Isothermic Transducer is not a redirect
Isothermic Tranducer is a triggered effect that deals damage. Like any other instance of damage, it will be boosted +2 by the two Obsidian Fields.
The amount of damage that Absolute Zero (AZ) deals is calculated from the amount of damage AZ takes - that is, the amount of life he lost. The initial number stated by the ability dealing damage before increases and decreases doesn't matter, only the final number.
So if AZ tries to deal himself 1 Fire damage, it will be boosted +2 and he will take 3 Fire damage. Then Isothermic Transducer will react and he will deal 3 damage +2 and deal 5 cold damage to the target of his choice.
(If he choose to deal that damage to himself and he has his Null-point Calibration Unit in play, he will heal 5 hp)
There are true redirects in the game, such as Visionary's Decoy Projection, Baron Blade's Elemental Redistributor, and Legacy's Lead from the Front. For a redirection effect, once you have determined who the damage is finally pointing at, recalculate the damage increases and decreases from scratch between the source and the final target.
For example, Obsidian Field is in play, Legacy had Fortitude (-1 damage taken) and Lead from the Front, and Tempest has Localized Hurricane (+1 damage taken) in play. Baron Blade tries to deal Tempest 1 damage. That would be 3 damage (+1 from the Hurricane, +1 from Obsidian Field), but Legacy redirects that damage to himself. So we recalculate from scratch as 1 (base damage) + 1 (Nemesis with Baron Blade) - 1 (Fortitude) + 1 (Obsidian Field) = 2 damage taken. The fact that Tempest would have taken 3 damage is irrelevant.*
- Well, mostly irrelevant. There are a few redirects that only trigger for certain amounts of damage. But once the redirect has been triggered, it doesn't matter if the final calculated damage is different from the amount that triggered the redirection.