I put new spark plugs in the MR2 the other day along with a new air filter ,simple job and didn't take long. Went for a drive later and after five miles the engine management light came on, back home I go and have a check that I haven't done anything stupid or disturbed something else , all looks fine but the light is still on , in desperation I put the old plugs back in but the light was still on , time to get the OBD reader out , turns out to be a faulty oxygen sensor, and nothing to do with the previous work , one new sensor and a reset and all is OK, then change the plugs again . Don't you love cars?
:) the joy of motoring at least it was simply and a cheap ish way to get sorted :)