P0387 OBD-II Diagnostic Guide Glow Plug/Heated Element Circuit B - Low Input

P0387 OBD-II Diagnostic Guide (Glow Plug/Heated Element Circuit B - Low Input)

Overview

• What the code means in general: P0387 is a DTC associated with the glow plug/heated element circuit "B" in OBD-II terminology. DTCs are stored by the powertrain control module (PCM/ECM) when the monitored parameter indicates a fault, and they are used to guide diagnostics and repairs.
• Where it fits in the system: This is a powertrain/emissions-related code. OBD-II monitors various electrical circuits including glow plug systems on engines that use glow plugs (primarily diesel engines; some gasoline engines use spark plugs instead and may interpret codes differently depending on OEM). The general concept is that the PCM detects an abnormal condition in a circuit and stores the corresponding DTC (emissions/powertrain monitoring context).
• Important note on code interpretation conflict: An OSS listing appears to map P0387 to (Crankshaft Position Sensor Bank B low signal), which conflicts with the common glow plug interpretation. The Wikipedia-based OBD-II references do not assign CKP-related interpretation to P0387. This discrepancy highlights the need to verify the exact OEM/engine-specific meaning in the vehicle's service documentation. When in doubt, confirm with the factory service information for the exact engine layout and DTC table.

Symptoms

• Difficulty starting in cold conditions or longer cranking before start.
• Noticeable hard starting or extended preheat cycle if the vehicle relies on glow plugs for cold starts.
• Check Engine Light (CEL) or Malfunction Indicator Lamp (MIL) on, sometimes accompanied by a limp-mode behavior.
• Possible rough idle on cold start or uneven engine operation during initial warm-up.
• No obvious fuel delivery symptoms; the issue is electrical/circuit related.

What to do first (scoping and safety)

• Safety: Park on a flat surface, engage parking brake, wear PPE as needed. If you'll be working near hot glow plugs or battery, wait until components cool. Disconnect the battery only if you need to isolate circuits or perform wiring repairs (reconnect before attempting starting/diagnostics).
• Tools you'll typically need: OBD-II scan tool capable of reading freeze-frame data, multimeter (V and ohms), stable power source for testing (12V battery or power supply), basic hand tools, factory service information for your exact engine (glow plug layout, relay locations, wiring diagrams).
• Confirm the code and its persistence: Use the scan tool to confirm current vs stored status, retrieve freeze-frame data, and note any related codes (P0380, P0381, etc.). DTCs are stored when faults are detected by the diagnostic system (Diagnostic Trouble Codes; Powertrain Codes).

Diagnostic Approach

1) Verify the fault and gather data

• Confirm P0387 is current (present on system scan) and record freeze-frame data (engine rpm, coolant temperature, battery voltage, ignition status, etc.).
• Look for any related or recent codes that could point to a common fault (for example, P0380 series or other glow plug-related information in OEMs). The general concept is that related codes help pinpoint the circuit.

2) Inspect the basic electrical health of the glow plug circuit (B circuit)

• Visual inspection: Inspect wiring harnesses and connectors for the glow plug circuit "B" path. Look for broken insulation, corrosion, moisture ingress, or loose connectors.
• Check fuses and relays: Locate the glow plug relay and associated fuse(s). Confirm fuse integrity and ensure the relay is operating as commanded by the PCM.
• Check the battery/charging condition: Glowing/heating circuits rely on adequate voltage. A weak battery or poor charging can cause a "low input" or weak activation condition.

3) Electrical testing of the circuit

• Access the glow plug circuit harness and measure supply voltage at the glow plug relay output or at the glow plugs themselves when the ignition/key is in the preheat/on position (as appropriate for the vehicle). Confirm the circuit is getting voltage when commanded.
• Test the glow plug relay coil and contacts:
  • With the engine off, apply the control signal from the PCM to the relay (or swap in a known-good relay if permissible) and verify that the relay clicks and the circuit is energizing as expected.
  • If the relay is energizing but voltage to the glow plugs is not present, investigate downstream wiring or the glow plug assembly.
• Wiring resistance path: If you have access to the harness, check continuity and resistance from the PCM/ECM connector to the glow plug(s) in circuit B. Look for high resistance, open circuits, or short to ground/short to V+.

4) Glow plug condition (individual element checks)

• Glow plug resistance: Remove power from the circuit, isolate individual glow plugs, and measure resistance of each glow plug element. Compare to manufacturer specifications (if available). Any open circuit or extremely high resistance indicates a failed glow plug and replacement is warranted.
• Visual plug condition: Inspect glow plugs for signs of scorching, deposits, or physical damage.

5) Determine if the issue is likely the glow plug system vs PCM control

• If all glow plugs test within spec and the relay and wiring check out, but the PCM/ECM control signal is absent or inconsistent, the fault may lie in the PCM input or control path (less common but plausible). In such cases, confirm there are no substrate faults on the PCM and verify accessory power and grounds to the PCM.

6) Clear codes and perform a controlled test drive

• After performing repairs or replacements, clear the DTCs with your scan tool and run a drive cycle (including cold starts if applicable) to verify the fault does not reappear and to ensure readiness monitors complete successfully.

7) If the code persists after the above checks

• Re-check for ECM-related issues: ensure the PCM ground paths are solid and that there are no voltage reference issues from the engine wiring harness.
• Re-test the glow plug system after a cooling-off period to re-confirm the initial findings.
• If you continue to see P0387 with no obvious glow plug circuit fault, escalate to OEM factory service information or a dealer-level diagnostic procedure; OEMs may have specific sequences or fault tree logic for this DTC.

Probable Causes

Note: In the absence of NHTSA data , these percentages reflect typical ASE-era field experience and common-sense weighting for glow plug/circuit faults. They are not official statistics.

• Glow plug(s) failure (open or high resistance): ~40-50%

• Glow plug relay failure or weak control signal from PCM: ~15-25%

• Wiring harness/connectors damage or poor connections (including grounds): ~20-30%

• Battery/charging system instability causing insufficient preheat voltage: ~5-10%

• ECM/PCM input fault or internal fault (less common): ~5%

• Other incidental or OEM-specific interpretations: ~0-5%

This diagnostic guide was generated using verified reference data:

• Wikipedia Technical Articles: OBD-II
• Open-Source OBD2 Data: N/A (MIT)

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

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