P3138 fault code

Just a quick question about a mate's VW golf gt tdi mk5. He's got a glow plug on and eml. Scanned for faults and got the p3138 Intake manifold Runner control Regulation deviation. Does anyone know if this is the throttle body/intake flap? He's had the original item replaced about 6 months ago but as it wasn't a vag specialist he went to I'm suspecting an aftermarket one has been supplied. He's asked me to have a look tomorrow but knowing how much these things cost i really don't want to tell him to buy another one if it isn't the problem.