how would I check a new pre wired eq for faults
 

Fitted a 3 band eq to my p/j style bass and it only works with the jack slightly pulled out and the eq controls don't seem to alter tone , every thing is new except the pick ups which have always worked fine before hope some one came help thanks in advance.

A lot of pre-amps use a TRS (stereo) jack to connect the battery to the circuit using a mono instrument plug.

If you can, insert a plug an check that the contacts are contacting the plug.

I have plugged a jack in and can see its touching all 3 terminals but as soon as the jack clicks in the sound goes, and it is a stereo jack it came pre wired

All I can suggest is to look at all of the solder joints. Thats all I got.