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faultcode:  16785    P0401     Exhaust Gas Recirculation (EGR)    Insufficient Flow

This faultcode is 'Generic OBD' (for each car manufacturer the same meaning / discription). The symptoms-causes-solutions on this page are specifically voor VAG-cars; they can slightly differ for other car brands.

Take care: the possible symptoms-causes-solutions on this page are not written by or for the VAG concern, they do come from countless other public sources. So completly and without mistakes cannot be guaranted. Keep thinking critically.

Possible symptoms:

Maybe drivability problems like knock tendency when the engine must work harder No 'drivability problem' at all Emission Warning Light is on MIL (Malfunction Indication Light) is on Other symptoms are also possible

Possible causes:

The EGR has insufficient flow the EGR valve is faulty (leaking, doesn't open enough) the EGR valve doesn't open because of lack of vacuüm narrowing of / carbon deposits on the exhaust channel EGR electrical circuit: short to Ground or Plus DPF EGR sensor damaged or wrond installed Control Module faulty (not likely but not impossible)

Possible solutions:

Clean the entire EGR Use a vacuümpump and pull the EGR valve open and check the rpm. Opening the EGR valve the rpm should change. Check all wiring and connections to/from the (DPF-)EGR Check (measuring blocks) / replace the (DPF-)EGR Check (measuring blocks) / replace the (DPF-)EGR sensor Check at the dealer for a software-update Check (measuring blocks) / replace the Control Module

Extra info:

The EGR system is being monitored any time the car drives. When the (DPF-)EGR sensor detects that the flow is to small this faultcode is set When only the EGR system is replaced the problem will be back soon. The (DPF-)EGR sensor is most probably the cause In a Golf-4 1.4 lt 16v this is a common fault. You can reset and clean whatever you want, the faultcode will come back. There is only one solution: replace the whole EGR (including the sensor) by a new modificated one and check at the dealer if there is a update for de Control Module For a Golf-4 1.4 16v 2002 (Marelli ECU) there is no tuner (at all) who can modify the Control Module in bypassing the EGR