VW Transporter in Limp Mode With a Lack of Power

P0200 – Turbocharger under boost
P0299 – Turbo under boost

We began by scanning for fault codes, which showed P0200 Turbocharger Under Boost. Monitoring the serial data on a road test confirmed the fault was active and revealed low charge pressure. A visual inspection of the intake system found both boost pipes had popped off the intercooler, so we re-secured them. On a second road test the low charge pressure was still present, along with a hissing noise under boost. A smoke test back in the workshop showed both pipes were still leaking and could no longer hold a seal, so they were replaced. With new pipes fitted, the van drove well and the limp mode and engine management light cleared, but the glow plug light then began to flash briefly. A fresh scan showed the fault had returned as P0299 Turbo Under Boost, pointing to a second fault. Further road testing confirmed boost was too low under full load. The N75 turbo control valve and vacuum pipes all tested fine, but exhaust back pressure was too high, causing the turbo to under-perform and likely explaining why the intercooler pipes had blown off in the first place. This pointed to a blocked DPF.