P0407 Exhaust Gas Recirculation (EGR) Sensor "B" Circuit Low

P0407 is an OBD-II fault code indicating that the engine control module (ECM/PCM) has detected an abnormally low electrical signal from the EGR Sensor "B" circuit.

The important word in this code is “Circuit Low.” It does not automatically mean that EGR flow is too low, the EGR valve is stuck closed, or the engine is receiving insufficient exhaust gas.

The code is specifically telling you that the electrical signal associated with the designated EGR Sensor B is lower than the ECM expects.

The exact identity of “Sensor B” is manufacturer-dependent. On one vehicle it may be an EGR valve position sensor, while on another it may refer to a different EGR-related feedback sensor or an integrated sensor inside the EGR assembly.

What Does the EGR System Do?

Exhaust Gas Recirculation (EGR) reduces combustion temperatures and helps control nitrogen oxide (NOx) emissions by sending a controlled amount of exhaust gas back into the intake.

Depending on the engine, the system may contain:

• EGR valve

• EGR position sensor

• EGR control solenoid or actuator

• EGR cooler

• Differential pressure sensor

• Exhaust pressure sensor

• Temperature sensors

• Vacuum-control components

• Electrically controlled EGR actuator

Modern diesel engines can have considerably more complex EGR systems than older gasoline engines.

The ECM needs feedback from the EGR system so it can determine whether the commanded EGR position or operation is actually occurring.

What Does “Sensor B Circuit Low” Actually Mean?

The ECM is monitoring an electrical signal and sees a voltage or signal level below the expected range.

For example, a sensor may normally operate within a particular voltage window. If the signal is pulled excessively toward ground, the ECM can determine that the circuit is “low.”

Possible electrical causes include:

• Signal wire shorted to ground

• Damaged wiring

• Corroded connector

• Loose terminal

• Poor sensor ground

• Missing or incorrect reference voltage

• Failed EGR sensor

• Internal failure of an EGR actuator containing the sensor

• Excessive resistance in the circuit

• Incorrect replacement component

• ECM/PCM driver or input circuit problem

The actual circuit design varies considerably between manufacturers, so a generic voltage value should not be treated as a universal specification.

P0407 Does Not Automatically Mean the EGR Valve Is Bad

This is one of the most important points when diagnosing P0407.

An EGR valve can be mechanically perfect while the sensor circuit connected to it has a problem.

For example, the valve may move normally, but a damaged signal wire may be touching ground. The ECM would then see a very low sensor signal and set P0407.

The opposite is also possible. A sensor can electrically fail while the EGR valve itself is mechanically clean and capable of moving correctly.

Therefore, replacing the complete EGR valve simply because P0407 is stored can result in an unnecessary repair.

Where Is EGR Sensor "B"?

There is no universal physical location for EGR Sensor B.

The letter “B” is a diagnostic designation defined by the vehicle manufacturer. It does not necessarily mean that there are two identical EGR sensors sitting next to each other.

Depending on the vehicle, Sensor B could be associated with:

• EGR valve position

• EGR actuator position

• EGR differential pressure

• EGR flow feedback

• Exhaust pressure

• Another EGR-related measurement

The vehicle's wiring diagram and manufacturer service information should therefore be used to identify exactly which sensor the ECM calls “B.”

This is particularly important on modern diesel engines where several sensors can participate in EGR control.

Common Causes of P0407

The most common causes can generally be divided into electrical, sensor, and mechanical/control problems.

Short to ground

A signal wire touching ground can pull the sensor signal down and produce P0407.

This can happen because of:

• Chafed insulation

• Melted wiring near the exhaust

• Incorrect previous repairs

• Harness rubbing against the engine

• Water or contamination inside a connector

Damaged or corroded connector

EGR components are often exposed to considerable heat, vibration, moisture and contamination.

A connector may look connected but still have:

• Corroded terminals

• Loose pins

• Poor terminal tension

• Damaged seals

• Oxidation

• Heat damage

A poor connection can produce a low or unstable sensor signal.

Failed EGR sensor

If the sensor itself has an internal electrical fault, its output may remain abnormally low.

This is particularly common when the sensor is integrated into the EGR valve or actuator.

Reference voltage problem

Many position sensors use a reference voltage supplied by the ECM. If that reference is missing or incorrect, the sensor cannot generate the expected signal.

A fault affecting the reference circuit can also produce several unrelated sensor codes at the same time.

Ground problem

A poor sensor ground can alter the signal seen by the ECM.

The ground should therefore be tested electrically rather than judged simply by visual inspection.

Wiring resistance or intermittent connection

A damaged conductor can have enough continuity to pass a basic ohmmeter test but still fail under operating conditions.

This is why voltage-drop testing and harness movement testing can be more useful than continuity testing alone.

EGR actuator or integrated electronics

Some modern EGR valves contain the actuator, position sensor and electronics in one assembly.

In such systems, a sensor fault may require replacement of the complete EGR assembly, but that should only be concluded after the external wiring and power/ground circuits have been tested.

Can a Dirty EGR Valve Cause P0407?

It can contribute to an EGR-related problem, but carbon buildup alone does not automatically explain a circuit-low code.

A mechanically sticking EGR valve is more naturally associated with EGR flow or position-performance problems.

However, if the EGR position sensor is integrated into the actuator, severe mechanical sticking can sometimes cause the reported position to disagree with the expected position and may contribute to related diagnostic faults.

The important distinction is:

Mechanical EGR problem: the valve does not move or flow correctly.

Electrical circuit problem: the ECM receives an abnormally low electrical signal.

P0407 primarily points toward the second situation.

Symptoms You May Notice

The symptoms depend heavily on how the vehicle's EGR system is designed and how the ECM reacts to the fault.

Possible symptoms include:

• Check Engine light

• Reduced engine performance

• Hesitation during acceleration

• Rough running

• Increased fuel consumption

• Increased emissions

• Unstable EGR operation

• Limp-home mode

• Poor low-speed drivability

• Difficult regeneration on some diesel applications

Some vehicles may show almost no noticeable driving symptoms.

The ECM can sometimes compensate for an EGR sensor problem or temporarily disable EGR operation while the engine continues to run normally.

How to Diagnose P0407 Correctly

A good diagnosis starts with determining which physical sensor the manufacturer calls EGR Sensor B.

1. Scan all control modules

Do not look only at the engine ECU.

Check the ECM, transmission module, body module and other relevant systems for related faults.

Additional EGR, 5-volt reference, sensor supply, CAN communication or power-supply codes can provide an important clue.

2. Check the freeze-frame data

Look at the conditions under which P0407 was stored:

• Engine RPM

• Coolant temperature

• Vehicle speed

• Engine load

• EGR command

• EGR position

• Sensor voltage

• Other relevant pressure readings

If the code appears immediately after starting, that can point toward an electrical problem.

If it appears only after the EGR begins moving, the diagnosis may need to include the actuator and mechanical operation.

3. Identify Sensor B

Use the manufacturer's wiring diagram or service information to determine:

• Sensor location

• Pin configuration

• Reference voltage

• Ground

• Signal wire

• Expected signal range

• Whether the sensor is integrated into the EGR valve

This prevents testing the wrong component.

4. Check the sensor signal

Monitor the sensor voltage or position value with a scan tool.

If the reported value is extremely low and does not change when the EGR is commanded to move, investigate the circuit.

If the sensor signal changes normally but the code returns, look more closely at intermittent wiring problems and the manufacturer's diagnostic criteria.

5. Test the reference and ground

If the sensor uses a reference circuit, verify the reference voltage at the sensor connector.

Then test the ground under operating conditions.

A ground can appear good with a simple continuity test and still have excessive voltage drop when the circuit is operating.

6. Inspect the signal wire for a short to ground

This is particularly important with a P0407.

Inspect the harness from the sensor toward the ECM for:

• Chafing

• Melted insulation

• Pinched wires

• Previous repairs

• Oil contamination

• Connector damage

• Contact with exhaust components

A signal wire shorted to ground can produce exactly the type of low input detected by the ECM.

7. Perform a wiggle test

With the scan tool displaying the EGR sensor value, gently move the harness and connectors.

If the signal suddenly changes, disappears, or becomes unstable, the wiring or connector deserves close attention.

8. Command the EGR system

If the vehicle supports bidirectional testing, command the EGR valve through different positions.

Watch both:

• Commanded EGR position

• Actual EGR position

If the command changes but the feedback remains near zero, investigate the sensor circuit, actuator and mechanical movement.

If both command and feedback behave normally but the code returns intermittently, wiring and connector faults become more important suspects.

9. Check the mechanical EGR system

Only after the electrical circuit has been evaluated should you move deeper into the mechanical system.

Inspect for:

• Carbon deposits

• Sticking valve

• Restricted EGR passages

• Damaged actuator mechanism

• EGR cooler restrictions

• Vacuum problems on vacuum-operated systems

The exact checks depend on the engine.

A Useful Diagnostic Clue: Several Sensor Codes at Once

If P0407 appears together with several other sensor circuit codes, do not immediately assume multiple sensors have failed.

A shared reference or ground circuit can affect several sensors simultaneously.

For example, a problem with a common 5-volt reference supply may cause multiple sensors to report abnormal values.

Likewise, an ECM ground or power-supply problem can create apparently unrelated sensor faults.

Looking at the entire fault-code list is therefore much more useful than diagnosing P0407 in isolation.

P0407 vs P0409

Both codes concern the EGR Sensor B/A naming system, but they describe different electrical conditions.

P0407 – EGR Sensor "B" Circuit Low

The ECM sees an abnormally low electrical signal from the designated EGR Sensor B circuit.

P0409 – EGR Sensor "A" Circuit

P0409 is a broader circuit malfunction involving the designated EGR Sensor A circuit.

The letters A and B are manufacturer-defined, so they should not be assumed to correspond to the same physical sensors across different vehicle brands.

P0407 vs P0405 and P0406

These codes are particularly useful to distinguish because they describe different electrical signal conditions.

P0405 – EGR Sensor "A" Circuit Low

The ECM detects a low signal from EGR Sensor A.

P0406 – EGR Sensor "A" Circuit High

The ECM detects a high signal from EGR Sensor A.

P0407 – EGR Sensor "B" Circuit Low

The ECM detects a low signal from EGR Sensor B.

The sensor designation is important. You should not assume that P0405 and P0407 refer to two identical sensors simply because both contain “EGR Sensor” in their descriptions.

Can You Drive With P0407?

If the engine runs normally and the only symptom is the Check Engine light, the vehicle may remain drivable for a limited period.

However, continued operation can lead to:

• Increased emissions

• Poor fuel economy

• Reduced performance

• EGR control problems

• Limp mode

• Problems with emissions testing

If the vehicle has severe power loss, heavy smoke, rough running, overheating, or other major symptoms, it should be diagnosed promptly rather than treated as a minor sensor fault.

Should You Replace the EGR Valve?

Not based on P0407 alone.

The correct repair depends on what testing reveals.

If the signal wire is shorted to ground, repair the wiring.

If the connector has poor terminal contact, repair the connector.

If the reference or ground circuit is defective, repair that circuit.

If the sensor itself has failed, replace the sensor if it is separately serviceable.

If the sensor is integrated into the EGR actuator and the assembly has been proven defective, replacement of the complete EGR assembly may be necessary.

If the EGR valve is mechanically stuck, the repair may involve cleaning or replacing the valve depending on its condition and the manufacturer's repair procedure.

Final Diagnostic Summary

P0407 means the ECM has detected a low electrical signal in the circuit of the designated EGR Sensor B.

It does not automatically mean:

• The EGR valve is dirty

• The EGR valve is stuck

• EGR flow is too low

• The sensor itself has failed

• The engine has a mechanical problem

The most effective diagnostic approach is to first identify exactly what EGR Sensor B represents on that vehicle, then check the sensor's reference, ground, signal and wiring. After the electrical circuit is proven healthy, evaluate the EGR actuator and mechanical system.

In practice, a P0407 diagnosis should answer one fundamental question:

Is the sensor actually producing a low signal, or is something in the circuit forcing the ECM to see a low signal?

That distinction prevents unnecessary EGR valve and sensor replacement and usually leads to a much faster repair.