P1389

Code

LINCOLN P — Powertrain

Glow Plug Circuit High Side Low Input

Brand: LINCOLN

Type: P — Powertrain

Views: UK: 14 EN: 38 RU: 19

AI status

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Page language: EN

Blown fuse or fusible link for glow plug supply
Faulty glow plug relay or high-side driver module
Open, short or high-resistance wiring/connectors between battery, relay/module and glow plugs
Corroded or damaged connector pins at relay, module or glow plugs
Individual failed glow plugs (high resistance/open) or shorted plugs
Faulty glow plug control module or PCM/ECM driver output

Retrieve freeze frame and DTC status from PCM/ECM; note conditions (battery voltage, engine temp)
Verify battery voltage at rest and while cranking (should be ~12–14V)
Visually inspect fuses, fusible links, glow plug relay and related connectors for corrosion, melting or damage
Inspect wiring harness along route from battery/relay/module to glow plugs for chafing, breaks, pinches
Measure voltage at glow plug power feed connector while glow command is active (should be near battery voltage)
Measure resistance/continuity of each glow plug to ground and compare between cylinders

Battery voltage supply: ~12–14.5 V (vehicle running/charging: up to ~14.5 V)
When glow is commanded ON: high-side power feed should be near battery voltage at relay/module and at glow plug terminals
When driver is active: control side may be switched to ground (low) by PCM — verify expected switching behavior with scan tool
Individual glow plug resistance: typically low (a few ohms); open/infinite or very high ohms indicates a failed plug

Connect a scan tool, confirm P1389 is stored and review freeze frame data (battery voltage, ambient/engine temp).
Visually inspect fuses/fusible links and replace if blown. Check glow plug relay and swap with identical relay if available to test.
With ignition ON and glow command active (use scan tool to actuate if possible), measure voltage at the glow plug relay power input and output and at the glow plug connector(s). Voltage should be near battery voltage; low voltage indicates upstream supply problem.
Check continuity from relay/module output to each glow plug terminal; repair any opens or high-resistance wiring/connectors.
Measure resistance of each glow plug to engine ground. Replace any glow plugs that are open or have significantly higher resistance than the others.
Inspect and clean ground connections for the glow plug module and engine block; retest.
If wiring, fuses, relays and glow plugs test good, test/replace the glow plug control module or investigate PCM driver output (replace module/PCM only after confirming wiring and components).
Clear codes and perform verified test drives/cold start checks to ensure code does not return.

⚠️ Status

Stored when the engine control module detects low-voltage input on the high-side glow plug supply circuit while a glow/heater command is present.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-2.5 hours

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P1389 — Glow Plug Circuit High Side Low Input