My sister-n-law has a Goodman GUPI075-3 furnace. The blower will not turn off even if thermostat is off. Does not ignite either.
If you cut the electric and turn it off, when you turn it back on, the inducer motor starts and runs for about 3 seconds and then the blower turns on immediately. Never tries to ignite.

There should be a small peep hole on the front panel which lets you look inside to see a code light. The color and blinking of the light is then compared to a chart inside that removable panel, it tell you what's wrong

This furnace looks pretty old. Don't see any lights anywhere. Is this a generic response or are you familiar with this specific model? Thanks

Most systems that use a inducer motor will have a control board that has a diagnostic light that blinks on system failure. You may have to remove the blower door and tape off the door safety switch, make the system call for heat to get the code. I know this is a generic response also but with out knowing if the pressure switch is closing --or if the main limit is open--or if the ignitor is working-- and so on.Its hard to say. If no Light flashing and you can use a volt meter we can walk you through it.