Daikin C5

What does Daikin C5 mean?

The C5 code means the indoor unit's gas-side heat-exchanger thermistor has returned an out-of-range resistance. The PCB reads this sensor alongside the liquid thermistor to control superheat and protect the compressor. When the value is open, shorted, or implausible, the PCB locks the unit out. As with C4, the thermistor itself, the harness, or the PCB analog input are the three real possibilities.

Symptoms

• C5 appears on the wired controller or via LED flash pattern
• Unit stops heating or cooling shortly after a start command
• Temperature reading on the diagnostic menu shows extreme high or low
• Fan may run while the compressor stays off
• Error returns within minutes after a power reset

Common causes

• Defective gas-pipe thermistor (open, shorted, or drifted element)
• Loose or corroded connector at the indoor PCB
• Damaged thermistor cable from vibration or installation damage
• Defective indoor unit PCB analog channel
• Thermistor not in proper contact with the gas line

Diagnostic steps

1. 01

   Identify the gas-pipe thermistor

   Follow the harness from the indoor PCB to the clip on the gas (larger-diameter) copper line. It will sit under an insulation wrap.

2. 02

   Inspect and reseat the connector

   Unplug at the PCB, check for oxidation on the pins, and reconnect. Confirm the locking tab clicks into place.

3. 03

   Measure thermistor resistance

   With the sensor disconnected, read its resistance at ambient and compare to the R-T table. A reading of infinity or near zero confirms failure.

4. 04

   Verify mechanical contact with the pipe

   Peel back the insulation and check that the thermistor bead is clipped tight to the copper gas line. Re-secure with the OEM clip.

5. 05

   Check the harness for damage

   Inspect the full cable run for cuts, kinks, or rodent damage. Replace the thermistor assembly if the jacket is compromised.

6. 06

   Replace the indoor PCB

   If the thermistor tests good and wiring is clean, C5 points to the PCB's analog input — replace the indoor PCB and reconfigure settings.

When to call a professional

Call a licensed technician when the thermistor sits behind a factory coil wrap or inside a concealed ducted unit, as disturbing refrigerant insulation and clips incorrectly can cause secondary leaks or sweat damage. Matching the thermistor curve and connector pitch to the specific model is not negotiable — a wrong sensor will read plausibly but with a skewed offset, producing superheat-control problems down the line. Let a pro verify the sensor curve and clip contact.