P0387: Crankshaft position sensor B signal too low
Crankshaft position sensor B circuit low input
Severity: Inspect soon
The crankshaft signal times fuel and spark, and TroubleCodes.net lists stalling or no-start among the symptoms for this family of codes, so have it checked soon.
P0387 in short
P0387 means the engine computer reports a low voltage problem with crankshaft position sensor B. The sensor, its wiring or its connector is the usual cause. Depending on the vehicle, the engine may start hard, run rough, stall or not start, so get it checked soon and avoid relying on the car until it is.
The crankshaft position sensor tells the engine computer how fast the engine turns and where the pistons are, so it can time fuel and spark. P0387 is set when it is not getting enough voltage from crankshaft position sensor B.
TroubleCodes.net and the Delaware DMV list agree on the generic title: Crankshaft Position Sensor B Circuit Low Input. The code points at the sensor B circuit but does not say which part failed.
Sensor, wiring and connector are the usual suspects, and the computer is checked last. Which engine is affected, and what 'B' refers to, depends on the maker, so use the vehicle's own service information.
Most likely causes
These are general causes for this code across vehicles. None is specific to one make or model.
Cause 1: Failed or misaligned crankshaft position sensor
Common general cause- Why it ranks here
- TroubleCodes.net lists a defective sensor and poor alignment with the reluctor wheel.
- What would change this
- Rises if the sensor tests open, shorted or gives no signal while cranking. Falls if the sensor tests normal.
Cause 2: Damaged wiring or connector between the sensor and the computer
Common general cause- Why it ranks here
- TroubleCodes.net lists harness damage, corrosion and loose or contaminated connectors for this code.
- What would change this
- Rises if the harness is chafed, corroded or wiggling it changes the signal. Falls if the circuit tests clean end to end.
Cause 3: Cranking speed too low for the sensor to read
Possible — needs testing- Why it ranks here
- TroubleCodes.net says to confirm battery and starter first, since slow cranking weakens the signal.
- What would change this
- Rises if the sensor and wiring test normal. Falls if either shows a fault.
What to test first
Start at step 1. Each result tells you what it means and where to go next.
Step 1: Read all stored codes and freeze-frame data with a scan tool, and note any other crankshaft or camshaft codes stored with P0387.
IfOther crank or cam codes are stored
It meansThe wiring or computer may be involved; note them for the shop.
IfOnly P0387 is stored
It meansLook at the sensor wiring next.
Step 2: With the engine off, inspect the crankshaft sensor connector and visible harness for damage, corrosion, oil or coolant contamination and loose pins.
IfDamage, contamination or a loose connector is found
It meansThe wiring or connector is the likely cause.
Points to: Damaged wiring or connector between the sensor and the computer
IfEverything looks sound
It meansTest the sensor itself.
Step 3: Have the sensor resistance and signal checked against the maker's specification, and have the harness tested for opens and shorts.
Warning:The signal is checked while the engine is cranked, close to belts and fans, and the sensor is often in a hard-to-reach spot near the transmission. Let a shop do it.
IfThe sensor reading or signal is out of specification
It meansThe sensor is the likely fault.
IfThe sensor is fine but the circuit has a fault
It meansRepair the circuit.
Points to: Damaged wiring or connector between the sensor and the computer
IfSensor and circuit both test normal
It meansConsider the other causes, including the computer.
Symptoms
- Long cranking or no start
- Misfire, stalling or lack of power
- Poor fuel economy in limp-home mode
- Sometimes no symptoms other than the light
How to fix
Step 1: With the engine off, look over the crankshaft sensor connector and visible wiring for damage or corrosion.
Parts and cost context
- Crankshaft position sensor:Test the sensor and wiring before replacing it. The sensor can be hard to reach, so labour varies; get a quote.
Labor varies by vehicle and shop; get a quote before you approve a repair.
Compare used part prices on PartPriceGuideOn your vehicle
We don't have reviewed vehicle-specific evidence for P0387 yet. The causes and tests above apply across vehicles.
Related codes
Sources and review
- Last reviewed
- Written by
- Saurabh K.
- Editorial review
- Pending
- Technician review
- Technician review pending
This page explains what a code usually means; it is not a substitute for a professional inspection. How we source and review · Report an error