York Error Code A4: Improper Thermostat Wiring

Severity
Warning
Type
digital display
Estimated repair cost: $50 — $150 (part only)
Warning: Your York furnace isn't working because it thinks the thermostat wiring is messed up. This usually means the system is locked out and needs a technician to fix it.

How to Fix York Code A4

  1. Step 1: Check the thermostat display. Make sure it's powered on and set to 'heat' mode.
  2. Step 2: Replace the thermostat batteries if it uses them.
  3. Step 3: Visually inspect the thermostat wiring at the thermostat itself (if easily accessible). Look for loose or disconnected wires. Do not touch any bare wires.
  4. Step 4: If the problem persists, contact a qualified HVAC technician to diagnose and repair the wiring issue.

How to Reset York Code A4

  1. Turn off the furnace power switch, usually located on or near the unit.
  2. Turn off the circuit breaker for the furnace. Wait 30 seconds.
  3. Restore power at the breaker and flip the furnace switch back on.
  4. Set the thermostat to call for heat. If the error code returns after 2-3 ignition attempts, the fault requires repair.

Resetting clears the code from the display but does not fix the underlying fault. If the code returns after reset, proceed to the diagnostic steps above.

What Does York Error Code A4 Mean?

The A4 error code on your York furnace indicates a problem with the thermostat wiring. The furnace's control board isn't seeing the correct signals from your thermostat, which tells it when to turn on and off. This can happen if the wires are loose, shorted, or connected to the wrong terminals. Because the furnace relies on these signals to operate safely and efficiently, it will shut down to prevent potential problems. Specifically, the A4 error often points to issues with the 24VAC control circuit. This circuit is responsible for communicating between the thermostat and the furnace's control board. If the board detects an unexpected voltage or a break in the circuit, it will trigger the A4 error and prevent the furnace from running. This is a safety feature to protect the furnace from damage and prevent it from running erratically.

Common Causes

  • Loose or disconnected thermostat wires at the furnace or thermostat.
  • Short circuit in the thermostat wiring.
  • Incorrect thermostat wiring configuration (e.g., wires connected to the wrong terminals).
  • Faulty thermostat.
  • Damaged or corroded thermostat wiring.

Diagnostic Steps

If... Check Expected
A4 error is displayed Measure voltage between R and C terminals at the furnace control board 24VAC
Voltage between R and C is absent or significantly low Check continuity of each thermostat wire from thermostat to furnace control board Continuity on all wires
Continuity is good, but voltage is low Check the 24VAC transformer output voltage 24VAC
Transformer output is good, but still no voltage between R and C Inspect the wiring harness and connections on the control board for damage or corrosion Clean, secure connections

Applicable York Models

Replacement Parts

Part OEM Number Est. Cost Where to Buy
Thermostat — — —
24VAC Transformer S1-02543714000 — Find Part
Furnace Control Board — — —

Related Codes on Same Models

Other codes that affect the same York model families as A4.

Code Description Severity
AF Low Flame Current Critical
R1 Flame Without Call Critical
R2 Pressure Switch Stuck Closed Warning
R3 Pressure Switch Stuck Open Warning
R4 High Limit Switch Open Warning
R5 Rollout Switch Open Critical
R6 Gas Valve Failure Critical
R7 Ignition Lockout Critical
R9 Reversed Polarity Critical