HVAC Technician Academy
Learn how to diagnose burners that ignite and then shut off by testing flame rectification, microamp DC signal, burner grounding, flame contact, wiring, and control-board response.
One common cause is weak flame proving. The burners ignite, but the control board does not receive a strong enough flame-rectification signal, so it closes the gas valve for safety. The root cause can be a dirty flame sensor, poor grounding, weak flame contact, wiring problems, burner contamination, or a control-board issue.
The furnace does not keep the gas valve open just because the burners ignited.
The control board must verify that flame is actually present.
If flame is not proven within the allowed ignition period, the board normally closes the gas valve.
Many modern gas furnaces use flame rectification to prove flame.
The flame itself becomes part of a very small electrical circuit.
The control board applies an AC signal to the flame-sensing circuit, and the flame allows a very small DC current to flow.
That signal is typically measured in microamps DC.
One microamp is one-millionth of an amp.
Because the flame signal is so small, seemingly minor problems can weaken it.
Examples include:
Where the furnace manufacturer supports the procedure, flame signal is measured by placing the meter in series with the flame-sensor circuit.
The meter must be set to the appropriate DC microamp range.
The exact test arrangement and acceptable signal depend on the furnace and control board.
Different ignition controls have different flame-signal requirements.
A technician should compare the measured signal with the manufacturer’s required or recommended range.
The stronger lesson is:
Oxidation or contamination on the sensor rod can weaken flame current.
A common symptom is:
Cleaning may restore a weak flame signal if contamination is the actual cause.
But repeated flame-sensor problems should trigger inspection of the entire flame-proving path.
Flame rectification depends on a complete electrical path through the burner assembly and furnace ground.
Poor grounding can produce weak or unstable flame signal even with a clean sensor.
Check:
A perfectly clean flame sensor cannot prove flame if the burner flame does not properly engulf it.
Possible causes include:
Observation can reveal valuable clues.
Confirm:
Inspect the flame-sensor wire from sensor to control board.
Look for:
If the flame is stable, grounding is good, the sensor and wiring are correct, and measured flame current meets manufacturer requirements, yet the board still fails to recognize flame, the control board becomes more suspicious.
The board should be near the end of the diagnostic path — not the beginning.
Technician Case File
Inducer: normal.
Pressure switch: proves normally.
Igniter: operates normally.
Gas valve: opens.
Burners: ignite cleanly.
Flame signal: weak and unstable.
Flame sensor is heavily oxidized.
Sensor is serviced according to manufacturer procedure.
Flame signal increases and burner operation remains stable.
Lesson: The ignition sequence was working. The failure occurred at the flame-proving step.
Flame sensor has already been replaced.
Furnace still lights and shuts off.
Flame signal: weak.
Inspection finds poor burner-ground continuity caused by corrosion at the burner assembly.
Ground path is corrected.
A new flame sensor could not overcome a poor electrical return path.
Burner flame: stable.
Sensor: clean and correctly positioned.
Grounding: verified.
Flame microamp signal: meets manufacturer requirement.
Control still fails to recognize flame reliably.
After the entire sensing circuit is proven, the control board moves much higher on the suspect list.
1. Verify the furnace reaches burner ignition.
2. Observe whether all burners ignite correctly.
3. Confirm flame fully contacts the sensor.
4. Inspect flame-sensor condition.
5. Inspect sensor wiring and terminals.
6. Verify burner and equipment grounding.
7. Measure flame current using the correct microamp DC procedure.
8. Compare the signal with manufacturer specifications.
9. Correct burner, grounding, sensor, or wiring problems as proven.
10. Consider the control board only after the rest of the flame-proving circuit has been verified.
| Pattern | Possible Direction | Next Check |
|---|---|---|
| Burners light then shut off quickly | Flame proving problem | Measure flame signal |
| Weak signal + dirty sensor | Sensor contamination | Service and retest |
| Weak signal + clean sensor | Ground, flame contact, wiring | Trace complete sensing path |
| Flame misses sensor | Burner/combustion issue | Inspect burner and gas conditions |
| Good flame signal + board drops gas valve | Possible control-board issue | Verify board inputs and manufacturer sequence |
Do not replace a flame sensor because the furnace lights and shuts off. Measure the flame signal and prove the entire flame-rectification circuit.
1. What electrical quantity is commonly used to evaluate flame signal?
2. Can poor grounding cause a weak flame signal?
3. Does a clean flame sensor guarantee good flame proving?
4. What symptom commonly points toward flame-proving failure?
5. When should the control board become a stronger suspect?
1. DC microamps.
2. Yes. The flame-rectification circuit depends on a proper electrical return path.
3. No. Flame contact, grounding, wiring, burner condition, and control behavior also matter.
4. Burners ignite normally and then shut off within a few seconds.
5. After flame, sensor, wiring, grounding, and measured flame signal have all been proven correct.
Return to the heating and combustion training hub.
Furnace Sequence →See where flame proving fits into the complete heating sequence.
Furnace Repair →Review the local furnace diagnostic service resource.
Flame-sensor testing involves live ignition controls, fuel gas, combustion products, hot surfaces, and potentially hazardous voltage. Only qualified personnel should perform live testing. Follow manufacturer procedures, use properly rated instruments and PPE, and investigate delayed ignition, rollout, abnormal flame, or combustion concerns before continued operation.
Flame-sensor contamination, grounding, burner conditions, wiring, and control-board faults can all produce similar symptoms. Measure the flame signal before replacing parts.
Furnace Diagnostic Service HVAC Technician Academy