Code
P1381
MITSUBISHI
P — Powertrain
Glow plug relay (+B short)
Views:
UK: 20
EN: 30
RU: 21
AI status
Completed
Completed
100%
Causes
- Shorted wiring harness to battery positive (+B) on the glow plug relay/control circuit
- Faulty or welded relay contacts inside the glow plug relay
- Stuck or shorted glow plug(s) creating abnormal current path
- Blown or corroded fuse, fusible link, or poor power distribution
- Damaged connector or pin (insulation abrasion, corrosion) feeding +B to the relay/control
- Faulty relay socket or aftermarket relay with incorrect pinout
Symptoms
- Engine hard to start when cold (diesel) or extended crank time
- Glow plug indicator or preheat warning lamp illuminated
- Reduced engine start performance or increased smoke at cold start
- Stored MIL (check engine) lamp with code P1381
- Battery drain (parasitic current) when vehicle is off if relay stuck closed
What to check
- Confirm battery voltage and state of charge (11.5–14.8 V normal range)
- Inspect fuses, fusible links and main power distribution for glow plug circuit
- Visually inspect glow plug relay, socket, wiring harness, and connectors for damage/corrosion
- Backprobe relay control and power terminals with key ON and during preheat to observe voltages
- Measure continuity between relay +B terminal and battery positive to detect unintended direct feed
- Disconnect glow plug relay and check if code or symptoms change (isolate relay/coil/load)
Signal parameters
- Battery supply to relay (+B): approx. 11.5–14.8 V with ignition ON
- Relay control output to glow plugs: near battery voltage when relay closed; near 0 V when open
- Relay coil/control circuit resistance (typical relay coil): tens to hundreds of ohms (manufacturer spec varies)
- Glow plug current draw: can be several amps per plug (total load often 10–50 A depending on engine and number of plugs); consult vehicle spec
- ECM control signal: should toggle per preheat strategy; an always-high battery voltage on control line indicates a +B short
Diagnostic algorithm
- Retrieve freeze frame and live data; confirm P1381 is current or historical and note related glow-plug codes.
- Visually inspect relay, relay socket, harness routing and connectors for heat damage, chafing, corrosion or aftermarket splices.
- With ignition OFF, disconnect negative battery terminal for safety if doing wiring repairs. If measuring current, follow safe procedures.
- With battery connected, backprobe the relay +B (power) terminal and the relay control terminal with ignition ON and during preheat. Verify expected voltages.
- Remove/replace the glow plug relay temporarily with a known-good relay or isolate the relay. If the short disappears, suspect the relay or socket.
- Disconnect glow plug harness at injectors/glow plugs and check for continuity to +B; measure each glow plug resistance to ground per spec to identify shorted plug(s).
- Check for unintended continuity between the control wire and battery positive using an ohmmeter (key OFF). A low resistance indicates a short to +B.
- Inspect and test fuses and fusible links feeding the glow plug relay; replace if blown and investigate cause of failure.
- If wiring and relay check good, inspect ECM/PCM connector for damage. If suspected ECM fault, confirm all wiring and loads are correct before replacing control module.
- Repair or replace damaged wiring, connector, relay, or faulty glow plugs as identified. Clear codes and perform functional test/start cycles to verify repair.
Likely causes
- Damaged harness where routing is near hot components or sharp edges causing insulation wear and contact with +12V
- Glow plug relay welded closed or internally shorted delivering constant battery voltage
- Corroded terminal at relay or battery feed causing erratic readings and false short detection
- Short in aftermarket accessories or recent repair wiring tapped into the +B feed
Fault status
Status
Glow plug relay circuit — short to battery positive (+B) detected. Circuit remains at battery voltage when it should be switched; ECM recorded fault P1381.
Repair difficulty: Medium
Diagnostic time: 1.0-3.0 hours
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