Crankshaft Position Sensor B Circuit Intermittent
Your engine's timing sensor is sending spotty signals to the computer, like a radio that keeps cutting out. The ECU can't reliably track piston position, which throws off ignition and fuel timing.
The ECM monitors voltage and signal frequency from Crankshaft Position Sensor B to confirm precise piston location for spark timing and fuel injection. It expects consistent signal transitions within specific voltage windows; intermittent dropouts or weak signals trigger this fault.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Sensor Signal Voltage | 0.2V to 4.8V with clean transitions | Voltage spikes, dropouts, or erratic transitions below 0.2V or above 4.8V |
| Signal Frequency Stability | Consistent pulses matching engine RPM | Intermittent signal loss or frequency variance >10% over 100ms window |
Code P0400 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P0400 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.