The engine computer uses an EGR sensor to check what the EGR valve is doing. When that sensor signal drops too low, the computer cannot trust it. A P0405 code means the EGR sensor A signal is below its allowed range.
This guide explains what the sensor measures, what pulls its signal low and how to test it.
P0405 Code Breakdown
Every OBD-II code follows the same five-character pattern set by the SAE J2012 standard. Each character narrows down which system has the fault. This table shows what each part of the code tells you.
| Character | Meaning | What It Tells You |
|---|---|---|
| P | Powertrain | The fault is in the engine or transmission systems. |
| 0 | Generic (SAE) code | The definition is the same on every OBD-II vehicle. |
| 4 | Auxiliary emission controls | The fault involves emission control systems such as EGR, secondary air, EVAP or the catalytic converter. |
| 05 | Specific fault | Exhaust Gas Recirculation Sensor A Circuit Low Input |
What the P0405 Code Means
The SAE J2012 standard defines the P0405 code as Exhaust Gas Recirculation Sensor A Circuit Low Input. EGR sensor A is usually the EGR valve position sensor on top of the valve, or on Ford vehicles the differential pressure feedback (DPFE) sensor that measures flow through the EGR pipe. Some vehicles use an EGR temperature or pressure sensor instead.
Most EGR sensors get a 5-volt reference and a ground from the powertrain control module (PCM) and send back a signal between about 0.5 and 4.5 volts. On a position sensor, the voltage changes as the valve opens. On a DPFE sensor, it rises as exhaust flows through the EGR pipe.
The PCM uses the reading to confirm that the EGR valve opens and flows as commanded. When the signal is out of range, it usually stops using EGR and sets a code.
Low input means the signal voltage dropped below the lowest value a working EGR sensor A should ever send. That usually points to a lost reference voltage, a signal wire shorted to ground or a sensor that has failed inside.
Symptoms of a Bad EGR Sensor A
Many vehicles run nearly normally. Common signs of a P0405 code include:
- Check engine light on
- Slight pinging under load
- Rough idle on some vehicles
- Failed emissions test
- EGR flow codes set at the same time
Because the PCM often turns EGR off when it cannot trust the sensor, the engine may feel fine. The main risk is higher combustion temperature and a failed emissions test.
Most Common Causes of the P0405 Code
Low input faults almost always come from an electrical problem:
- Open 5-volt reference. Without its supply, the sensor cannot produce a signal.
- Signal wire shorted to ground. Chafed insulation touching metal pulls the voltage down.
- Failed EGR sensor A. An internal short drags the signal low.
- Corroded or loose connector. High resistance on the reference pin lowers the signal.
- Shorted 5-volt reference. Another sensor on the same reference can pull it down.
- PCM fault. Rare.
Tools You Will Need
You can do most of this testing at home with basic tools:
- OBD-II scan tool that can command the EGR valve
- Hand vacuum pump for vacuum-operated valves
- Digital multimeter
- Carbon cleaner and a small scraper or brush
- Wiring diagram for your engine
How to Track Down the P0405 Code
Find out what sensor A is on your engine. Then check it at the connector before you replace anything.
Check Live Data
With the key on and engine off, look at the EGR sensor A voltage. Then command the EGR valve, if your scan tool allows it, and watch whether the reading changes smoothly.
Check the Reference Voltage
Unplug the EGR sensor A and turn the key on. The reference pin should read close to 5 volts. If it reads 0 volts, unplug other sensors on the same reference one at a time. If the voltage comes back, the last sensor you unplugged is shorted.
Check the Signal Wire for a Short
With the battery disconnected and the sensor and PCM unplugged, check the signal wire for continuity to ground. Any continuity means a short. Inspect the wiring where it passes near metal edges and hot parts.
Check DPFE Hoses on Ford Vehicles
If sensor A is a DPFE sensor, check its two hoses for cracks and blockage, and make sure they are on the correct ports. A melted or plugged hose can push the reading out of range.
Repairs That Fix the P0405 Code
Match the repair to what your tests found. Then clear the code and drive under the same conditions that set it.
- Repair the connector or wiring.
- Replace EGR sensor A if it fails tests. On many electric EGR valves, the position sensor is part of the valve.
- Replace cracked DPFE hoses on Ford vehicles.
- Repair a shorted reference shared with other sensors.
After the repair, clear the codes and check that the sensor reading changes smoothly when the EGR valve is commanded. Then drive at a steady cruise so the PCM can run its EGR flow test and confirm the fix.
Common Mistakes to Avoid
These mistakes waste money or leave the real fault in place:
- Replacing the sensor without checking the reference voltage
- Missing a shorted sensor elsewhere that drags the shared reference down
- Replacing the EGR valve when only the sensor or wiring is bad
- Ignoring cracked DPFE hoses
Typical Repair Costs
The cost to fix a P0405 code depends on the cause. This table shows typical ranges for parts and labor.
| Repair | DIY Cost | Typical Shop Cost |
|---|---|---|
| EGR passage cleaning | $10 to $30 | $150 to $400 |
| EGR valve | $60 to $300 | $250 to $600 |
| DPFE or EGR pressure sensor | $30 to $120 | $120 to $300 |
| EGR vacuum solenoid | $20 to $80 | $100 to $250 |
| Vacuum hose | $5 to $20 | $50 to $120 |
A DPFE sensor or separate position sensor is inexpensive. If the sensor is built into an electric EGR valve, the whole valve must be replaced, which costs more.
Is It Safe to Drive with a P0405 Code?
Usually, yes. Most engines run normally, though EGR may be turned off. Avoid heavy loads if the engine pings.
Get it fixed before your next emissions test.
Related Codes
These codes often show up with this one or point to the same system:
| Code | Meaning |
|---|---|
| P0406 | Exhaust Gas Recirculation Sensor A Circuit High Input |
| P0401 | Exhaust Gas Recirculation Flow Insufficient Detected |
| P0403 | Exhaust Gas Recirculation Circuit Malfunction |
| P0404 | Exhaust Gas Recirculation Circuit Range/Performance |
Fixing EGR Sensor A
A P0405 code means EGR sensor A voltage is too low. Lost reference voltage, shorts to ground and failed sensors cause most cases.
- Find out what sensor A is on your engine.
- Check reference, ground and signal.
- Inspect DPFE hoses if fitted.
- Replace the sensor only after the wiring tests good.
Frequently Asked Questions
Is the P0405 Code Always a Bad Sensor?
No. A lost 5-volt reference or a signal wire shorted to ground can pull the signal low. Check the reference at the connector first, since a wiring fix costs less than a sensor or EGR valve.
What Is a DPFE Sensor?
DPFE stands for differential pressure feedback EGR. Ford and some other makers use it to measure EGR flow by comparing pressure on both sides of a small restriction in the EGR pipe. It connects to the pipe with two hoses.
Where Is the EGR Valve?
On most gasoline engines, it bolts to the intake manifold or throttle body area, with a pipe from the exhaust manifold. On diesels, it is often near the intake with an EGR cooler beside it. Your service manual shows the location.
How Long Until the Code Comes Back After a Repair?
The PCM usually runs its EGR test during steady cruising or deceleration, which may take a few drives. If the repair worked, the code should stay away and the readiness monitor should show complete. If the fault is still there, the code can return within a few days.
Do Diesels Have the Same EGR Problems?
Diesels often have worse EGR problems, because their exhaust carries more soot. Diesel EGR systems also use an EGR cooler, which can clog or leak coolant. Soot buildup in the intake is common on high-mileage diesels that do a lot of short trips.
Can a Bad EGR Sensor Affect Fuel Economy?
Slightly. When the PCM cannot trust the sensor, it may turn EGR off. Without EGR, combustion runs hotter and the PCM may retard timing to prevent knock, which can cost a little power and fuel economy.