Fires can exist only on a livable or owned business lot. Fires on other lots immediately extinguish.

Each burn cycle there is a 1% chance a fire will ignite a random sim on an adjacent tile, and a 100% chance it will ignite all sims on the current tile. Walkbys are immune to fires from the same tile but apparently not from adjacent tiles.

Firefighters, the Grim Reaper, ghosts and babies are immune to fire and if marked to burn will immediately unmark themselves before the fire starts. Toddlers are confusing. They will not actually burn, but they can apparently die if sitting in a fire. However I believe that is simply an incorrect branch in the code and not intended.

Sims on fire lose 200 points per hour from all motives except environment (plantsims lose water and love plus lose 100 sunshine per animation loop) but motives can't drop below -90. This is because unlike some other death types, motive loss doesn't kill burning sims. Instead the sim has 5-10 animation cycles (randomly determined) and if still on fire at the end, the sim dies. The only hope is for some other sim, including firefighters, to extinguish the burning sim.

So how do fires start? I have found the following, though there may be more.

• Cooking - Each food has a Cooking skill range. The minimum skill is the minimum Cooking a sim needs to prepare it. The maximum skill is 1-4 points above this, depending on the food. Where the sim's skill falls in or out of this range determines the chance of burning the food or causing a fire:
  • <0%: sim can't even attempt to cook the food
  • 0%-40%: 24% chance of burning food, 6% chance of fire
  • 41%-80%: 8% chance of burning food, 2% chance of fire
  • 81%+: 4% chance of burning food, 1% chance of fire

  If that explanation is confusing (and it is), then the short version is "The higher your cooking skill, the less chance of fire. And Lobster Thermidor ALWAYS has a 6% of causing a fire."

• Flambe - Foods like Baked Alaska which require a flambe step do another food failure check, so have two opportunities to create fire.
• Fireplace - Fireplaces have a 1/300 chance each minute of spontaneously starting a fire. This means they will cause fires approximately every 5 hours of continuous burning.
• Fire Jet - Every cycle of flaming (I think) each object adjacent to the fire jet has a 5-15% chance of bursting into flames, depending on the object's flammability. Sims who are teens or older have a 5% chance of catching fire.
• Electrocution - When repairing an object, if the sim is shocked there is a 15% chance a fire will be created in the object's tile. Multi-tile objects will get fires in each tile. If the sim is standing directly on the same tile, the sim will catch fire.
• Train Set - Each time you start the train set, there is a 2% chance it will burst into flame. The operating sim will always be lit on fire, and each tile of the train set will burn as well.
• Spontaneous Combustion - I was skeptical that this was real, but I finally found the code. It's a bit complicated but basically there is a 15 minute window out of every 2-hour cycle where if a teen or older sim is overheated (body temperature over 99) there is a 0.1% chance the tile the sim is in will burst into flame. This will cause the sim to catch fire as well. Needless to say, this is very, very rare even if you try to create the effect.
• Counterfeiting Machine - In each counterfeiting session, the first $80 produced are safe. After that, there is a 0.5% chance for every $4 produced that a fire will start. This means a fire starts approximately every $880. The using sim is not automatically set on fire.

Although lightning lists a 2% chance to cause a sim to catch fire, the actual lightning strike routines never check it for sims.