P0407 means the powertrain control module has detected a below-specification voltage signal from the EGR system’s designated "B" sensor, which reports EGR valve position or differential pressure. A sustained low reading usually indicates the sensor circuit is reporting near 0 V, so the PCM may disable EGR control and store the code.
The sensor’s internal element can fail, stick, or produce a permanently low output, causing the PCM to see a circuit-low condition.
The signal, 5 V reference, or ground wire between the sensor and PCM may be shorted to ground, chafed, or broken, pulling the signal voltage low.
Corroded, bent, or moisture-filled terminals in the sensor connector can increase resistance or create a partial short, resulting in a low-voltage reading.
If the PCM reference voltage to the sensor is lower than specified, the sensor output will also read low and set P0407.
On differential-pressure-style EGR sensors, clogged, cracked, or incorrectly connected hoses can produce an implausibly low signal.
A rare PCM failure that misreads the sensor or supplies an incorrect reference voltage; this should only be considered after wiring, connectors, and the sensor are ruled out.
The definition is universal; the diagnostics are engine-specific. Open your vehicle for the codes documented for it:
Short trips are usually possible, but EGR function may be disabled, leading to higher emissions, possible rough idle, and an emissions-test failure; it should be diagnosed soon.
No. Wiring shorts, connector problems, reference-voltage faults, and sometimes EGR hose issues can set the code; the sensor itself is only the most common cause.
Usually no. PCM failure is uncommon for this code. Verify the sensor, wiring, connectors, and reference voltage first before considering a control module.
Not necessarily. The code refers to the sensor circuit; the valve may be fine, though heavy carbon buildup can affect position-sensor readings on some engines.