Exhaust Gas Recirculation Sensor B Circuit Low
P0407 indicates the Engine Control Module detected a low voltage signal from the EGR (Exhaust Gas Recirculation) Sensor B circuit. This sensor monitors exhaust gas recirculation flow, and a low signal typically means a short to ground, wiring issue, or failed sensor preventing proper EGR system operation.
What You Might Notice
- Check Engine Light (MIL) illumination
- Rough idle or unstable engine operation
- Increased engine emissions or visible exhaust smoke
- Possible increase in fuel consumption
- Reduced engine performance under load
Most Common Causes
- 1
Short to ground in EGR signal circuit
The EGR sensor signal wire may be shorted to ground, causing the voltage to drop below the expected threshold. This is commonly caused by damaged insulation or improper routing.
- 2
Bad EGR valve or sensor
The EGR valve or EGR position sensor has failed electrically, producing an incorrect low voltage signal to the PCM.
- 3
Wiring issues and loose connections
Loose terminals, corroded connectors, or chafed wiring in the EGR sensor circuit can create intermittent or continuous signal loss.
- 4
PCM or module malfunction
Less commonly, the Powertrain Control Module itself may have failed, though this is typically diagnosed after ruling out circuit and component issues.
How to Diagnose It
- 1
Check Short to ground in EGR signal circuit — The EGR sensor signal wire may be shorted to ground, causing the voltage to drop below the expected threshold. This is commonly caused by damaged insulation or improper routing.
- 2
Check Bad EGR valve or sensor — The EGR valve or EGR position sensor has failed electrically, producing an incorrect low voltage signal to the PCM.
- 3
Check Wiring issues and loose connections — Loose terminals, corroded connectors, or chafed wiring in the EGR sensor circuit can create intermittent or continuous signal loss.
- 4
Check PCM or module malfunction — Less commonly, the Powertrain Control Module itself may have failed, though this is typically diagnosed after ruling out circuit and component issues.
How to Fix It
- 1
Inspect and repair EGR sensor wiring
Visually inspect the EGR sensor B circuit wiring harness for signs of damage, chafing, or loose connections. Repair any damaged insulation and reseat all connectors securely. Test continuity with a multimeter.
- 2
Test EGR sensor voltage
Use a scan tool or multimeter to measure the actual voltage output from the EGR sensor B circuit. Compare to manufacturer specifications; if voltage is below range, the sensor or its circuit is faulty.
- 3
Replace EGR sensor or valve
If wiring tests pass but voltage remains low, replace the EGR valve or EGR position sensor (depending on vehicle design). Follow OEM procedures for removal and installation.
- 4
Command EGR valve diagnostics and PCM evaluation
Use a scan tool to command the EGR valve on and monitor response. If the valve does not respond or signals remain low, verify PCM ground and power circuits, then replace the PCM if all other components test normal.
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