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P0406 is specific to the EGR position sensor voltage - not EGR flow itself. The sensor is reporting a position voltage above 4.5V, suggesting it thinks the valve is fully open (or the signal is shorted to voltage). Check wiring first, then the EGR valve assembly. Common on Ford and GM trucks. See top-rated scanners on Amazon ↗
🗺️ Where Is the Problem?
Blueprint view - P0406 EGR position sensor A reporting high voltage signal
These are statistical causes across ALL vehicles - your exact car may rank differently
For example, on a Honda 4-cyl the downstream O2 sensor causes P0406 64% of the time, but on a GM 5.3L V8 the catalytic converter is the cause 71% of the time. Get a probability ranking built specifically for your year, make, model, and mileage.
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🎯 Top Causes & Probability
45%
#1 - Most Likely
EGR Position Sensor Short to Voltage
A wiring short between the EGR position sensor signal wire and a voltage source drives the reported position to near 5V, indicating a fully-open valve regardless of actual position. Inspect the EGR wiring harness for chafed insulation near the exhaust manifold or firewall.
🔩 Part
$5–$30
👨🔧 Labor
$60–$150
⚡ DIY Difficulty
Medium
35%
#2 - Check First
EGR Sensor Failure at Max Position
The EGR position sensor's internal resistive track has failed at the maximum position end, outputting maximum voltage even when the valve is closed. On integrated EGR valve/sensor assemblies, the entire valve assembly requires replacement. Confirm by measuring position sensor voltage with the EGR valve known to be fully closed.
🔩 Part
$40–$200
👨🔧 Labor
$60–$150
⚡ DIY Difficulty
Medium
15%
#3 - Less Common
Wiring Fault
A damaged wire, corroded connector, or pushed-back terminal in the EGR sensor circuit can create a high-resistance fault that appears as high voltage to the PCM. Inspect the full harness from the EGR valve connector back to the PCM for damage, especially near heat sources.
🔩 Part
$5–$40
👨🔧 Labor
$60–$150
⚡ DIY Difficulty
Medium
5%
#4 -
Carbon Fouling of EGR Position Sensor
Carbon deposits from exhaust gas can infiltrate the EGR position sensor mechanism, contaminating the sensor's resistive track and causing erratic or high voltage readings. This typically requires valve/sensor assembly replacement since the sensor is usually sealed within the valve body.
🔩 Part
$40–$200
👨🔧 Labor
$60–$150
⚡ DIY Difficulty
Medium
🚗 Most Affected Vehicles
🔧 Step-by-Step Diagnosis
- Measure EGR Sensor Voltage at Idle - With key on, engine off, probe the EGR position sensor signal wire. The voltage should read near 0.5–1.0V with the valve closed. A reading above 4.5V confirms the high voltage condition - check for a short to voltage next.
- Disconnect EGR and Recheck Voltage - Unplug the EGR connector and recheck voltage on the signal wire (harness side). If voltage is still near 5V with the sensor disconnected, there is a wiring short to voltage - not a sensor fault.
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