P0387 – Crankshaft Position Sensor “B” Circuit Low

P0387 is a diagnostic trouble code that points to an issue in the starting circuit, most often related to the crankshaft position or starter circuit low input, depending on the manufacturer. When this code sets, your ECU is seeing a voltage or signal that’s out of range and can’t reliably control starting or timing. You might notice hard starting, extended cranking, or a no-start when the engine is hot or cold. In this guide, you’ll learn what P0387 means, common causes, symptoms, how to diagnose it, and realistic repair options.

What Does P0387 Mean?

P0387 is typically defined as “Crankshaft Position Sensor B Circuit Low Input” or a similar “circuit low” description for a starting/position sensor, depending on the brand. In plain terms, the ECU is not seeing the proper signal level from a key sensor or circuit used to control starting and ignition timing.

This code is common on many diesel and gasoline engines that use multiple crank sensors or reference signals. A “circuit low” fault usually points to a wiring, connector, or sensor problem rather than a purely mechanical failure inside the engine.

Quick Reference

• Code: P0387
• Meaning: Sensor or start circuit “B” low input (often crankshaft position related)
• Typical Symptoms: Hard start, long crank, occasional no-start, rough idle
• Risk Level: Moderate – can leave you stranded if it worsens
• Common Fixes: Sensor replacement, wiring repair, connector cleaning, starter/ground checks

Real-World Example / Field Notes

In the shop, I often see P0387 on higher-mileage trucks and SUVs, especially those used for towing or short trips. One example was a diesel pickup that cranked longer than normal and occasionally just clicked on cold mornings. The scan tool showed P0387 and a weak signal from the crank sensor “B.” The wiring harness near the starter was rubbed through on a bracket, causing a low voltage signal. After repairing the harness and securing it away from the metal edge, the truck started instantly and the code never returned.

Symptoms of P0387

• Hard starting: Engine cranks longer than normal before it fires.
• Intermittent no-start: Sometimes you just get cranking or a click with no start.
• Rough idle: Engine may stumble or idle unevenly after starting.
• Reduced power: ECU may enter a limited-power or “limp” mode to protect the engine.
• Stalling: Engine can stall at idle or low speeds if the signal drops out.
• Check Engine Light on: MIL illuminated, possibly with other starting or sensor codes.
• Poor fuel economy: Incorrect timing and fueling can waste fuel over time.

Common Causes of P0387

Most Common Causes

• Faulty crankshaft position sensor “B” (or related start/position sensor)
• Damaged wiring harness to the sensor or starter circuit (chafed, corroded, or broken wires)
• Loose, corroded, or oil-soaked electrical connector at the sensor
• Weak battery or low system voltage affecting sensor signal levels
• Poor engine or chassis ground connections near the starter or sensor

Less Common Causes

• Failing starter drawing excessive current and pulling voltage low
• Internal ECU (PCM) fault or water intrusion at the control module
• Incorrect aftermarket sensor with wrong resistance or output characteristics
• Mechanical damage to the reluctor ring or tone wheel used by the sensor
• Previous wiring “repairs” with poor splices, tape-only joints, or wrong gauge wire

Diagnosis: Step-by-Step Guide

You’ll want a basic tool set, a quality digital multimeter, a scan tool capable of live data and Mode $06, and ideally a wiring diagram for your specific vehicle. A test light and back-probing pins are also helpful. If you’re not comfortable working around the starter or under the vehicle, consider having a professional handle the undercar checks.

1. Confirm the code: Scan the ECU and verify P0387 is present. Note any companion codes (such as other crank sensor, cam sensor, or voltage codes) because they can point you in the right direction.
2. Check battery and charging system: Test battery voltage (should be around 12.6V engine off, 13.5–14.5V running). Low voltage can cause a “circuit low” reading. Clean and tighten battery terminals.
3. Perform a visual inspection: Locate the sensor or circuit labeled “B” in your service info. Inspect the harness from the sensor to the main loom and around the starter for rubbing, burns, or oil saturation. Repair obvious damage.
4. Inspect connectors: Unplug the sensor connector. Look for green corrosion, bent pins, moisture, or loose terminals. Clean with electrical contact cleaner and apply dielectric grease if appropriate.
5. Check reference voltage and ground: With the key on, use a multimeter to verify the sensor is getting proper reference voltage (often 5V) and a solid ground. A missing reference or bad ground will trigger P0387.
6. Test sensor resistance/output: Compare the sensor’s resistance to factory specs. On some systems, you can monitor live data and watch the RPM signal while cranking. If the RPM stays at zero or drops out, the sensor or wiring is suspect.
7. Wiggle test the harness: While monitoring live data or the sensor signal with a meter, gently move the harness and connector. If the signal cuts in and out, you’ve likely found a broken wire or loose terminal.
8. Inspect grounds and starter circuit: Check engine-to-chassis ground straps and main grounds near the starter. High resistance at these points can pull voltage low when cranking and confuse the ECU.
9. Check Mode $06 data (if available): Some scan tools let you see misfire and sensor test results. Look for crank sensor test values close to the failure threshold, which can confirm a marginal sensor.
10. Evaluate the ECU only last: If wiring, sensor, grounds, and starter current draw all check out, an internal ECU fault becomes more likely. These are rare and should be confirmed by a professional.

Pro tip: Always clear the code after any repair or test, then drive or perform several start cycles. If P0387 returns under the same conditions, you may have missed an intermittent wiring issue or a heat-related sensor failure.

Possible Fixes & Repair Costs

Common fixes for P0387 include replacing the affected crankshaft position or start-related sensor, repairing or replacing damaged wiring, cleaning or tightening connectors and grounds, and in some cases replacing a failing starter or ECU. Typical repair costs range from about $120–$350 for a sensor replacement, $150–$400 for wiring and connector repairs, and $300–$700+ if a starter is involved. An ECU replacement, while less common, can run $600–$1,200 or more. Labor rates, vehicle design, and parts brand all affect the final bill.

Can I Still Drive With P0387?

Often you can still drive with P0387, but it’s not something you should ignore for long. Many vehicles will start and run, just with harder starting, occasional stalling, or reduced performance. The bigger risk is that the problem becomes intermittent and leaves you with a no-start situation at the worst possible time. If you’re noticing worsening symptoms, avoid long trips and get it diagnosed as soon as you can.

What Happens If You Ignore P0387?

Ignoring P0387 can lead to more frequent no-starts, possible stalling in traffic, and extra stress on the starter and battery from repeated long cranking. Over time, you may end up replacing more parts than necessary because a simple wiring or sensor issue was allowed to worsen. It’s cheaper and safer to address the code early.

Related Sensor Crankshaft Codes

Compare nearby sensor crankshaft trouble codes with similar definitions, fault patterns, and diagnostic paths.

• P0337 – Crankshaft Position Sensor “A” Circuit Low
• P0392 – Camshaft Position Sensor “B” Circuit Low Bank 2
• P0367 – Camshaft Position Sensor “B” Circuit Low Bank 1
• P0807 – Clutch Position Sensor Circuit Low
• P2962 – Intake Air Metering Control Valve Position Sensor Circuit Low
• P2618 – Crankshaft Position Signal Output Circuit Low

Key Takeaways

• P0387 usually indicates a low input problem on a crankshaft or start-related circuit, often labeled “B.”
• Most causes are electrical: bad sensor, damaged wiring, poor connectors, weak grounds, or low system voltage.
• Symptoms range from hard starting and rough idle to intermittent no-start and stalling.
• Basic testing with a scan tool and multimeter can often pinpoint the issue without guesswork.
• Fixing P0387 early helps prevent getting stranded and avoids extra wear on the starter and battery.

Vehicles Commonly Affected by P0387

P0387 shows up frequently on many trucks, SUVs, and diesel applications. It’s commonly seen on Chevrolet and GMC pickups and SUVs, Ford F-Series trucks and vans, Dodge/Ram trucks, and various Jeep models. Some Hyundai, Kia, and Nissan vehicles with multiple crank or start sensors can also log this code. High-mileage work trucks, off-road vehicles, and vehicles exposed to harsh weather or road salt tend to be more prone due to wiring and connector corrosion.

FAQ

Can I clear P0387 and keep driving if the car seems fine?

You can clear the code and drive, but if the underlying issue isn’t fixed, P0387 will likely come back. Use clearing as part of diagnosis, not as the final solution. If the code returns quickly, you need to address the sensor or wiring problem.

Is P0387 always caused by a bad crankshaft sensor?

No. While a failed sensor is common, many P0387 cases are caused by wiring damage, poor grounds, low battery voltage, or corroded connectors. Always test the circuit before throwing parts at it.

Can a weak battery cause P0387?

Yes, a weak battery or poor charging system can drop voltage low enough during cranking to trigger a “circuit low” code like P0387. That’s why checking battery condition and charging voltage is one of the first diagnostic steps.

How urgent is it to fix P0387?

You usually don’t need to stop driving immediately, but you should schedule diagnosis soon. The code often starts as an intermittent issue and then gets worse, eventually leaving you with a no-start or frequent stalling.

Can I replace the sensor myself to fix P0387?

If you’re comfortable with basic tools and can safely raise and support the vehicle (when needed), you can often replace the sensor yourself. However, you should still inspect wiring and connectors. If access is tight or you’re unsure, having a professional handle it can save time and frustration.