Error codes/ scan tool

Hey fellow volvians had my c30 close to a year been one of my fav cars, have done few small mods so and no troubles ( pod, downpipe, high flow cat, springs just the beginners stuff) yesterday i thought i would use the old air intake and extend it with duct right up to the pod for more cold air as summer is coming also in the process move the computer to a more cooler aswell. Went for a small drive all g, then went for a boot up the freeway took off at the lights hit 3rd went into limp mode, pulled over checked the engine bay all good restart the car no limp mode yay the on the way get orange check engine. Today discounted the battery to reset everything again all good boot through the hills everything is sweet, then just a small trip to the shop orange engine light back on and lack off power, looking on to try read codes without scanner not getting much luck. So i want to know is there a proper way to read engine light codes without a scanner or should i go buy one and witch one, thanks regards volski