1995 Toyota Camry Engine Missing

My car just recently started to run awful, it is idling erratically and when I put it in drive it idles very low (almost shuts off but doesn't). Car has new spark plugs (old plugs showed no oil), new rotor cap, used another distributor cap (old but working), plug wires are ok, performed compression test ( all cyl at around 180), car not blowing white smoke, no oil in radiator, no water in engine oil, 3 months ago replaced radiator and thermostat, I have been losing a little oil due to rear main seal (needs replacement). The check engine has recently just turned on, code 401, check and filter and vacumn hoses ok, I believe that I hear an unusual hissing noise, I have check the throttle sensor? Loosen both screws and try to adjust, it can cause a change but car still missing, can you help please! Thanks in advance, fm