P0406 is an exhaust gas recirculation (EGR) trouble code that can show up on many modern vehicles when the engine computer sees an EGR sensor signal that’s higher than it should be. While it sounds intimidating, it usually comes down to a bad EGR valve, position sensor, wiring issue, or carbon buildup. In this guide, you’ll learn what P0406 actually means, the most common causes, symptoms to watch for, and how a mechanic would diagnose and repair it so you can decide what to do next.
What Does P0406 Mean?
P0406 is defined as “EGR Sensor A Circuit High.” In plain language, your engine control unit (ECU) is seeing an EGR position sensor signal voltage that is too high for the operating conditions. The ECU expects the EGR valve to be in a certain position, and the feedback from the sensor is telling it the valve is open more than commanded, or stuck in the wrong position.
This code is part of the EGR (Exhaust Gas Recirculation) system monitoring. The EGR system routes a small amount of exhaust gas back into the intake to reduce combustion temperatures and NOx emissions. When the sensor signal is out of range on the high side, the ECU flags P0406 and usually disables EGR operation to protect the engine and emissions system.
Quick Reference
- Code: P0406 – EGR Sensor A Circuit High
- System: Exhaust Gas Recirculation (EGR)
- Severity: Low to moderate (emissions and drivability)
- Common Fixes: EGR valve/sensor replacement, wiring repair, carbon cleaning
- Driveability: Often drivable, but not recommended long term
Real-World Example / Field Notes
In the shop, I see P0406 a lot on higher-mileage vehicles that spend their lives in city traffic. One example: a mid-2000s sedan came in with a check engine light, slightly rough idle, and poor fuel economy. Scan tool showed P0406 stored and the EGR command was 0%, but the EGR position sensor was reading almost fully open. After checking wiring and power/ground, I pulled the EGR valve and found heavy carbon buildup holding the pintle partially open and fouling the sensor. A new EGR valve and a quick cleaning of the passages solved the issue, and fuel mileage improved immediately.
Symptoms of P0406
- Check engine light on: The MIL (Malfunction Indicator Lamp) will usually be illuminated and P0406 stored as a confirmed code.
- Rough idle: Excessive or uncontrolled EGR flow can cause unstable idle, shaking, or occasional stalling at stops.
- Hesitation or stumble: You may feel hesitation, flat spots, or surging during acceleration if EGR is flowing when it shouldn’t.
- Reduced fuel economy: Incorrect EGR operation can lead to inefficient combustion and more trips to the gas station.
- Loss of power: Some vehicles will feel sluggish, especially at low RPM, when the EGR system isn’t behaving correctly.
- Failed emissions test: With EGR disabled or malfunctioning, NOx emissions can increase and cause an inspection failure.
- Occasional pinging/knock: In some cases, lack of proper EGR can raise combustion temps and cause light spark knock under load.
Common Causes of P0406
Most Common Causes
- Faulty EGR valve with built-in position sensor: Many modern EGR valves have an integrated position sensor. Internal failure or wear can cause the sensor to send a constant high-voltage signal.
- Carbon buildup in EGR valve or passages: Heavy carbon can hold the valve open or partially open, making the sensor report an incorrect position.
- Short to voltage in EGR signal wire: Damaged insulation, rubbed-through wiring, or corrosion can cause the signal wire to see battery voltage, triggering a “circuit high” condition.
- Poor electrical connections: Loose, corroded, or water-contaminated connectors at the EGR valve or harness can skew the sensor signal.
- Failed EGR position sensor (separate unit): On some older systems, the EGR position sensor (DPFE or similar) is separate from the valve and can fail on its own.
Less Common Causes
- ECU/PCM internal fault: Rare, but a damaged ECU input circuit can misread the EGR sensor voltage as too high.
- Incorrect or low-quality replacement parts: Cheap aftermarket EGR valves or sensors sometimes send incorrect feedback signals right out of the box.
- Previous wiring repairs done poorly: Twisted wires, no solder/heat-shrink, or incorrect pin locations can cause intermittent high-voltage readings.
- Vacuum-related issues on older systems: On vacuum-operated EGR valves with electronic feedback, a stuck vacuum solenoid or misrouted hoses can hold the valve open and confuse the sensor.
- Software/firmware issues: In rare cases, a manufacturer TSB (Technical Service Bulletin) may call for an ECU reflash to correct sensitive EGR diagnostics.
Diagnosis: Step-by-Step Guide
To properly diagnose P0406, you’ll want at least a basic scan tool that can read live data, a digital multimeter (DMM), and some hand tools. An advanced scan tool with Mode $06 data and bi-directional controls is ideal, but not strictly required if you’re methodical.
- Confirm the code and check for companions. Connect a scan tool, read all stored and pending codes, and note any related EGR or sensor codes (like P0404, P0405). This helps narrow the fault.
- Inspect the EGR valve and connector. With the engine off, locate the EGR valve. Check the connector for broken locks, corrosion, bent pins, or signs of water intrusion. Inspect the harness for chafing or melted sections.
- Check live data for EGR position. In the data stream, look at EGR command (%) and EGR position or feedback voltage. If the ECU commands 0% but the sensor shows high position or voltage, you’ve confirmed the basic issue.
- Verify reference voltage and ground. Using a multimeter and wiring diagram, check for proper 5V reference and a good ground at the EGR sensor connector with the key on. Poor power or ground can skew readings.
- Test the signal wire for shorts. Backprobe the EGR signal wire. If you see near battery voltage when the valve should be closed, unplug the EGR valve. If the voltage drops to normal with it unplugged, the valve/sensor is likely bad. If it stays high, suspect a wiring short to power.
- Manually move or command the EGR valve. On some vehicles, you can use a scan tool to command the EGR open/closed while watching the position feedback. The voltage or position should change smoothly. No change or erratic readings point to a bad sensor or sticking valve.
- Inspect for carbon buildup. Remove the EGR valve and check for heavy carbon deposits on the pintle and in the passages. If the valve is stuck open with carbon, clean the passages and re-test the valve or replace if worn.
- Check Mode $06 (if available). Some scan tools show EGR monitor test results in Mode $06. This can confirm that the EGR monitor is failing due to position feedback out of range.
- Perform a wiggle test on wiring. With the engine running and watching live data, gently move the harness near the EGR valve. If the position signal jumps or drops out, you’ve likely found a wiring or connector issue.
- Rule out ECU issues last. Only after confirming good power, ground, wiring, and a known-good valve/sensor should you suspect an ECU fault. At that point, professional diagnosis is recommended.
Pro tip: If you replace the EGR valve, always clear the code and perform a proper drive cycle. Many ECUs need specific conditions (coolant temp, speed, load) to re-run the EGR monitor and confirm the repair.
Possible Fixes & Repair Costs
Most P0406 repairs involve addressing the EGR valve, sensor, or wiring. Common fixes include cleaning or replacing the EGR valve, repairing damaged wiring or connectors, and occasionally replacing a separate EGR position sensor or DPFE sensor. Typical repair costs at a shop range from about $150–$350 for cleaning and wiring repairs, $250–$600 for an EGR valve replacement (parts and labor), and $600+ if extensive diagnostics or ECU-related work is required. Costs vary based on vehicle make, engine layout, parts quality (OEM vs aftermarket), and local labor rates.
Can I Still Drive With P0406?
In most cases, you can still drive with P0406 without immediate catastrophic damage, but it’s not ideal. The ECU often disables EGR operation when this code is active, which can increase combustion temperatures and NOx emissions. You may notice reduced fuel economy, mild drivability issues, and a guaranteed check engine light. Short trips to get the vehicle inspected or repaired are usually fine, but you shouldn’t ignore the problem for weeks or months, especially if you live in an area with emissions testing.
What Happens If You Ignore P0406?
If you ignore P0406 long term, you risk higher combustion temperatures, increased NOx emissions, and potential damage to related components such as the catalytic converter. Carbon buildup can worsen over time, making future repairs more difficult and expensive. You’ll likely fail emissions inspections, and any existing drivability issues (rough idle, hesitation) may gradually get worse.
Related Codes
- P0474 – Exhaust Pressure Sensor Intermittent
- P0473 – Exhaust Pressure Sensor High
- P0472 – Exhaust Pressure Sensor Low
- P0471 – Exhaust Pressure Sensor Range/Performance
- P0470 – Exhaust Pressure Sensor
- P0469 – EVAP Purge Flow Sensor Circuit Intermittent
- P0468 – EVAP Purge Flow Sensor Circuit High
- P0467 – EVAP Purge Flow Sensor Circuit Low
- P0466 – EVAP Purge Flow Sensor Circuit Range/Performance
- P0465 – EVAP Purge Flow Sensor Circuit
Key Takeaways
- P0406 means the ECU sees an EGR position sensor signal that’s too high, usually due to a bad valve/sensor or wiring issue.
- Symptoms include a check engine light, rough idle, hesitation, reduced fuel economy, and potential emissions test failure.
- Most common causes are a faulty EGR valve with integrated sensor, carbon buildup, or a short to voltage in the signal wire.
- Diagnosis involves checking live data, testing power/ground and signal circuits, and inspecting the valve for carbon or sticking.
- Repairs typically range from $150–$600 depending on whether you need cleaning, wiring repair, or a new EGR valve.
- Driving short-term with P0406 is usually possible, but ignoring it long-term can lead to higher emissions and more expensive repairs.
Vehicles Commonly Affected by P0406
P0406 is common across many brands that use electronically controlled EGR systems. You’ll often see it on domestic vehicles such as Ford (F-150, Escape, Focus), GM/Chevrolet (Silverado, Malibu, Impala), and Chrysler/Dodge/Jeep models. It also shows up on various Asian makes including Toyota, Honda, Hyundai, and Kia, especially on higher-mileage sedans and SUVs. Diesel pickups and commercial vehicles with complex EGR systems are frequent offenders as well, since they tend to accumulate heavy carbon deposits over time.
FAQ
Can P0406 clear itself?
Sometimes P0406 will go away on its own if the condition is intermittent, such as a loose connector making temporary contact. However, if there’s a real fault like a bad EGR valve or wiring short, the code will usually return quickly after a few drive cycles. It’s best to diagnose and fix the root cause rather than hoping it disappears.
Is P0406 serious enough to stop driving immediately?
P0406 is generally not an emergency “pull over now” code, but you shouldn’t ignore it. The vehicle will usually remain driveable, but you may notice drivability issues and increased emissions. Plan to have it checked and repaired soon, especially if the engine runs rough or the code returns immediately after clearing.
Can a dirty EGR valve cause P0406?
Yes, a dirty or carbon-clogged EGR valve is one of the most common causes of P0406. Carbon can hold the valve open, causing the position sensor to report an incorrect (high) reading. In some cases, thorough cleaning of the valve and passages can restore normal operation, but if the sensor or valve mechanism is worn, replacement is the better option.
Can I just replace the EGR valve to fix P0406?
Replacing the EGR valve often fixes P0406, especially when the sensor is integrated into the valve. However, you should still inspect and test the wiring, connectors, and power/ground circuits first. If the problem is a shorted signal wire or bad connector, a new valve won’t solve the issue and you’ll waste money on unnecessary parts.
Does P0406 always mean the EGR sensor is bad?
No, P0406 only tells you the ECU is seeing a high signal from the EGR sensor circuit, not exactly why. The cause could be a bad sensor, stuck valve, wiring short to power, poor ground, or in rare cases a faulty ECU. That’s why basic electrical testing and live data checks are important before replacing any parts.