Exhaust Gas Recirculation Sensor “A” Circuit High

What P0406 means

The EGR system routes a metered amount of inert exhaust back into the intake to lower peak combustion temperature and cut NOx. To verify the valve actually responds, the PCM reads an EGR feedback sensor — on this platform the differential pressure feedback EGR (DPFE) sensor that is integrated into the EGR System Module (ESM). That sensor is fed a 5-volt reference (VREF) and returns a signal (DPFEGR) proportional to EGR flow and valve position, referenced to a signal-return ground (SIGRTN). P0406 sets when the averaged DPFEGR voltage climbs above the maximum the PCM has calibrated for it. A DPFEGR PID reading above roughly 4.5 V with the ignition on, engine off or running, indicates a hard fault. Because the feedback value is pinned high, the module can no longer trust the EGR reading, so it disables or limits EGR and turns on the malfunction indicator lamp. P0405 is the mirror-image code for the same circuit reading too low.

Symptoms

• Check-engine / malfunction indicator lamp on with P0406 stored
• DPFEGR (EGR sensor) PID reading pegged high — above about 4.5 V with the key on
• Rough or slightly unstable idle if EGR ends up disabled or stuck open
• Light spark knock or pinging under load as EGR dilution is lost
• Elevated NOx and a failed emissions inspection because the EGR monitor has flagged a fault
• Often little obvious driveability change, since the fault is in the feedback circuit rather than the valve mechanism

Common causes

• Open in the DPFEGR signal circuit between the sensor and the PCM, which lets the signal drift or bias high
• Open in the SIGRTN (signal-return/ground) circuit, removing the sensor's reference and driving the reading high
• VREF (5-volt reference) shorted to voltage, pushing the signal supply above normal
• DPFEGR signal wire shorted to voltage or to the VREF/battery circuit
• Damaged or failed differential pressure feedback EGR (DPFE) sensor within the EGR System Module
• Corroded, spread, or backed-out terminals at the EGR sensor / ESM connector

Severity & driving advice

Severity: Moderate — Usually drivable, but EGR is typically disabled, so expect possible light knock or rough idle and a guaranteed emissions-test failure until fixed.

Can I drive? You can generally keep driving short-term, but with EGR disabled the lost dilution can cause light knock under load, and the code will fail an emissions test. Repair before an inspection or heavy use.

Diagnostic approach

1. Scan and confirm the fault — Retrieve P0406 along with any companion codes (P0405 for the low case, or EGR flow codes) and record the freeze-frame data. Watch the DPFEGR / EGR sensor PID: a reading greater than about 4.5 V with the ignition on, engine off or running, confirms a hard high-side fault rather than an intermittent one.
2. Inspect the sensor and connector — Unplug the DPFE sensor / EGR System Module connector and look for corrosion, spread or backed-out terminals, and chafed harness near the module. A poor ground or open at the connector alone can bias the signal high, so repair any bad terminal before deeper testing.
3. Check the 5-volt reference (VREF) — With the key on, back-probe the VREF supply at the sensor connector; it should read close to 5 V. A reference that is high because VREF is shorted to voltage will drag the DPFEGR signal up with it and set P0406. If VREF is out of range, trace that circuit before condemning the sensor.
4. Isolate the DPFEGR signal and SIGRTN circuits — With the sensor unplugged, watch the DPFEGR PID: if the signal stays high with the sensor disconnected, the fault is in the wiring — an open SIGRTN (signal return) or a DPFEGR wire shorted to voltage. Check SIGRTN for continuity back to the PCM and test the signal wire for a short to VREF or battery voltage.
5. Test the signal wire for a short to voltage — With the PCM disconnected, check the DPFEGR signal circuit for continuity to any powered wire or to battery positive. A short to voltage here is a classic cause of a stuck-high reading. Repair any short, open, or high-resistance connection found in the harness.
6. Verify the sensor, then follow the pinpoint test and confirm — If VREF, SIGRTN, and the signal wire all test good, the differential pressure feedback EGR sensor inside the ESM is the likely fault; substitute a known-good sensor/module or complete Ford Pinpoint Test HH for the EGR System Module. Clear the code and drive a cycle that enables EGR to confirm P0406 does not return and the DPFEGR PID reads sensibly.

FAQ