P0408

P0408 is the high-side counterpart to P0407 — EGR position sensor B reads above the maximum voltage threshold, typically above 4.8V. Same fault topology as P0406: internal sensor short to 5V reference, external harness short, or mechanical pintle jammed at full-open driving an honest-high reading. Diagnostic ladder: backprobe the B-signal pin at rest, unplug the connector to see if the short clears on the sensor side or stays on the harness side, and verify the supply/ground integrity at the harness. On dual-sensor EGRs, compare A-signal to B-signal on a scan tool in live data — if A reads sensible (0.7V closed, sweeping to 4.2V open with command) and B reads pegged-high, the B sensor is failed independently and the valve assembly typically has to be replaced because B isn't a serviceable sub-component on most platforms. On diesel EGRs specifically (Cummins 6.7L, Powerstroke 6.7L, VW/Audi TDI, Sprinter OM642), soot accumulation between the sensor PCB and the actuator housing can short B-signal to the supply rail through conductive soot films — same diagnosis path, same replacement-only outcome. Verify mechanical pintle freedom before electrical replacement on every P040X position code: a stuck-open valve gives an honest sensor reading that looks like an electrical fault and gets sensors replaced unnecessarily.

Vehicle-specific patterns: 2011-2016 Ford 6.7L Powerstroke throws P0408 from the same soot-contaminated dual-sensor EGR assembly that produces P0407 — replace the assembly, don't bother cleaning. 2007-2014 Cummins 6.7L throws P0408 from internal sensor short on the secondary feedback channel; full valve replacement, ~$500-$900 part. 2010-2014 VW/Audi 2.0L TDI throws P0408 alongside DPF and EGR-cooler codes when the cooler fails internally and routes hot soot toward the sensor — the cooler-and-valve pair replaces together as a job. 2009-2014 Ford 6.4L Powerstroke (earlier generation) throws P0408 from the well-documented EGR cooler failure that floods the intake with coolant and shorts the position sensor — diagnose the underlying cooler before installing a new valve or you'll cook the new one in 500 miles. Estimated repair: $200 (light-duty) to $2,800+ (6.4L Powerstroke EGR cooler + valve + intake cleaning).