Game is quite glitchy but once you get past it its not that bad.
Found that the reason codes don't work (especially on the first level or for saves with 3 lives) is because letter J cannot be inputted or causes an error.
Apart from that here is the decoded the save system-
First 2 letters are the level (JH/TR/OP/FN/AZ/PB/VT/ER/WS)
Next 2 are life amount (WM=7, XD= 6, LA= 5, BY = 4, FJ=3, LK = 2, GT = 1) ***
Next is fire amount (Z=0, D=1, L = 2, E=3, ?, I=5, C=6, X=7, F=8).
Next 1 is shield (V= no shield, W=Shield).
Last letter is speed (S=No speed, F = speed).
*** Contrary to what the game says, you can go over 5 health. Code XD gives 6, WM gives 7 health.
Tl;Dr
Use _ _ LAFWF
To start any level with 5 health, 8 fire, shield and speed (e.g. TRLAFWF for third level)