black diagnostic box just shows solid red light.

I tried using the diagnostic jobby on my 1992 240, but when i press the button, the led just goes red, even if i dont plug the lead into any of the 8 sockets. Could there be a fault,thinking in the short leadt, or am i doing something wrong.

The correct method for using the diagnostic unit is a bit tricky! I can't remember the correct method but,,, first plug short lead into socket 1,2 etc and then press button for two seconds but not longer than four seconds.and then count the lights as they flash.now I know this isn't correct but I couldn't find the right post for you,all I am trying to say is you don't just plug in the lead or just push the button,there is a set procedure.only ever did it once and it came up no faults.even though my lambda light was on. Regards Brian.

Numpty! totally forgot about just holding it for a few seconds. there's me at the top of the drive holding it in shouting at the car.

thats it as far as i remember.there is a full description and a list of 3 digit codes in the 700/900 series FAQ's on the voc main page.

diagnostic box 

just checked the FAQ's re you having trouble revving over 3000rpm and it mentions failing ecu as a cause.

The various holes are for the different systems , fuel - Ignition - SRS - climate - ABS- etc This socket does far more than just reading codes , there are up to 4 modes The first mode is code reading and erasing the first button press of ONE second reads the code and a second press of ONE second reads the next code in that system etc The other modes do activating components like injectors , idle valve electric fan etc . another mode does reading sensor outputs . on 400 series you program the remote fobs with another mode . mode 1 = one button press for one second mode 2 = 2 button presses of one second intervals etc . you will find the full details and all the codes listed in the Pocket data book for your car available from your volvo dealer .

managed to get it to work, showing me 2 - 1 - 2. Signal missing/faulty from oxygen sensor. could this be a wiring fault, or a dead lambda sensor? will do the volt meter check on it later on.

Glad to hear you sorted the use of the little blackbox! The fault could well be the wire has come off the lambda sensor,easily seen under the car or maybe from in the engine bay.it's on the downpipe right near the bottom of the exhaust.It could also be the unit itself,is the lambda light on on the dashboard?Be aware when ordering a new sensor they do come with two or three wire varieties,check which yours is before ordering,I'm sure other things including the ECU may be at fault but I would start with the cheaper items to try and remedy the fault.If possible borrow an ECU that is known to be good before spending very large amounts of money. Regards Brian.