P0407
Exhaust Gas Recirculation Sensor B Circuit Low
If your code reader dashboard displays the generic DTC error code P0407, your vehicle is currently experiencing an active failure related to "Exhaust Gas Recirculation Sensor B Circuit Low". Proper system troubleshooting is required to safely clear this warning.
Driver's Summary
The diagnostic trouble code P0407 indicates an active fault in the exhaust gas recirculation sensor b circuit low circuit or component. On the road, this usually shows up as check engine light, engine knocking under load. You can typically drive short distances, but ignoring this code long-term will cause accelerated component wear and higher repair costs.
Symptoms
Check engine light, engine knocking under load
Common Causes
- Short to ground in EGR sensor B wiring
- Faulty EGR sensor B
- Carbon buildup restricting EGR flow
- Corroded connector
How to Fix
- 1 Repair shorted circuit
- 2 Replace EGR sensor/valve
- 3 Clean EGR passages
- 4 Clean electrical contacts
Technical Explanation
The PCM triggers P0407 after its internal monitoring routine detects that a specific circuit or sensor has exceeded its acceptable operating range. The PCM commands the relevant emission control valve or solenoid and then verifies system response through a dedicated feedback mechanism — either a position sensor, a downstream pressure sensor, or changes in O2 sensor activity patterns. Once confirmed, the code is stored as a permanent DTC and the MIL is activated. The freeze frame snapshot — recording RPM, load, coolant temperature, and fuel trim at fault detection — is also saved and is critical for accurate diagnosis.
Is It Safe to Drive?
You can drive short distances, but the symptoms — check engine light, engine knocking under load — indicate the affected system is compromised. Leaving this unresolved will lead to progressively worse fuel economy and potential damage to components beyond the original fault.
Mechanic's Pro Tip
The most common mistake with P0407 is replacing the sensor without verifying the reference voltage and ground integrity first. Use a scan tool to monitor the sensor's live output; a truly failed sensor shows a stuck, flatlined reading — a sensor that fluctuates but reads slightly off usually indicates a wiring or vacuum issue, not a dead sensor. Always spray electrical contact cleaner on the connector pins before condemning the sensor.
EGR Valve/Sensor: $150 - $350