Which Check First Gas Pressure Vs Flame Sense No Heat Decision Tree

Why this matters

On a gas furnace no-heat call where the burners light and drop out, both gas pressure and flame sensing can be at fault, and the order you check them in changes what you can trust. The deciding factor is whether the burner flame is actually established and stable when it drops, because flame-sense diagnosis is only valid against a real, properly sized flame. A weak or lifting flame from low gas pressure produces a low or erratic flame-rectification signal that looks exactly like a dirty flame sensor. Clean the sensor on a furnace that is actually starving for gas and it still drops out, and you have masked a pressure or combustion fault. This tree sequences observation and gas-pressure verification ahead of flame-sense measurement so the microamp reading means what you think it means.

Symptom presentation

The classic presentation is ignition followed by flameout: the igniter glows, the gas valve opens, burners light, and a few seconds later the control closes the gas valve and may retry or lock out. This is a flame-proving failure, and it has two main causes that present identically: the control is not seeing adequate flame (sensing fault) or the flame itself is inadequate (gas, combustion, or burner fault). The principle is observe the flame before measuring the signal: a healthy flame with a low signal is a sensing fault, a poor flame with a low signal is a combustion fault.

Quick checks

1. Watch the burners through the sight port during the trial: do they light fully and burn steady blue, or light weakly, lift, roll, or flutter before dropout?
2. Confirm the ignition sequence: igniter, valve open, flame established, then flameout timing relative to the trial-for-ignition period.
3. Check the obvious combustion-air and venting items: blocked flue, restricted combustion air, a tripped pressure switch chattering on a marginal inducer.

Isolation tree

Step 1: Observe the flame first

Direct observation gates the whole tree:

• Strong, full, steady blue flame that still drops out: the flame is good, so the failure is in sensing. Go to flame-sense measurement (Step 3).
• Weak, small, yellow, lifting, or rolling flame: the flame itself is inadequate. Do NOT blame the sensor yet. Go to gas-pressure and combustion verification (Step 2).
• Burners do not light at all: this is an ignition or gas-delivery problem upstream of flame proving, a different path (igniter, valve power, gas supply).

Step 2: Verify gas pressure and combustion when the flame is poor

With a manometer on the manifold and inlet taps:

• Inlet (supply) pressure: confirm it is within the range on the rating plate (commonly around 7 inWC for natural gas, higher for propane); low inlet pressure starves every burner and points to supply, regulator, or undersized piping.
• Manifold pressure: confirm it matches the rating-plate value for the fuel (commonly about 3.5 inWC natural gas); a low or unstable manifold reading points to the gas valve or regulator.
• Combustion air and burner condition: a blocked flue, restricted combustion air, or fouled/rusted burners produce a poor flame that drops out regardless of sensor condition.

A flame starved by low gas pressure cannot produce a stable rectification signal, so fixing pressure first is mandatory before judging the sensor.

Step 3: Measure flame sense only against a verified-good flame

With a confirmed full, steady flame, put a meter in series with the flame-sensor lead in microamp DC mode:

• Read the flame-rectification current during the trial. Compare to the manufacturer's minimum (often roughly 1 to 6 microamps depending on the control; many controls drop out below about 0.5 to 1 microamp).
• A signal below minimum on a good flame points to a coated or oxidized flame rod, a poor ground, a cracked porcelain, or a marginal control.
• Clean the rod with a non-abrasive pad, confirm a solid burner-ground bond, and re-read; the signal should rise well clear of the dropout threshold.

Why a poor flame fakes a sensor failure

Flame rectification works because a flame conducts current in one direction only, and the control reads that small rectified current as proof of flame. The size and stability of that current depend on the flame fully and steadily enveloping the sensor rod. A flame starved by low gas pressure is small, may lift off the burner, and may flutter, so the rod is intermittently or partially in the flame and the rectified current drops below the control's threshold. The control reads inadequate flame and shuts the gas valve, identical to what a coated sensor produces. The only way to tell them apart is to confirm the flame itself is full and steady before trusting the microamp number, which is why observation and gas-pressure verification precede the sensor measurement.

Confirming diagnosis

• Sensing fault: full steady flame, low microamp reading that recovers after cleaning the rod and confirming ground, furnace then completes the heat cycle.
• Gas/combustion fault: poor flame correlated with out-of-range manifold or inlet pressure or a restricted combustion path, with the flame and signal both normalizing after the pressure or airflow correction.

Remediation

• Sensing: clean or replace the flame rod, restore a clean burner ground, replace a cracked sensor or a marginal control board.
• Gas pressure: correct the inlet supply (regulator, piping, supply pressure), set the manifold pressure to the rating plate, and verify with a manometer.
• Combustion: clear the flue and combustion-air restriction, clean or replace fouled burners, confirm the inducer and pressure switch operate correctly.

References

• NFPA 54 National Fuel Gas Code (ANSI Z223.1)
• ANSI Z21.47 / CSA 2.3 Gas-Fired Central Furnaces
• NFPA 70 National Electrical Code, Article 725 Class 2 Control Circuits
• UL 60730-1 Automatic Electrical Controls for Household and Similar Use