P0406

P0406 is the high-side counterpart to P0405 — the EGR position sensor A signal is reading high, above the maximum voltage threshold the ECM expects, typically above 4.8V on a 0-5V sensor. The fault is signal-wire shorted to the 5V reference internally in the sensor, externally in the harness, or the sensor's wiper has lost contact with its resistor track and is floating at supply voltage. Diagnostic ladder: backprobe the position sensor signal pin at rest and read voltage — anything above 4.5V with the valve commanded closed is a fault. Unplug the sensor connector with the ignition on; if signal voltage at the harness side drops to a normal floating value (typically 2.5V or so on a pulled-up circuit), the sensor itself is shorted internally — replace it. If signal voltage stays pegged at 5V with the sensor unplugged, the short is in the harness between the sensor and the ECM — inspect for chafed insulation against the engine, manifold, or strut tower. Carbon-bound valves can also drive P0406 if the pintle is jammed in the fully-open position and the position sensor honestly reports max travel — pull the valve and verify mechanical freedom before condemning the sensor electrically. On vehicles with a separate EGR position sensor (not integrated into the valve), the sensor is replaceable by itself for $40-$80; on integrated-sensor valves (most GM linear EGR), the whole assembly goes.

Vehicle-specific patterns: 2003-2010 GM 3.5/3.9L V6 and 4.8/5.3L truck V8s throw P0406 from the same integrated-sensor failure pattern as P0405 — the internal wiper loses contact and reads pulled-up high; whole valve replaces. 2000-2007 Chrysler/Dodge 3.3/3.8L V6 minivan throws P0406 from a stuck-open carbon-coked valve combined with an honest high-position reading — clean or replace the valve, the sensor itself is fine. 2002-2009 Ford 4.6L 2V (Crown Vic, Mustang) throws P0406 from a chafed EGR sensor harness against the upper intake — wrap the chafe point and the code clears. 2005-2012 Nissan Pathfinder/Frontier VQ40DE throws P0406 from corrosion in the EGR position sensor connector where it picks up engine-bay moisture — clean pins, dielectric-grease, and verify the code stays gone. Estimated repair: $30 (harness fix) to $250+ (full valve replacement).