Just a simple question for you experts. Which is the correct procedure? Plug in the code reader before starting the engine or after starting the engine. Once before after plugging in the code reader the engine ran poorly and now I can't remember which way I did it.

Thanks guys

 

My code reader is powered by the current in the OBD connector so I plug mine in with the ignition switch off. If I only want to read codes I can turn the ignition on without starting the engine and read/clear codes. If I am looking for diagnostic I can continue on and start the engine and take readings with the engine running.

There are so many makes and models out there, I would recommend you do a web search for your model and try to find an online owners/operation manual.

 

if you have a Droid cell phone, download the Torque Pro app and order an ELM reader off ebay. Great little code reading, real time info. Can be used on MULTI OBDII cars.
this is what I got and it works fine. ELM327 V1.5 Bluetooth OBD2 Diagnostic Tool OBDII Code Reader Scanner For Android

 

I plug mine in before starting the engine, then crank her up. Works like a charm. Have the super duper harbor freight OBDII $100