P0400
Crankshaft Position Sensor B Circuit Intermittent
Powertrain Ignition System Crankshaft Position Sensing 🟡 Moderate — Fix within a week
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What This Actually Means
In plain language — no jargon

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.

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Symptoms You May Notice
3 known symptoms for this code
Check Engine Light illuminates intermittently
Engine hesitation or stumbling during acceleration
Hard starting or rough idle
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Embedded Systems Insight
What the ECU/ECM is actually computing

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.

Voltage & Parameter Thresholds

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
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Diagnostic & DIY Fix Guide
Check these in order — from cheapest to most complex
1
Crankshaft Position Sensor B connector and wiring
Inspect connector for corrosion, loose pins, or damaged wires; reseat connector firmly and clean terminals with electrical contact cleaner.
2
Crankshaft Position Sensor B
Remove sensor from engine block, check air gap (typically 0.020-0.050 inches) against reluctor ring, clean sensor tip, and reinstall.
3
Engine wiring harness and shielding
Trace sensor wiring for abrasion, pinches, or separation from shielding; repair or re-route away from high-heat and high-voltage components.