P0409: EGR Sensor A Circuit

What does P0409 mean?

Exhaust gas recirculation (EGR) routes part of the exhaust back into the intake to lower combustion temperature and NOx. EGR sensor A is usually the valve position sensor (on older Fords the DPFE pressure sensor). P0409 is its general circuit code, for a missing or implausible signal that the module does not classify as low or high. No manufacturer document available to us uses it – GM, Ford and Saab report sensor A as P0405/P0406, Ford intermittent faults as P046D – so the description follows CARB, which requires input circuits to be checked for continuity and range. EGR is usually switched off.

The code tells you which monitor failed, not which part is broken. Test before replacing parts.

Symptoms

• Check engine light on
• EGR switched off
• Otherwise usually unremarkable

Common causes

• High Connector or wire of the position sensor open or loose
• Medium EGR valve with integrated sensor faulty
• Low 5 V reference or ground disturbed
• Low Engine module

Likelihood is a general estimate across vehicles; your make and model may differ.