Crankshaft Position Sensor B Circuit Low Input
The engine's crankshaft position sensor B is sending a weak or no electrical signal to the computer, like a dimly lit dashboard warning light that's hard to read. The ECU can't properly detect engine timing, which disrupts fuel injection and ignition.
The ECU monitors the voltage signal from crankshaft position sensor B, which typically alternates between high and low states as the engine rotates. When the signal voltage drops below the expected threshold or remains continuously low, the ECU cannot accurately determine crankshaft position for fuel and spark timing.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Sensor Signal Voltage | 0.2V to 4.8V (alternating square wave) | Below 0.2V or stuck at ground |
| Signal Frequency | Proportional to RPM (typically 50-500 Hz) | No signal or erratic frequency |
Code P0388 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, P0388 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.