OBD-II trouble code
P0389: Crankshaft Position Sensor 'B' Circuit Intermittent
The second crank sensor's signal keeps dropping out and coming back. Mounted near the starter and exhaust, this sensor lives in a heat-soak zone that makes 'fails hot, works cool' its defining pattern.
Quick facts
- System
- Powertrain
- Category
- Sensors / Timing
- Severity
- High severity
- Drivable
- No — stop driving until repaired
- Repair cost range
- $150 – $700
- DIY difficulty
- Intermediate DIY
What does P0389 mean?
P0389 records an event rather than a state: the second crankshaft position sensor's signal was present and valid, disappeared or turned erratic, and then recovered. By the time anyone connects a scan tool, the circuit may measure perfectly — which is what separates this code from every other member of its family. Its siblings describe faults you can find on demand. This one describes a fault you have to catch.
The pattern to interrogate first is temperature, because the 'B' sensor's home is a heat-soak trap. On the many engines that mount it at the bellhousing, it sits inches from the exhaust and directly above the starter — and the worst thermal moment in that location is not a long motorway run but the fifteen minutes after shutdown, when airflow stops and heat from the manifolds and starter migrates into everything nearby. Marginal electronics fail by temperature: a winding whose insulation has a microscopic break opens as it expands, a cracked solder joint inside the sensor lifts, a spread connector terminal loses its grip. So the classic biography of this code is a car that starts perfectly cold, runs a short errand, and then cranks endlessly or stalls when restarted hot — recovering completely once it sits and cools. An owner who can say 'it only does it hot' has done half the diagnosis in one sentence.
The other distinctive suspect list for an intermittent at this location is mechanical motion. The engine rocks on its mounts with every throttle application, and a 'B' harness with insufficient slack, or one re-secured badly during previous work, gets tugged at each torque event. Signal dropouts that correlate with hard acceleration, gear changes, or rough roads are the fingerprint of a wiring fault being physically moved, and a collapsed engine mount that exaggerates the rocking has ended more than one long hunt for this code. There is also a genuinely mechanical intermittent unique to flywheel-reading sensors: a cracked flexplate whose gap opens with thermal expansion and load, corrupting the signal only in the conditions that flex it.
Diagnosis therefore leans on evidence capture rather than static measurement: freeze frame data for the conditions at the moment of failure, a scope or graphing meter left connected while an assistant recreates them, wiggle tests at full engine rock, and heat applied deliberately rather than waited for. An intermittent found is a repair; an intermittent guessed at is a parts bill followed by the same code.
Common causes
- Sensor failing under heat soak — internal winding break or cracked solder joint that opens as temperature rises
- Spread, corroded or oil-wicked connector terminal making contact only intermittently
- Harness with inadequate slack being tugged by engine rock under torque
- Chafed insulation making momentary contact with ground over bumps or vibration
- Deteriorated or collapsed engine mount amplifying movement at the harness
- Air gap at the margin — correct when cold, excessive when hot parts expand
- Cracked flexplate flexing under load and heat where the sensor reads the flywheel
- Intermittent shield or ground connection letting ignition noise swamp the signal at certain loads
Symptoms
- Hot-restart failure: starts perfectly cold, cranks endlessly or stalls after a short stop, recovers once cooled
- Momentary stumble or cutout under hard acceleration or over rough pavement
- Stalls that leave no evidence — the engine restarts and runs normally immediately afterwards
- Tachometer flicker coinciding with the stumble in some implementations
- Code that returns days or weeks after being cleared, never immediately
- Long periods of flawless running between events, particularly in cool weather
Diagnostic steps
- 1.Mine the freeze frame first: coolant temperature, RPM, load and vehicle speed at the moment the code set are the conditions to recreate, and they usually declare the fault heat-driven or motion-driven before any tool touches the car.
- 2.Interview the symptom pattern honestly — 'only when hot', 'only under acceleration', 'only over bumps' each sends the diagnosis down a different branch.
- 3.Recreate heat deliberately rather than waiting: run to full temperature, shut down for ten to fifteen minutes to let heat soak the bellhousing area, then attempt a restart with a scope on the 'B' signal. This is the highest-yield single test for this code.
- 4.Perform a wiggle test with the engine idling and the scope connected: flex the harness along its run, rock the engine against its mounts with brake and drive engaged, and watch for dropouts.
- 5.Apply controlled heat to the sensor body with a heat gun while monitoring the signal — a sensor with an internal thermal fault will reproduce on the bench of its own bore.
- 6.Inspect the connector under magnification for terminals that have lost tension, and for the fretting corrosion that intermittent contact leaves behind.
- 7.Check the engine mounts; excessive rock multiplies every marginal connection's exposure, and a failed mount can be the root cause rather than a bystander.
- 8.Where the sensor reads the flexplate and events correlate with load and heat together, inspect the plate through the access cover before concluding — the mechanical intermittent looks electrical on every meter.
Repair cost
$150 – $700
The repair itself is rarely the expense — a sensor is $30 to $150, a terminal or harness dressing $100 to $250, and fitted totals land between $150 and $450 for most outcomes. What inflates this code's real-world cost is diagnostic time: an intermittent that will not perform on cue can absorb $150 to $300 across visits, and the failed shortcut — replacing the sensor on hope — adds a part without removing the fault roughly as often as not. Bring the shop your pattern (hot, load, bumps) in writing; a good description shortens the expensive part of this job more than any tool.
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 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.