# P0406: Exhaust Gas Recirculation Sensor "A" Circuit High **Verdict:** Safe to drive Emissions - Generic OBD-II code, applies to all gasoline cars and light trucks sold in the US since 1996. - **Typical cost:** $100-$750 - **Urgency:** Keep driving - **Most likely:** Open sensor ground or failed EGR sensor - **DIY?:** Steps 1-3 ## Quick answer P0406 means the engine computer sees a voltage from EGR sensor "A" that is higher than a working sensor should produce. That sensor reports EGR valve position or EGR flow, depending on the engine. An open ground, a wire shorted to voltage, or a failed sensor is the usual cause. ## What does P0406 mean? To control exhaust gas recirculation accurately, the engine computer needs to know what the EGR system is actually doing. Depending on the engine, EGR sensor "A" is either a position sensor mounted on the EGR valve or a pressure sensor, such as Ford's DPFE, that measures the pressure difference created by exhaust flowing through the EGR tube. Most are three-wire sensors with a 5-volt reference, a ground and a signal wire. P0406 is a "circuit high" code. The computer sets it when the signal voltage rises above the highest value the sensor should ever send, close to the full reference voltage or above it. That usually happens when the sensor loses its ground, when the signal wire touches a reference or 12-volt wire, or when the sensor fails internally. Because it is an electrical check, many vehicles set it quickly once the reading is seen. Some sensors may also read high if the valve is stuck wide open. ## What are the symptoms of P0406? - Check engine light on, often without other symptoms - Rough idle or hesitation on some engines - Pinging under load if the computer disables EGR - Slightly lower fuel economy on some vehicles ## What causes P0406? Ranked from most to least likely. Check them in this order: the cheap, common causes first. 1. **Failed EGR position or pressure sensor** (most common). Heat from the nearby EGR valve and exhaust slowly damages the sensor's internal circuit, and a failed sensor can send a voltage near the full reference value. On engines where the position sensor is part of the EGR valve, the valve is replaced as a unit. Separate pressure sensors, such as Ford DPFE units, are replaced alone. 2. **Open sensor ground circuit** (common). If the sensor loses its ground, through a broken wire, a loose pin or a corroded engine ground, its signal floats up toward the reference voltage. A shop measures voltage drop on the ground wire with the key on. Ground problems sometimes affect several sensors at once, so look for other circuit-high codes. 3. **Signal wire shorted to voltage** (common). When the signal wire rubs through and touches the 5-volt reference or a 12-volt wire in the same harness, the reading jumps high. Heat near the exhaust and harnesses pinched during earlier repairs are typical causes. Water inside a connector can also bridge pins. Inspecting the harness and connector for melted or chafed spots usually finds it. 4. **EGR valve stuck wide open** (less common). On systems where the position sensor sits on the valve, a valve held fully open by carbon or a broken part may read at the top of the range. This usually comes with a rough idle or stalling, and may set P0404 too. Commanding the valve with a scan tool and watching the reading shows whether it moves. 5. **Engine computer input fault** (rare). If the sensor, ground and wiring all test good and the reading at the computer connector is still wrong, the computer's input circuit may have failed. This is uncommon. A shop checks for software updates and confirms the fault at the computer before replacing it, since a new computer must usually be programmed. ## How do you diagnose P0406? 1. Read all codes, noting other circuit-high codes that point to a shared ground fault. 2. Identify whether the engine uses an EGR valve position sensor or a DPFE-type pressure sensor. 3. With the key on, check the reference, ground and signal voltages at the sensor connector. 4. Inspect the harness for chafing, melted spots or pinched wires, and test the ground for voltage drop. ## How do you fix P0406, and what does it cost? | Fix | Typical cost | What it involves | | --- | --- | --- | | Repair sensor ground or wiring | $100-$300 | Fixes an open ground, a short to voltage or corroded connector pins. | | Replace EGR pressure (DPFE) sensor | $100-$300 | For systems with a separate pressure sensor; replace old hoses at the same time. | | Replace EGR valve with built-in sensor | $250-$750 | Needed when the position sensor is integrated into the valve. | Full price breakdowns: [egr valve replacement cost](/cost/egr-valve-replacement). ## Is it safe to drive with P0406? > **Safe to drive.** Usually safe to drive while you arrange the repair, because most engines run normally. If the engine idles roughly, stalls, or pings heavily under load, have it checked soon. ## Will P0406 fail an emissions test? P0406 with the check engine light on fails an OBD-based emissions test in most states. The computer may disable EGR when its feedback reads high, so the EGR monitor can stay not ready, and tailpipe-tested cars may show higher NOx. Repair it, clear the code and drive until the monitors complete before testing. ## After the repair: clearing P0406 Clear the code, then check the sensor voltage with the key on; it should sit in its normal resting range rather than near 5 volts. Drive with some steady cruising once the engine is fully warm so the EGR test can run. If P0406 stays away and the EGR monitor completes, the repair worked. ## How to prevent P0406 - Keep EGR sensor wiring routed and clipped away from exhaust heat and sharp brackets. - Check engine ground straps for corrosion during routine maintenance, especially in road-salt states. - Make sure connectors are fully latched and dry after any engine work or washing. - Address EGR carbon buildup early so the valve does not stick open. ## P0406 by make | Make | What to know | | --- | --- | | Ford | Many Ford vehicles from the 1990s and 2000s use a separate DPFE pressure sensor for EGR feedback, and replacing it is often a simple bolt-on job. | ## Codes often seen with P0406 - [P0405](/codes/p0405): Exhaust Gas Recirculation Sensor "A" Circuit Low - [P0404](/codes/p0404): Exhaust Gas Recirculation "A" Control Circuit Range/Performance - [P0401](/codes/p0401): Exhaust Gas Recirculation "A" Flow Insufficient Detected - [P0403](/codes/p0403): Exhaust Gas Recirculation "A" Control Circuit - [P0400](/codes/p0400): Exhaust Gas Recirculation "A" Flow ## Frequently asked questions ### Can I drive with a P0406 code? Yes, most cars can be driven normally with P0406 while you arrange a repair. The computer may switch off EGR, which can cause light pinging under load and higher NOx emissions. If the engine idles roughly or stalls, a stuck-open valve may be involved, so get it checked soon. ### What does EGR sensor circuit high mean? It means the voltage from the EGR position or pressure sensor is above the range a working sensor should produce. The computer cannot trust it, so it may disable EGR. An open sensor ground, a wire shorted to voltage or a failed sensor are the most likely causes. ### How much does it cost to fix P0406? Most P0406 repairs cost $100 to $750 at an independent shop. A wiring or ground repair runs about $100 to $300, and a separate DPFE-type pressure sensor $100 to $300. If the sensor is built into the EGR valve, a new valve costs about $250 to $750. ### Can a bad ground cause P0406? Yes, a sensor that loses its ground often reads high, because its signal floats up toward the reference voltage. Broken ground wires, loose connector pins and corroded engine ground straps are common culprits. If several sensors show circuit-high codes together, a shared ground is the first thing to check. ### What is the difference between P0406 and P0404? P0406 means the EGR sensor's voltage is above its electrical limit, while P0404 means the valve is not reaching its commanded position. P0406 points mainly to the sensor or its wiring. P0404 more often points to a sticking, carbon-fouled valve, though the two can appear together. ### Can I fix P0406 myself? You can often handle it with a multimeter and basic tools. Check the connector, look for damaged wiring, and test reference, ground and signal voltages against service information. Separate DPFE sensors bolt on easily. If the sensor is built into the valve, replacing the valve is a moderate job. ### Can a stuck EGR valve cause P0406? It can on some engines where the position sensor sits on the valve. A valve stuck fully open may read at the top of the sensor range. That usually comes with rough idle or stalling at stops, and P0404 may also be stored. Commanding the valve with a scan tool shows whether it moves. ### Does P0406 affect fuel economy? It can a little. When the computer disables EGR, it often adjusts ignition timing to prevent knock, which can cost some efficiency on certain engines. Many drivers notice no change at all. The bigger concerns are a failed emissions test and possible pinging under load. ### Will P0406 go away on its own? Sometimes the light turns off if the fault was a loose connector that reseated, but a failed sensor or damaged wire will keep setting the code. Clearing it without a repair resets the readiness monitors, so the car will show not ready if you test right after. ### Will P0406 pass inspection? No, P0406 with the check engine light on fails an OBD-based emissions test in most states. The EGR monitor may also stay not ready while EGR is disabled. Fix the circuit, clear the code and complete a drive cycle with steady cruising before going for the test. ## Sources - [SAE J2012: Diagnostic Trouble Code Definitions](https://www.sae.org/standards/content/j2012_201812/) - [EPA: On-Board Diagnostics (OBD)](https://www.epa.gov/state-and-local-transportation/vehicle-emissions-board-diagnostics-obd) ## Other emissions codes - [P0400 Exhaust Gas Recirculation "A" Flow](https://diagxcar.com/codes/p0400) - [P0401 Exhaust Gas Recirculation "A" Flow Insufficient Detected](https://diagxcar.com/codes/p0401) - [P0402 Exhaust Gas Recirculation "A" Flow Excessive Detected](https://diagxcar.com/codes/p0402) - [P0403 Exhaust Gas Recirculation "A" Control Circuit](https://diagxcar.com/codes/p0403) - [P0404 Exhaust Gas Recirculation "A" Control Circuit Range/Performance](https://diagxcar.com/codes/p0404) - [P0405 Exhaust Gas Recirculation Sensor "A" Circuit Low](https://diagxcar.com/codes/p0405) - [P0410 Secondary Air Injection System "A"](https://diagxcar.com/codes/p0410) - [P0411 Secondary Air Injection System "A" Incorrect Flow Detected](https://diagxcar.com/codes/p0411) - [P0420 Catalyst System Efficiency Below Threshold (Bank 1)](https://diagxcar.com/codes/p0420) - [P0421 Warm Up Catalyst Efficiency Below Threshold (Bank 1)](https://diagxcar.com/codes/p0421) ## Symptoms - [Why is my car idling rough?](https://diagxcar.com/symptoms/rough-idle) - [Why is my engine knocking?](https://diagxcar.com/symptoms/engine-knocking-noise) ## Repair costs - [EGR valve replacement cost](https://diagxcar.com/cost/egr-valve-replacement): $250-$850 --- Source page: https://diagxcar.com/codes/p0406 Updated: 2026-11-03 General information, not a substitute for a qualified mechanic.