OBD-II trouble code
P0387: Crankshaft Position Sensor 'B' Circuit Low Input
The second crank sensor's circuit is stuck at a low voltage — a wiring-flavoured complaint, on a sensor whose harness lives in one of the harshest neighbourhoods on the vehicle.
Quick facts
- System
- Powertrain
- Category
- Sensors / Timing
- Severity
- High severity
- Drivable
- No — stop driving until repaired
- Repair cost range
- $120 – $600
- DIY difficulty
- Intermediate DIY
What does P0387 mean?
P0387 sets when the voltage on the second crankshaft position sensor's circuit stays below the window the powertrain control module considers valid. Where a range/performance code (P0386) complains about a believable signal with the wrong shape, a low-input code complains about the level itself: the line is being held down, has collapsed, or was never driven up in the first place. That framing matters because it steers the diagnosis toward electrical causes — shorts to ground, opens, connector faults, a dead output stage — and away from the trigger wheel, which cannot pull a circuit's voltage down no matter how damaged it is.
What gives this code its particular character is the neighbourhood the 'B' circuit runs through. On the many engines that mount the second sensor at the bellhousing or low on the block, its harness spends its life in the hottest, oiliest, most mechanically trafficked part of the underbody. It routes past the exhaust, where a displaced heat shield lets insulation bake until it cracks and the conductor leans on grounded metal. It passes the transmission-to-engine junction, where every clutch job, transmission removal and engine mount replacement disturbs it — a harness pinched between bellhousing and block during reassembly is a genuinely common origin for this code, and the fault appears immediately after the work, which makes the service history the single most useful document in the diagnosis. And it lives where a leaking rear main seal or valve cover drips on it, because oil-swelled insulation eventually splits.
Sensor technology decides one fork in the reasoning. A magnetic (two-wire) sensor generates its own voltage, so a permanently low line means a shorted winding, a short to ground in the run, or an open that leaves the module's reference with nowhere to go. A Hall-type (three-wire) sensor must be fed: its supply and ground have to be present before the signal wire can do anything, so a missing 5-volt reference or a corroded ground produces a flatlined signal that convicts the feed, not the sensor. Determining which type the 'B' position uses on your engine is a wiring-diagram question and it halves the fault tree before the first measurement.
A permanently low circuit is, in one respect, the merciful version of this fault: it is present all the time, so it can be found on a bench-calm morning with a multimeter rather than chased through the intermittent misery of its sibling P0389.
Common causes
- Signal wire shorted to ground where heat-damaged or chafed insulation contacts the block, bellhousing or a heat shield
- Harness pinched during clutch, transmission or engine mount work at the bellhousing junction
- Open circuit in the 'B' signal run leaving the line at ground potential
- Missing 5-volt reference to a Hall-type sensor, from a supply fault or shared-reference short elsewhere
- Shorted winding inside a magnetic pickup dropping its output to nothing
- Oil-soaked connector wicking into the terminals and shorting the signal to ground
- Corrosion at the sensor connector building resistance until the signal collapses
- Damaged shield drain wire grounding the signal conductor instead of the shield
Symptoms
- Check engine light with otherwise normal driving on engines that run happily on the primary sensor
- Long cranking before start where the 'B' signal provides start synchronisation
- No-start after recent clutch or transmission work — the signature of a disturbed harness
- Stalling with no restart on architectures that require both crank signals
- Code returning immediately after clearing, reflecting a hard, permanent fault
- Tachometer needle dead or erratic in some implementations
Diagnostic steps
- 1.Start with the service history. If the code appeared after clutch, transmission, engine mount or starter work, inspect the harness at the bellhousing junction before testing anything — a pinched or trapped loom there is a frequent and fast finding.
- 2.Identify from the wiring diagram whether the 'B' sensor is magnetic or Hall-type; the two produce a low circuit through different mechanisms and are tested in different orders.
- 3.For a Hall-type: verify the sensor's supply and ground at its connector, key on. No supply means the fault is upstream, and note that a shared 5-volt reference shorted by a different sensor elsewhere on the engine can present as this code.
- 4.For a magnetic type: measure winding resistance against specification. A short reads near zero; an open reads infinite; both flatline the signal.
- 5.With the sensor disconnected, check the signal wire for continuity to ground. Continuity with both ends open convicts the harness run — trace it past the exhaust and the bellhousing seam.
- 6.Inspect connector terminals for oil intrusion; where oil has wicked in, the leak feeding it needs identifying too, or the repair has a countdown timer.
- 7.Scope the signal at the module connector while cranking to confirm the repair end-to-end rather than at the sensor alone.
- 8.Clear the code and re-test through a full warm-up: heat-dependent shorts close as the exhaust reaches temperature, and a fault that passed cold can return in ten minutes.
Repair cost
$120 – $600
Harness and connector repairs — the most common outcome for a low-input code — run $120 to $350 depending on how much loom has to be opened and re-wrapped. A replacement sensor is $30 to $150 in parts, with fitted cost governed by access: $150 to $300 for a reachable unit, up to $600 where the 'B' position hides behind mounts or requires working blind at the bellhousing. If oil contamination is found, price in fixing the leak that delivered it; a rear main seal is its own substantial job, but ignoring it re-buys this repair on a schedule.
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DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.