# P0405: Exhaust Gas Recirculation Sensor "A" Circuit Low **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:** Failed EGR sensor or open reference wire - **DIY?:** Steps 1-3 ## Quick answer P0405 means the engine computer sees a voltage from EGR sensor "A" that is lower than it should ever be. Depending on the engine, that sensor is an EGR valve position sensor or a pressure sensor that measures EGR flow. A failed sensor, a broken reference wire, or a short to ground is the usual cause. ## What does P0405 mean? The EGR system routes a little exhaust back into the intake to cut nitrogen oxide emissions, and the engine computer needs feedback to know how much is flowing. Some engines use a position sensor on top of the EGR valve that reports how far it has opened. Others, including many older Ford vehicles, use a differential pressure feedback EGR (DPFE) sensor that measures the pressure drop across a small orifice in the EGR tube. Either way, sensor "A" is usually a three-wire sensor fed a 5-volt reference by the computer, with a ground and a signal wire that rises and falls as EGR changes. P0405 is a "circuit low" code. It sets when the signal voltage drops below the minimum a working sensor could produce, which usually means an open reference wire, a signal shorted to ground or a dead sensor. Because it is an electrical check, many vehicles set it quickly once the condition is seen. ## What are the symptoms of P0405? - Check engine light on, often with no driving change - Slight hesitation or roughness on some engines - Pinging under load if the computer shuts off EGR - Other sensor codes stored when a shared 5-volt feed is shorted ## What causes P0405? 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). The sensor sits on or near the hot EGR valve or tube, and heat breaks down its electronics over time. Ford DPFE sensors on many 1990s and 2000s vehicles were a well-known failure point. On many engines the position sensor is built into the EGR valve, so the whole valve is replaced when the sensor fails. 2. **Open 5-volt reference or signal wire** (common). If the reference wire breaks or its connector pin loses contact, the sensor has no power and its signal falls to near zero. Wires routed near the exhaust become brittle and crack. A shop checks for about 5 volts at the reference pin with the key on and looks for continuity back to the computer. 3. **Signal wire shorted to ground** (common). A signal wire with rubbed-through insulation touching the engine or a bracket pulls the voltage low. So can water or corrosion inside the connector, especially on cars in wet or road-salt climates. Wiggle-testing the harness while watching the sensor voltage on a scan tool often shows the reading drop as the short makes contact. 4. **Shared 5-volt supply pulled down by another sensor** (less common). On many engines the computer feeds several sensors, such as the throttle position and MAP sensors, from one 5-volt reference. A shorted sensor elsewhere can pull that feed down, setting P0405 alongside codes such as P0107 or P0122. Unplugging sensors one at a time until the reference returns finds the culprit. 5. **Engine computer reference fault** (rare). If the 5-volt reference is missing at the computer itself with all sensors unplugged, the computer's internal regulator may have failed. This is rare and sometimes follows a wiring short that overloaded the circuit. A shop confirms it carefully, and repairs any short first, before replacing and programming a computer. ## How do you diagnose P0405? 1. Read all codes and look for other sensor-low codes that point to a shared 5-volt reference problem. 2. Identify whether your engine uses an EGR valve position sensor or a DPFE-type pressure sensor. 3. With the key on, check for a 5-volt reference, a good ground and the signal voltage at the sensor connector. 4. Inspect and wiggle-test the wiring between the sensor and the engine computer for opens and shorts. ## How do you fix P0405, and what does it cost? | Fix | Typical cost | What it involves | | --- | --- | --- | | Replace EGR pressure (DPFE) sensor | $100-$300 | For systems with a separate pressure sensor; replace cracked sensor hoses too. | | Repair EGR sensor wiring or connector | $100-$300 | Fixes an open reference wire, a grounded signal wire or corroded pins. | | Replace EGR valve with built-in sensor | $250-$750 | Needed when the position sensor is part of the valve. | Full price breakdowns: [egr valve replacement cost](/cost/egr-valve-replacement). ## Is it safe to drive with P0405? > **Safe to drive.** Usually safe to drive while you arrange a repair, since most engines run normally. If other sensor codes appear and the engine runs poorly or goes into reduced power, get it checked soon. ## Will P0405 fail an emissions test? P0405 with the check engine light on fails an OBD-based emissions test in most states. With a bad feedback signal, the computer may disable EGR, which can also keep the EGR monitor from completing. In areas with tailpipe testing, NOx can rise. Fix the circuit, clear the code and complete a drive cycle first. ## After the repair: clearing P0405 Clear the code and check the sensor voltage with the key on; it should sit in the normal range rather than near zero. Then drive with some steady cruising once fully warm so the EGR test can run. The repair worked when P0405 does not return and the EGR monitor reports complete. ## How to prevent P0405 - Keep the EGR sensor harness clipped away from exhaust heat after any repair work. - Replace cracked or brittle DPFE sensor hoses before they split or melt. - Fix other sensor wiring faults promptly, since a shared 5-volt short can affect the EGR sensor. - Use dielectric grease on connectors in wet or salty climates if service information allows it. ## P0405 by make | Make | What to know | | --- | --- | | Ford | Many Ford vehicles from the 1990s and 2000s use a DPFE sensor to measure EGR flow, and a failed DPFE is a frequent cause of EGR sensor codes on them. | ## Codes often seen with P0405 - [P0406](/codes/p0406): Exhaust Gas Recirculation Sensor "A" Circuit High - [P0404](/codes/p0404): Exhaust Gas Recirculation "A" Control Circuit Range/Performance - [P0401](/codes/p0401): Exhaust Gas Recirculation "A" Flow Insufficient Detected - [P0107](/codes/p0107): Manifold Absolute Pressure/Barometric Pressure Circuit Low - [P0122](/codes/p0122): Throttle/Pedal Position Sensor/Switch "A" Circuit Low ## Frequently asked questions ### Can I drive with a P0405 code? Yes, in most cases you can drive normally while you arrange a repair, because the engine usually runs fine. The computer may stop using EGR, which can cause pinging under load and higher NOx. If several sensor codes appear and the engine runs poorly, get it checked sooner. ### What is EGR sensor A? EGR sensor A is the feedback sensor the engine computer uses to monitor the EGR system. On many engines it is a position sensor on top of the EGR valve. On others, including many older Fords, it is a pressure sensor called a DPFE that measures exhaust flow through the EGR tube. ### How much does it cost to fix P0405? Most P0405 repairs cost $100 to $750 at an independent shop. A separate DPFE-type pressure sensor runs about $100 to $300, and a wiring repair $100 to $300. If the position sensor is built into the EGR valve, replacing the valve costs about $250 to $750. ### What is the difference between P0405 and P0406? P0405 means the EGR sensor voltage is too low, while P0406 means it is too high. Low readings usually come from an open reference wire, a short to ground or a dead sensor. High readings usually come from an open ground, a short to voltage or a failed sensor. ### Can I replace a DPFE sensor myself? Yes, on most engines a DPFE sensor is an easy job. It usually mounts with two bolts or a bracket and connects with two small hoses and one electrical plug. Note which hose goes on which port before removing them, replace cracked hoses, and clear the code afterward. ### Why do I have P0405 and throttle sensor codes together? When P0405 appears with codes such as P0122 or P0107, a shared 5-volt reference is probably being pulled low. One shorted sensor or wire can affect every sensor on that feed. Fixing that one short usually clears all the codes, so replacing several sensors is rarely needed. ### Can P0405 cause engine knocking? It can on some engines. When the computer cannot trust EGR feedback, it may shut off EGR, which raises combustion temperature and can cause pinging under load. Use the recommended fuel octane, avoid heavy loads, and fix the sensor circuit to restore normal EGR operation. ### Does P0405 mean the EGR valve is bad? Not necessarily. The code points to the sensor circuit, which may be a separate sensor, the wiring or a sensor built into the valve. Only when the position sensor is part of the valve does the fix mean a new valve. Test the reference, ground and signal first. ### Will P0405 go away on its own? It may go off for a while if the fault is intermittent, such as a loose connector, but it usually returns. A wire or sensor that has failed will keep setting the code. Clearing it without a repair only resets the readiness monitors, which delays any inspection. ### Will P0405 pass inspection? No, P0405 with the check engine light on fails an OBD-based emissions test in most states. The EGR monitor may also be not ready if the computer disabled EGR. Repair the circuit, clear the code and drive with steady cruising until all monitors report complete before testing. ## 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) - [P0406 Exhaust Gas Recirculation Sensor "A" Circuit High](https://diagxcar.com/codes/p0406) - [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 engine knocking?](https://diagxcar.com/symptoms/engine-knocking-noise) - [Why does my car hesitate when I accelerate?](https://diagxcar.com/symptoms/car-hesitates-when-accelerating) ## Repair costs - [EGR valve replacement cost](https://diagxcar.com/cost/egr-valve-replacement): $250-$850 --- Source page: https://diagxcar.com/codes/p0405 Updated: 2026-11-03 General information, not a substitute for a qualified mechanic.