In extremely rare cases, the transistor inside the DME that drives the fan output fails. This is more common on early N54-equipped 335i cars if the wiring harness short-circuited to ground. However, 95% of D98E codes are not the DME.
Corrosion or damage to the wiring harness leading to the ABS/DSC unit can break the communication signal. Technicians often inspect the DSC valve relay and wiring harness for integrity.
The D98E was equipped with a Garrett T15 turbocharger. Unlike some entry-level diesels of the time which were naturally aspirated, the D98E utilized forced induction to provide usable torque. In higher-spec trims, such as the "tds" models, the engine utilized an intercooler to lower intake temperatures, thereby increasing air density and power output.