Check engine light came on 1996 acura slx code reads p0401

acura transmissions are only good for about 100k miles

if it has a transmission dip stick , in through the dipstick tube
if no dip stick it is a sealed system and only accredited transmission shops can check and fill the transmissions

Check eBay for a used factory manual set. We do not have all these diagrams, they are not shared on the web, only bits here and there.

have the fault codes read. Check that the engine immobiliser is not the problem.( Battery in key flat)

Maybe need to change the spark plugs

Any vehicle that backfires is not getting the correct ratio of air to fuel. your ignition timing could be incorrect, or could be running rich with fuel

The check engine light comes only when the computer control unit (called the PCM) senses a fault in the electronic controls of the engine and/or the emission system, there are about 200 possible faults that turn the light on, to know what has developed a problem you must have the system tested, this is called an OBD2 fault code test, Kragen and Autozone parts stores do this testing for free.

i wold check the knock sensor on the motor to make sure it hooked up, or bring it to your local parts store and have the check the codes

is the filter tight? did someone forget to remove the old filter O ring gasket? that is more common than u think, check it before u spend at least $100-150 on the housing.

you will see six wires running from the engine to a central box.....trace them from the box to the engine and at the end of each wire will be a spark plug. You may have to remove a hard plastice shell from the the top of the engine.
screw drives and deep socket are all the tools you should need

Good Luck