Why does my furnace light and then shut off a few seconds later?

The board expects a minimum flame-sense microamp signal (flame rectification). Low current shuts the gas for safety. Technicians measure microamps, correct grounding or polarity, verify manifold pressure, confirm flame carryover, and clean or replace the sensor.

Blower runs but air is lukewarm — what should be checked?

Check temperature rise vs. nameplate, verify gas input and burner carryover, inspect the indoor coil, confirm blower speed/profile, and measure external static pressure to rule out airflow restrictions.

Furnace Repair Markham | Same-Day Heating Service

Our Diagnostic Playbook (On-Site, Step-by-Step)

Safety & setup: Gas shut-off verification, vent/intake visual, CO alarm presence, filter/return survey, thermostat and low-voltage scan.

Measurements that matter: Live ignition test; flame-sense microamps; temperature rise vs. nameplate; inducer/blower amperage; pressure/vacuum on switches; control-board fault tree; combustion analysis where applicable.

Options & approval: Findings explained plainly; you get a fixed repair quote with choices.

Repair & verification: Parts installed or settings corrected; we re-run readings to confirm safe, stable operation.

Clean wrap-up: Workspace tidied; results and maintenance pointers reviewed with you.

Pricing — Clear, Predictable, Approved by You

Quotes reflect:

Time to confirm the root cause (not just clearing symptoms),

Parts & labour (OEM or approved equivalent),

Access constraints (attic/crawlspace/mech room, venting corrections),

After-hours demand during extreme cold (disclosed upfront).
You always approve the fixed price before work begins.

60-Second Checklist (Try This Before You Call)

Thermostat: Heat mode; fan Auto; fresh batteries; setpoint above room temp.

Filter: Replace if clogged or too high-MERV for your duct capacity.

Registers/returns: Confirm open and unblocked (esp. in finished basements).

Power: Furnace switch ON; breaker not tripped; nearby GFCI (condensate pump) not popped.

Condensate (90%+): Check that exterior discharge isn’t frozen and the pump runs.

Still cold? Call 416-335-0881 — we’ll guide you and dispatch.

These are the most frequently asked questions about HVAC, Sizing, and More. Please feel free to contact us if you don’t see your question answered below.

General Questions
Mechanical Questions

Why does my furnace light and then drop out a few seconds later?

The board expects a minimum flame-sense microamp signal (flame rectification). Low current shuts the gas for safety. Techs measure microamps, correct grounding/polarity, verify manifold pressure, confirm flame carryover, and clean/replace the sensor.

Why do pressure-switch errors spike on frigid nights?

Iced intake/exhaust, water in tubing/traps, weak inducer, or vent length/slope out of spec. Pros verify inducer negative pressure with a manometer, clear ports, correct slope, and match the manufacturer’s venting tables.

Blower runs but air is lukewarm — what gives?

Check temperature rise vs. nameplate, verify gas input and burner carryover, inspect the indoor coil, and confirm blower speed/profile. Measuring external static pressure rules out airflow restrictions.

My board cycles through different codes — is that normal?

Stacked faults can cascade. A proper diagnostic captures history codes, then validates power/grounding, low-voltage circuits, safeties/sensors, and combustion/airflow before declaring the unit fixed.

What causes pressure-switch faults, and how do technicians diagnose them?

A pressure switch proves safe venting/combustion air before ignition. Faults are often due to:
Blocked or iced intake/exhaust (common in Ajax deep freezes)
Cracked/loose tubing or water in the tube/trap on condensing units
Weak inducer motor or blocked inducer port

Vent length/slope outside spec
Pro diagnosis: Tech measures inducer negative pressure with a manometer, verifies the switch’s make/break with a meter, clears ports, checks tubing/traps, and confirms venting against the manufacturer’s tables. Never bypass a pressure switch—it’s a safety device.

Why is my furnace tripping the high-limit or short-cycling?

Pro diagnosis: Tech measures external static pressure with a manometer, checks temperature rise, verifies blower speed/programming, inspects coil and heat exchanger, and balances airflow. Fixes can include filter right-sizing, coil cleaning, return upgrades, or ECM adjustments.

What does the flame sensor do, and why does the furnace shut off a few seconds after lighting?

The flame sensor confirms flame via microamp DC (flame rectification). If current is low, the control board shuts gas for safety—often seen as a burner lights, then drops out after 2–5 seconds.

Why is there water around my furnace (especially on 90%+ units)?

High-efficiency furnaces produce condensate. Water leaks usually trace to: Clogged condensate trap or blocked drain line Frozen exterior discharge or neutralizer in cold snaps Improper vent/drain slope, sagging vinyl lines, or missing trap prime Failed condensate pump or cracked collector box