How to check engine trouble code

I have an 1989 nissan maxima 3.0l with problems trouble codes came up its 12 34 what is it ???? Thank you for your time

You can still pull codes without a scan tool on this vehicle using either a analog volt meter or by just watching the check engine light

to use the mil...
turn ignition on
connect a jumper wire between the STI and sig rtn of the DLC

to use an analog meter
set vom @ 0-15 dcv range
connect positive lead of vom to bat+
connect negitive vom lead to sto of dlc

with either method you need to make sure the engine is at normal temp before starting tests

start the engine and watch your outputs
once you receive the pass code then push and release the brake pedal, turn the steering wheel about one revolution and press/release the OD button if you have one and finally snap the throttle

assuming you have done everything correctly, you should hear some stuff happen under the hod and you'll start getting you codes

all this being said, it is a lot easier, especially if you've never done this before, just to go to an auto parts store... most will pull codes for you for FREE!

Unless your Check Engine light is on - you will most of the time not get a code. You can take your car to one of the after market parts houses like -Auto Zone or Advance Auto and Advance I know will check for free. There is a hook up for their machine on your car. Before that - try unhooking your negative side battery post - wait about 20 minutes or longer -30 or so and then reattach and run the car for about 25 minutes and see if things don't right themselves. If you need an oil change -DO IT NOW.
While I have your attention please take down the vin number and call the dealer - ask them about any SAFETY RECALLS or RECALLS or SERVICE BULLETINS.