Quick Answer
P0406 means your EGR position sensor is reporting a voltage signal that’s higher than the engine computer expects. The most common fix is replacing the EGR position sensor or cleaning the EGR valve, as carbon buildup or a faulty sensor causes this low-severity emissions code.
What Is the P0406 Code?
The P0406 diagnostic trouble code (DTC) indicates that your vehicle’s engine control module (ECM) has detected an abnormally high voltage signal from the exhaust gas recirculation (EGR) position sensor. This sensor monitors how much the EGR valve is open and reports this position to the engine computer. When the signal voltage exceeds the maximum acceptable threshold, the code is triggered.
The EGR system is a critical emissions control component that reduces nitrogen oxide (NOx) emissions by recirculating a portion of exhaust gases back into the intake manifold. The position sensor tells the ECM exactly how much the valve is open so it can adjust fuel injection and ignition timing accordingly. A faulty signal disrupts this balance and causes the check engine light to illuminate.
What Does P0406 Mean?
P0406 is specifically a “EGR Position Sensor Circuit High” code. Here’s what’s happening:
- Sensor Malfunction: The EGR position sensor is sending a voltage reading that exceeds the maximum calibrated range (typically above 4.5 volts on most vehicles)
- Signal Interpretation: The ECM interprets this high signal as the EGR valve being more open than physically possible
- Emissions Impact: The engine computer cannot properly control EGR flow, affecting emissions control and engine performance
- Low Severity: Unlike drivability codes, P0406 is an emissions-related code that won’t typically cause immediate engine damage or safety hazards
Common Symptoms of P0406
- Check Engine Light: Illuminated on the dashboard (most common symptom)
- Rough Idle: Engine may stumble or shake at stops
- Hesitation During Acceleration: Slight delay in throttle response
- Increased Emissions: Vehicle may fail an emissions test
- Knocking or Pinging: Pre-ignition noise under load (less common)
- No Noticeable Drivability Issues: Many drivers experience only the check engine light with no performance symptoms
- Reduced Fuel Economy: Slight decrease in MPG due to improper EGR control
Possible Causes of P0406 (Ranked by Frequency)
1. Faulty EGR Position Sensor
The most common cause of P0406. The sensor’s internal potentiometer can fail, causing it to send an out-of-range high voltage signal even when the EGR valve is in its normal operating range. This is typically a wear-out failure after 80,000+ miles.
2. Carbon Buildup on EGR Valve
Excessive carbon deposits can cause the EGR valve to stick partially open. The sensor then reports a higher-than-normal position, triggering the code. This is especially common in vehicles with high mileage or those that primarily operate in stop-and-go traffic.
3. Wiring Issues or Corroded Connectors
Corroded or loose connections on the EGR position sensor circuit can cause erratic voltage readings. Damaged wiring insulation may cause the signal wire to short to power, creating an artificially high voltage.
4. EGR Valve Stuck Open
A mechanically stuck EGR valve that remains partially or fully open will cause the position sensor to report a high reading. This can result from carbon buildup, a failed solenoid, or internal valve damage.
5. Engine Control Module (ECM) Issue
Rarely, a faulty ECM or corrupted calibration can misinterpret normal sensor signals as out-of-range. This is typically only diagnosed after other components have been ruled out.
6. Power Supply Problems
If the sensor’s reference voltage is higher than specified (typically 5V), the sensor output will also be high. This can indicate a problem with the ECM’s voltage regulator or power distribution circuit.
Diagnostic Steps for P0406
Step 1: Retrieve and Document the Code
Use an OBD-II scanner to confirm P0406 is present. Check for any additional codes that might provide clues (such as P0400, P0401, or P0405). Note the freeze frame data showing engine conditions when the code was set.
Step 2: Visual Inspection
Inspect the EGR position sensor connector and wiring harness for:
- Corrosion or green/white oxidation on connector pins
- Loose or partially disconnected connectors
- Damaged or pinched wiring
- Water intrusion or moisture in the connector
Step 3: Check Sensor Voltage with a Multimeter
With the ignition on (engine off), connect a multimeter to the EGR position sensor signal wire. The voltage should typically be between 0.5V and 4.5V depending on valve position. If it reads above 4.8V consistently, the sensor is likely faulty.
Step 4: Test EGR Valve Movement
With the engine running and the vehicle safely supported, observe the EGR valve position sensor reading using a live data scanner. The voltage should change smoothly as the engine transitions between idle and load. If the reading stays high or doesn’t change, the valve may be stuck or the sensor is failing.
Step 5: Inspect the EGR Valve
Remove the EGR valve and inspect it for carbon buildup. A heavily carbonized valve may need cleaning or replacement. Check that the valve stem moves freely and isn’t stuck in the open position.
Step 6: Test the Reference Voltage
Measure the 5V reference voltage supplied to the EGR position sensor. It should be stable at 5.0V ±0.2V. If it’s higher, there may be an ECM power supply issue.
Step 7: Clear the Code and Test Drive
After repairs, clear the code using your scanner and perform a test drive to verify the fix. The code should not return within 50-100 miles of normal driving.
Repair Cost Estimates
EGR Position Sensor Replacement
- Parts Cost: $50–$200 (varies by vehicle make/model)
- Labor Cost: $100–$300 (1–2 hours)
- Total Estimate: $150–$500
EGR Valve Cleaning
- Parts Cost: $0–$50 (cleaning supplies)
- Labor Cost: $200–$400 (2–3 hours)
- Total Estimate: $200–$450
EGR Valve Replacement
- Parts Cost: $150–$500
- Labor Cost: $200–$500 (2–4 hours)
- Total Estimate: $350–$1,000
Wiring Repair or Connector Replacement
- Parts Cost: $10–$50
- Labor Cost: $75–$200 (0.5–1.5 hours)
- Total Estimate: $85–$250
Can I Still Drive with P0406?
Safety Assessment: LOW SEVERITY
P0406 is a low-severity emissions code that does not pose an immediate safety risk. Your vehicle will likely remain drivable with normal performance in most cases.
Driving Considerations:
- Safe to Drive: Yes, in most cases. The code affects emissions control, not engine safety systems.
- Performance Impact: Minimal to none. You may notice slight roughness at idle or reduced fuel economy, but the engine will continue to run.
- Emissions Test: Your vehicle will likely fail an emissions inspection until the code is resolved.
- Warranty: If your vehicle is under warranty, have it diagnosed and repaired promptly to maintain coverage.
- Long-Term Driving: While not immediately dangerous, prolonged driving with P0406 may increase emissions and slightly reduce fuel economy. Repair within a few weeks is recommended.
When to Stop Driving:
Stop driving immediately if you experience:
- Severe engine knock or detonation
- Complete loss of power or stalling
- Visible smoke from the engine bay
- Extreme overheating
These symptoms would indicate a more serious underlying issue beyond a simple P0406 code.
Frequently Asked Questions
Q: Can I clear the P0406 code myself?
A: Yes, you can clear the code using an OBD-II scanner, but this is only a temporary fix. The code will return if the underlying problem isn’t addressed. Clearing the code without diagnosis is not recommended, as it prevents you from identifying the root cause. Always diagnose first, repair second, then clear the code.
Q: What’s the difference between P0406 and P0405?
A: P0405 is “EGR Position Sensor Circuit Low,” meaning the voltage is too low. P0406 is “EGR Position Sensor Circuit High,” meaning the voltage is too high. Both codes indicate sensor or circuit problems, but the direction of the fault differs. The diagnostic and repair steps are similar, though the causes may vary.
Q: Will a faulty EGR sensor cause my car not to start?
A: No, P0406 alone will not prevent your vehicle from starting. The EGR system is a secondary emissions control system. However, if the EGR valve is stuck open due to carbon buildup, it could cause rough starting or difficulty starting in cold weather. The engine will still start, but may run rough.
Q: How often do EGR position sensors fail?
A: EGR position sensors typically last 80,000–150,000 miles before failure. Failure rates increase significantly after 100,000 miles. Vehicles operating in dusty environments or with poor maintenance may experience earlier failures. Regular maintenance and timely sensor replacement can help prevent P0406.
Q: Can I drive to a mechanic with P0406 active?
A: Yes, it’s safe to drive to a mechanic with P0406 active. The code is low-severity and won’t cause engine damage during normal driving. However, avoid aggressive driving or extended highway trips until the issue is diagnosed and repaired. If you experience rough idle or hesitation, drive cautiously and have it checked soon.