC systems, it goes to the air handler's contactor relay.
If the outdoor unit hums but the fan or compressor does not start, the start capacitor is likely failed. This is the most common non-refrigerant cause of E13 in summer. A technician can test and replace the capacitor quickly — it is usually a $50-$150 repair including labor.

Do not attempt to discharge or replace capacitors yourself. They store electricity even after power is removed and can cause severe electrical shock.

When to Call a Professional

If the breaker and fuse are fine and the wiring looks correct, refrigerant or capacitor issues need a licensed HVAC technician. Refrigerant handling requires EPA certification. Capacitor replacement involves working near high-voltage components and is best left to a technician.

Frequently Asked Questions

My AC turns on but the house does not cool. Is that E13?

Not exactly. E13 means the AC did not turn on at all in response to a cooling call. If the AC turns on but cooling is poor, that is a different problem — likely low refrigerant or an oversized or undersized system — and Nest may eventually show E13 if the temperature never drops enough. But the first diagnostic step is different: check whether the outdoor unit is running and whether cool air is coming from the vents.

Is it normal for the AC to not cool in extreme heat?

Somewhat. On extremely hot days (above 100°F / 38°C), air conditioners work much harder than normal and may struggle to reach the set temperature. However, an AC should still be actively running and reducing the indoor temperature, even if it cannot reach the exact setpoint. If the AC is running but losing ground — the temperature is rising despite continuous AC operation — the system may be undersized for the space or have low refrigerant. Nest should not show E13 in this case since the AC is responding, just not keeping up.

How often should I check my capacitor?

Capacitors are consumable components that typically last 10-20 years but can fail earlier in hot climates where the outdoor unit works hard. A technician checks the capacitor during a seasonal tune-up as a standard part of the inspection. Signs of a failing capacitor include the outdoor fan spinning slowly, the unit humming without starting, and the unit struggling to start on hot days. A preventive capacitor replacement during a tune-up costs $50-$100 and can prevent a mid-summer breakdown.