What Does P0388 Mean?
The crankshaft position sensor B circuit is reporting a high input signal to the ECM. This indicates the voltage from the CKP sensor B exceeds the expected maximum threshold. On a Hall-effect sensor, the signal may be stuck high; on a magnetic sensor, the output amplitude may be excessive due to a gap that is too close. The ECM may miscount teeth or misinterpret position data, causing ignition timing errors, misfires, and rough engine operation.
Common Causes
30%
CKP sensor B signal wire shorted to the 5V reference or battery voltage
25%
Sensor air gap too close causing excessively high signal amplitude that saturates the ECM input
20%
Internal CKP sensor B fault producing an abnormally high output
15%
Open ground return wire causing the signal to float high
10%
ECM input circuit pull-up resistor or protection diode failure
Diagnostic Steps
1
Measure the CKP sensor B signal voltage with the engine off and ignition on. For a Hall-effect sensor, the resting state should be defined (typically low). A constant 5V or battery voltage reading indicates a short-high or open ground.
2
Check the sensor B ground return wire for continuity from the sensor connector to the ECM ground pin. An open reading (OL) confirms a broken ground wire, which causes the signal to float to the reference voltage.
3
With the sensor disconnected, check for shorts between the signal wire and the 5V reference wire or any B+ wiring. Even brief contact between these wires can cause a high input reading.
4
For magnetic sensors, verify the air gap is not too small. An excessively close gap (below ~0.3mm) generates voltages that can exceed the ECM's input clamp, causing signal distortion and misreading.
5
Capture the sensor B waveform with an oscilloscope and compare peak voltage to manufacturer specifications. If the waveform shows clipping at the top, the signal is exceeding the ECM's input range.
Estimated Repair Cost
$80 - $450
Parts + labor, varies by vehicle and location
The crankshaft position sensor B circuit is reporting a high input signal to the ECM. This indicates the voltage from the CKP sensor B exceeds the expected maximum threshold. On a Hall-effect sensor, the signal may be stuck high; on a magnetic sensor, the output amplitude may be excessive due to a g...
The most common cause of P0388 (Crankshaft Position Sensor B Circuit High Input) is: CKP sensor B signal wire shorted to the 5V reference or battery voltage
Typical repair costs for P0388 range from $80 to $450, depending on the vehicle, location, and whether you do it yourself or go to a shop.
A high CKP signal can cause timing calculation errors leading to misfires and rough operation. While less likely to cause sudden stalling than a low signal, incorrect timing data can cause engine damage from detonation if timing advances too far. Have the vehicle repaired soon; drive gently and avoid high loads until fixed.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0388 to identify the root cause.
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Quick Info
Category
Powertrain
System
Ignition / Engine Timing System
Difficulty
Type
Generic (SAE)
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