OBD-II trouble code
P0386: Crankshaft Position Sensor 'B' Circuit Range/Performance
The second crank sensor is sending a signal, but the pattern doesn't add up — and because 'B' often reads the flywheel, the suspect list includes the wheel it is reading, not just the sensor doing the reading.
Quick facts
- System
- Powertrain
- Category
- Sensors / Timing
- Severity
- High severity
- Drivable
- No — stop driving until repaired
- Repair cost range
- $150 – $2,000
- DIY difficulty
- Advanced DIY
What does P0386 mean?
P0386 is the plausibility complaint of the second crank sensor family. The circuit is electrically alive — this is not a dead sensor or a shorted wire — but the pulse train arriving at the powertrain control module does not describe a crankshaft behaving the way the module knows crankshafts behave. Pulses missing where teeth should be, extra edges where there are none, spacing that jitters, or a tooth count per revolution that simply comes out wrong.
A range/performance complaint shifts suspicion away from the wiring and toward the mechanical interface: the gap between the sensor tip and the wheel, the condition of the wheel's teeth, and the sensor's own declining output. And here the 'B' position adds something its 'A' sibling usually escapes. On many engines the second sensor reads the flywheel or flexplate at the back of the crankshaft — often the ring gear teeth or slots machined into the plate itself. That wheel leads a harder life than the dedicated trigger wheel at the front of the engine. The starter pinion slams into the ring gear at every start, and worn or burred teeth in the section the starter favours distort the pattern once per revolution. On automatic-transmission vehicles, flexplates crack — a classic, well-documented failure — and a crack that lets the plate flex changes the sensor gap cyclically, producing exactly the kind of believable-but-wrong signal this code describes, sometimes accompanied by a knock that mimics a rod bearing. Oil from a leaking rear main seal finding its way onto the pickup completes the list of rear-of-engine hazards that the front sensor never meets.
There is also a subtlety in how this code gets set: on engines where the module continuously cross-references the two crank signals, P0386 can mean not 'B is malformed in isolation' but 'B disagrees with A'. That is worth knowing because it changes the standard of proof — a signal that looks passable on a scope can still be the loser of a comparison against a healthier channel, and the module's freeze frame data showing when the disagreement occurs (cold, hot, under load, at start) often points at thermal or mechanical origins better than any static test.
The cost question hangs on which side of the interface is at fault. A sensor is cheap. The wheel it reads may not be: ring gear or flexplate work on most vehicles means removing the transmission, and that decision deserves visual confirmation — through the inspection cover or starter aperture — before anyone commits to it.
Common causes
- Cracked flexplate flexing under load and cyclically altering the sensor gap
- Worn, burred or damaged ring gear teeth in the section the starter engages
- Weak output from an ageing magnetic pickup that no longer clears the module's threshold at low RPM
- Excessive or uneven air gap after clutch, flywheel or transmission work
- Metallic swarf accumulated on the sensor tip fattening and blurring the pulses
- Oil mist from a leaking rear main seal coating the sensor face
- Intermittently poor connection adding or dropping edges without failing outright
- Damaged or missing teeth on a dedicated 'B' trigger wheel where one is fitted
Symptoms
- Check engine light with intermittent rough running or hesitation
- Misfire codes stored alongside on engines that use the 'B' signal for misfire detection
- Extended cranking or occasional failure to catch on the first attempt
- Stalling that clusters around hot restarts or specific RPM ranges
- A metallic knock or rattle at idle that changes with load — the acoustic signature of a cracked flexplate
- Symptoms that worsen at low RPM, where a weak magnetic signal is smallest
Diagnostic steps
- 1.Read the freeze frame before touching the car: engine temperature, RPM and load at the moment of the fault sketch the conditions to reproduce, and they distinguish thermal faults from mechanical ones.
- 2.Establish what the 'B' sensor reads on this engine — a dedicated wheel or the flywheel/flexplate. The remainder of the diagnosis branches on this answer.
- 3.Listen at idle for a rattle or knock that changes when the transmission is loaded in gear; a cracked flexplate frequently announces itself acoustically before it fails the sensor.
- 4.Scope the 'B' signal over several seconds of cranking and running. Look for the once-per-revolution disturbance that marks localized wheel damage, as opposed to uniform degradation that marks the sensor.
- 5.Compare against the 'A' channel on a dual-trace scope where possible — the module is comparing them, and seeing what it sees settles which channel is lying.
- 6.Inspect the sensor tip on removal for swarf and oil; both are repairable findings that cost nothing but cleaning and a seal.
- 7.Rotate the engine by hand and inspect the ring gear or trigger wheel through the sensor aperture or inspection cover, tooth by tooth through the starter-engagement zone.
- 8.Only after the wheel is visually cleared should a marginal sensor be condemned on waveform amplitude — the sensor is the cheap conclusion, so it must be the proven one.
Repair cost
$150 – $2,000
A sensor, a cleaning or a gap correction lands at $150 to $400 fitted. The number that separates this code from its siblings is the mechanical tail: a cracked flexplate or damaged ring gear means transmission removal, and that is $800 to $2,000 on common vehicles once the plate and labor are counted. That is precisely why the inspection step matters — a $150 sensor thrown at a $1,500 flexplate problem returns in days with the same code, and the second bill still includes everything the first one did.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with crankshaft position sensor replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an advanced DIY job. It typically requires specialty tools, scan-tool access, lifting equipment, or careful sequencing to avoid causing new failures. Plan for extended downtime and have a backup vehicle. Most owners are better served by a shop that has done this repair before.