See Exactly Where Ignition Fails
Follow each stage from inducer to flame proving, so the step that drops out intermittently isn't missed.
System Diagnostics
Find Out Why the Furnace Keeps Locking Out
From short cycling to lockouts after ten minutes of run time, ACLogics tells you what to check next until the cause is confirmed, so the fix sticks.
FAULT SYMPTOM
The furnace fan stays on constantly and will not turn off
CHECK 01
Disconnect the G (fan) wire at the furnace control board. Does the blower fan shut off?
Yes, blower shuts off
DIAGNOSIS
Thermostat or G-wire short. Check for a shorted thermostat wire or a faulty thermostat setting.
No, blower keeps running
CHECK 02
Check the furnace control board status LED. Is it flashing a limit switch error code?
Yes, limit code flashingNo, normal or solid LED
Complete diagnostic tree — every branch, every fault path
Try ACLogics Free — iPhone & AndroidHow a Call Goes
01
Describe When It Fails
During ignition, right after the flame proves, or after several minutes of run time.
02
Rule Out Causes in Order
Get the checks that separate a control fault from venting, gas supply or airflow first.
03
Leave Proof of the Fix
Get a summary of the cause and a report with your readings and the final check.
What You'll Stop Guessing About
Know what failed before you order the part.
Know Which Safety Tripped and Why
Narrow it to the pressure switch, rollout or high limit with guided voltage and continuity checks, before you replace a switch.
Stop Limits Tripping for Good
Match temperature rise to blower performance to find why limits trip under real load.
Frequently Asked Questions
Burners light, then drop out in 3-5 seconds, even after cleaning the flame sensor. What is next?
Check furnace grounding and neutral integrity. Poor ground return can mimic a bad flame sensor by distorting microamp signal to the board.
Limit code appears only after the blower door is installed. Why would that happen?
Door position can change return-air behavior. Recheck filter loading, blower speed tap, and static pressure with the system fully closed.
Inducer starts but pressure switch closes only sometimes. Is the switch itself bad?
Not always. Inspect condensate trap routing, vent termination, and collector box port restrictions before replacing the switch.
Can I jump a limit or pressure switch just to keep the call moving?
No. Bypassing safeties creates risk and hides the true fault. Use controlled diagnostic measurements instead of defeating protection devices.
Know What to Check Next on Your Next Call
Get the checks in order, confirm the cause, and fix it right the first time.