York Error Code AF: Low Flame Current

Severity
Critical
Type
digital display
Estimated repair cost: $75 — $200 (part only)
Warning: This error indicates a problem with the furnace's flame, which could lead to a dangerous buildup of unburned gas. Shut down the furnace immediately and call for professional service.

How to Fix York Code AF

  1. Step 1: Turn off the furnace. Locate the furnace's power switch (usually on the side of the unit) and switch it to the 'off' position.
  2. Step 2: Check the gas supply. Ensure that the gas supply valve to the furnace is fully open. It's typically a yellow handle running parallel to the gas pipe when open.
  3. Step 3: Reset the furnace. Wait at least five minutes, then turn the power switch back on. If the error reappears immediately, proceed to the next step.
  4. Step 4: Call a qualified HVAC technician. Due to the potential safety hazards associated with gas furnaces, it's best to have a professional diagnose and repair the issue.

How to Reset York Code AF

  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 AF Mean?

The 'AF' error code on your York furnace indicates a low flame current. This means the furnace's control board isn't detecting a strong enough electrical signal from the flame sensor. The flame sensor is a safety device that proves the burners have lit. If the sensor doesn't detect a proper flame, the furnace should shut off the gas valve to prevent a dangerous accumulation of unburned gas inside the unit. This error can occur intermittently or cause a complete lockout of the furnace, preventing it from heating your home. Because it relates to gas combustion, it's important to address this issue promptly and safely.

Common Causes

  • Dirty or improperly positioned flame sensor
  • Weak gas pressure or a problem with the gas valve
  • Poor electrical connection to the flame sensor or control board
  • Faulty flame sensor
  • Cracked heat exchanger (can dilute the flame)
  • Control board malfunction

Diagnostic Steps

If... Check Expected
Furnace is attempting to start, inducer motor running Measure flame sensor microamps (uA) during burner operation Typically 2-10 uA DC. A reading significantly below this indicates a weak flame signal.
Flame signal is weak Inspect and clean the flame sensor with steel wool or fine emery cloth. Sensor is clean and free of deposits. Re-test flame signal.
Flame signal still weak after cleaning Measure gas pressure at the gas valve with a manometer Proper gas pressure according to the furnace's rating plate (typically around 3.5" WC for natural gas).
Gas pressure is within range Check the flame sensor wiring for continuity and proper connection to the control board. Wiring is intact and connections are secure.

Applicable York Models

Replacement Parts

Part OEM Number Est. Cost Where to Buy
Flame Sensor 024-27675-000 — Find Part
Gas Valve — — —
Ignition Control Board — — —

Related Codes on Same Models

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

Code Description Severity
A4 Improper Thermostat Wiring Warning
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