A gas furnace that lights, runs for a few seconds, then drops out often has a flame sensor problem rather than a gas problem. The sensor is a metal rod sitting in the burner flame. It does not measure heat. It completes a small circuit through the flame, and the control board reads the current that passes as proof that the burners are lit. When that proof is weak, the board closes the gas valve, usually within seconds of ignition, and the cycle repeats until the control locks out.
What the meter is actually reading
Flame rectification works because the flame passes current in one direction. Many boards send a low voltage to the sensor and measure the small DC current that returns through the flame. That current is measured in microamps. The minimum signal the board needs varies by manufacturer, so the number printed on the board label or in the service manual is your reference, not a figure from a forum post.
A weak reading does not automatically mean a bad rod. Oxide buildup, a loose connection, a cracked ceramic insulator, or a burner flame that sits in the wrong place can each lower the signal. The rod is simply where the problem shows up first.
Take the reading the right way
Set a multimeter to DC microamps. Shut off power to the furnace and the gas supply as the manufacturer directs. Disconnect the sensor wire from its board terminal. The meter goes in series, so one lead attaches to the wire and the other to the terminal you just freed. Restore power, call for heat, and let the flame settle for about thirty seconds before you record the value. Some meters have a fuse that a stray voltage reading can blow, so confirm the leads are in the microamp jack before you start. If your board has a test point or your furnace has a dedicated microamp adapter, use that instead.
Log before you clean
Record the reading first, then clean, then re-read. Use fine emery cloth or fine steel wool on the rod and skip any abrasive that leaves residue behind. Wipe the rod with a clean dry cloth, avoid bending it, and reseat it so the tip sits in the flame the way the original did. Write down the date, the outdoor temperature, the reading at ignition, the reading after thirty seconds, and whether the cycle held. Keep that sheet near the furnace.
The log tells you more than a single number. If the reading jumps after cleaning and then slides back over a week or two, the rod is probably not the root cause. Deposits are returning from something upstream, such as a burner that is not lighting evenly or combustion air that is not where it should be. In that case the sensor is reporting a symptom, and the cleaning only bought time.
When to stop and call service
Stop working and call a licensed technician if the control locks out repeatedly, if you smell gas, if the flame looks lazy, yellow, or tends to lift off the burners, or if you see signs that flame is rolling out of the burner area. Those conditions involve combustion and gas safety, and a cleaned rod does not address them.
Why the paper trail matters
ACHR News reports that SMART Local 49's new training center in Lubbock is "already drawing strong interest" and expanding apprenticeship opportunities for the local trade, as described in its coverage of the Planting Roots piece on SMART Local 49 in West Texas. That is useful background for homeowners who expect to hire technicians over the coming years. Whatever the training pipeline looks like, a technician arriving at your door will work faster with a dated log of readings than with a description from memory.
If you are tracking this for an older parent who lives alone and depends on that furnace, the heat is part of the household plan, not just a repair item. A before-we-talk checklist for families considering home care can help you decide who checks the log, who calls for service, and who has access to the house when a technician needs to come in.