Short cycling(?) HVAC furnace - 15 min cycles ?

This concerns an old (1990s?) Carrier gas-powered forced-air HVAC with fan/ventilator

Product 58MCA120-13120

Model 58MCA120-20

Series 131 No pilot light

The first time I used heating this Winter (two days ago) it ran for 40 minutes and produced hot air normally, but then shut down abruptly(air stopped, rumbling stopped).
After 1 minute, it started rumbling again, showing that new cycle began.

Since then, every single time that I activate heat on thermostat---
Heating system starts up, rumbles for 5 minutes, no flame appears, and shuts down without emitting any hot air.
Silence for 20-30 minutes.
After this pause, it starts up again, rumbles, produces flame within unit, emits hot air for 15 minutes before shutting down.

From my research I take it that either exhaust pipe is blocked (unlikely, as it ran for 40 min straight on first day), or that safety switch and/or the flame sensor is misoperating.

Apologies for poor photos: Can anyone guide me?
Thank you in advance.