My mom's oil furnace runs for about 1-3 minutes and shuts off

The fan normally runs for a few minutes or so after the gas shuts off.

This removes the heat from the furnace and transfers it into the living space instead of just letting it possibly cause overheating of the furnace until it dissipates. If it is a fan that blows outside, that fan also needs to run for a few minutes to clear exhaust gasses from the furnace - to keep them from dissipating into your home.

On a conventional furnace of today, your furnace may have a flame sensor which is typically located in front of an end burner opposite the ignitor. It is connected by 1 wire. This metal rod needs to be removed by 1/4'' screw in most cases and cleaned. Clean with steel wool or even a clean greenie scrubber will work. Wipe clean and re install. Many problems can cause these symptoms, but this is a very common one.

When you say 'older furnace' I am assuming old enough to where there is no circuit board. They will usually have a thermally activated 'fan-limit' switch with a bi-metal sensing device. Most older furnaces had either a Honeywell or a White-Rodgers controller and they look almost identical. With the furnace door off, look towards the top of the furnace vestibule (above the flame area) for a rectangular box approximately 3 1/2" wide by 5" long. Tap the side of the box with a screwdriver and if the thermostat setpoint has been satisfied and the gas burners have been off for more than three minutes, the fan should then shut off. The switch contacts are either stuck together OR the bimetal sensing device is weakening from thousands of cycles.

If it continues to run, squeeze the sides of the 'lid' of the box and remove the cover. Inside you'll see a round moveable dial with three pointers, marked 'fan off, fan on, and limit'. The 'limit' should remain at or near the 200 degree setting, the 'fan on' can be anywhere around 140 and the 'fan off' can be anywhere near 100 degrees. It is the 'fan off' position thats giving you the grief.

Turn the thermostat up again and sit there and watch the dial do its thing and yo'll understand better how this control works. Watch it rotate past the 'fan on' setting and observe the fan turn on. Turn the thermostat below room temperature and watch the gas shut off and the dial rotate back towards the 'fan off' setting. If it hangs up again before twisting past the setpoint, you can slightly increase the setting to compensate for the weakening bi-metal element. But just a tiny bit, not a great amount or you will have the fan shutting off with a lot of residual heat still in the heat exchanger. The idea of the delay in shutting off the fan is to remove all that residual heat and transfer it to the house.

There are other brands that utilized 'timed on' fan controls, and there are a few other variances of controllers but what's described above should cover the vast majority of older furnaces.

Remember there is ELECTRICITY inside the fan control and any adjustments should be done with the power OFF !!!!

see the following steps:
f the burner doesn't run, set the thermostat a few degrees higher than normal to see if it comes on. If it still doesn't come on, check switches, and breakers and fuses (see the Electrical Service Panel Fix-It Guide). If the unit has a reset button, press it. Also oil the motor at any oil ports (see below). If the unit doesn't want to start and run, first check the c to make sure that the ignition is getting power. If there is power, check the built-in safety controls (see below) that may turn the system off if they perceive problems.
If the burner cycles too often, replace the filter (see below). Also, oil and adjust the blower (see the Forced-Air Distribution Fix-It Guide).
If the burner runs but won't fire, make sure the oil valves are open and that there's oil in the tank. Check the tank level with a clean stick.
If the burner smokes or squeals, shut off the unit, let it cool, and fill the oil cups. Recheck them after the motor has run for an hour.
If the chimney smokes even after the flue has warmed up, the unit is wasting fuel; call for professional repair.
Safety Controls
Oil furnaces and other heat systems include safety devices that monitor operation and turn off the unit if something goes wrong. In some cases, the safety device can be the problem.
If you encounter an operating problem with an oil furnace, first reset the safety and try again.
If the burner kicks off again, shut off all power at the electrical service panel ; the burner motor and ignition may be protected by separate fuses or breakers.
If the sensor has a photocell, wipe it with a clean rag or tissue and see if the furnace starts.
If the safety is a stack switch mounted on the flue, remove the screw holding the unit to the stack, slide it out, and wipe off the sensor.
If the furnace won't start after three tries, seek professional assistance. Unburned oil can accumulate in the combustion chamber and "flash back."

We need to get a few details cleared up:

Is it the Furnace (Oil burner) that is running, (on idle you called it..)
or is it the FAN that is running?
What are you tapping, that shuts off what?
Is the fan that blows the air around the home shutting off, or the burner?
(When the burner is running, there is a loud roar!-- the fan should be much quieter..)

If it is the fan that is running,-- I might ask, is the air coming from the registers too cold?-- for with the fan running, you are getting all the heat from the oil that was burned. But naturally, you don't want it to be uncomfortable.. So-- you can open the cover on that Fan control switch, and raise the temperature at which the fan turns ON and OFF..

Let us know what you find.

Mack B

check for dead animal down furnace stack

if the unit has a flame (gas valve opening)the problem may be the fan going bad or fan relay -- sounds like unit going off on high limit control

Could be a defective thermostat in the plenum.
When a gas furnace shuts down the fan(blower) will run until the plenum cools to the safe range. Usually 1 and 1/2 minutes.
Intermittent operation is a defective thermostat or relay.

That is an old style t-stat i would replace it with a more up to date model. Your problem may be in the limit switch. But yes to answer your question most fans can be adjusted just depends on if it has a manual limit that looks like a silver box on the interior of your furnace or if it has a mother board control system.