Ignition Sequence Mapping
Validate each stage from inducer call to flame prove, so missed transitions and intermittent breaks are visible.
System Diagnostics
Furnace Troubleshooting for Normal but Tricky Failures
From short cycling to late-stage lockouts, ACLogics guides your team through sequencing, measurement, and confirmation so fixes stick.
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 on App StoreThe Three-Step Workflow
01
Describe the Lockout Pattern
Enter exactly when the failure occurs: during ignition, after flame prove, or after several minutes of run time.
02
Run Ordered Tests
The assistant prioritizes checks that separate control faults from venting, gas delivery, and airflow faults.
03
Apply the Fix With Evidence
Get a root-cause summary and a report that documents readings, decisions, and final verification.
Built for Precision
Everything you need to diagnose with confidence and accuracy.
Safety Circuit Clarity
Pinpoint pressure switch, rollout, and high-limit behavior with guided voltage and continuity checks.
Airflow-Driven Limit Analysis
Correlate temperature rise and blower performance to find why limits trip under real operating 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.
Ready to Diagnose Faster?
Get the precise sequence and support you need on every call.
