I forgot to reconnect MAF wires and started the engine ( MAzda3 2005 2,0 l) . The CEL light came on. I have a OBDII code reader, I used it to clear the codes, but it returned the code "ERROR". My reader works fine, I used it on another vehicule after, and it reads the codes allright.

Does anyone have a clue to this ?

 

I have no clue but i know how to fix it. Disconnect - side from battery and turn on your head light. wait 15 to 20 min. turn off your head light connect - side battery that will reset your ECU and drive around in city for like 5 to 10 mile and see.
Good luck

 

Twiggy, not sure you did this............but sometimes if the car is already started and plug in the reader it can return an error. I would try again, but plug in the reader, then start the car.
What is the type(manufacturer of reader)? Is it CAN II bus compatible? Have you used it on this car before?