Run capacitor
Common no-start and weak-compressor failure point. Match the microfarad and voltage rating.
Carrier 42 Error Code — What It Means
Carrier code 42 is an inducer motor fault — the control board fired the inducer but didn’t see the expected speed feedback or pressure switch closure within the startup window. The inducer (draft motor) must pull combustion gases before ignition begins; if it fails to prove airflow, the board locks out to prevent unsafe combustion. You’ll see 4 long flashes followed by 2 short flashes on the status LED.
Common Causes
- Failed inducer motor — Bearings seize or windings burn out, motor doesn’t reach operating speed; most common cause on units over 10 years old.
- Blocked flue or condensate — Partially blocked exhaust vent or condensate line creates back pressure that prevents the pressure switch from closing even when the motor runs.
- Faulty draft pressure switch — The hose, port, or diaphragm on the pressure switch fails; motor runs fine but switch never proves.
- Defective control board — Board doesn’t send proper voltage to the inducer or misreads the tach signal.
Step-by-Step Fix
- Check the flue and intake pipes — Inspect both PVC pipes (on 90%+ units) or the metal flue collar for blockages: bird nests, ice, debris. Clear any obstruction.
- Listen to the inducer on call for heat — Motor should spin up before the ignitor glows. If silent, check 120V at the inducer connector with a multimeter; present voltage + no spin = bad motor.
- Inspect the pressure switch hose — Remove the small rubber hose between the inducer housing and pressure switch. Look for cracks, kinks, or condensate pooling. Blow it clear with compressed air.
- Test the pressure switch — With the inducer running, use a manometer or jumper the switch terminals momentarily. If jumping clears the fault, replace the switch.
- Reset the system — Cut power at the disconnect for 30 seconds, restore, and call for heat. Confirm LED shows solid green or normal flash pattern.
Parts Often Needed
| Part | Notes |
|---|---|
| Inducer motor assembly | Match HP and RPM to model; Carrier OEM or aftermarket (Fasco, Rotom) |
| Draft pressure switch | Single or dual-port; verify cracking pressure spec on the label |
| Pressure switch hose | Standard 3/16” or 1/4” silicone tubing |
Technician Tips
Carrier uses different diagnostic systems across furnaces, heat pumps, chillers, mini-splits, VRF equipment, rooftop units, and communicating controls. Confirm the full model number and whether the fault came from the indoor board, outdoor board, thermostat, or equipment controller before matching it to a repair path.
Start with power, wiring, airflow or water-flow, sensor, condensate, and safety-circuit checks before ordering control boards. If the same code returns after one reset and the basic cause is not obvious, test the named circuit instead of repeatedly cycling the equipment.
Common replacement parts for Carrier systems:
- Carrier flame sensor (part #HH18ZA641)
- Limit switch kit (part #HH18HA600)
- Pressure switch (part #HH18HA719)
Need Replacement Parts?
Before ordering, match the HVAC part to the model number, electrical rating, and failure symptom. Check current prices on Amazon - prices and availability change frequently.
When to Call a Pro
If the flue is clear, the switch tests good, and the motor runs but the fault persists, the control board may have a failed tach input circuit. Board replacement requires verifying compatibility and should be done by a licensed technician on systems still under warranty.