P138B

Code

LAND ROVER P — Powertrain

System voltage of the candle control module incandescence

Brand: LAND ROVER

Type: P — Powertrain

Views: UK: 8 EN: 14 RU: 13

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Page language: EN

Causes

Low battery voltage or poor battery condition
Faulty alternator or charging system causing over/under voltage
Blown fuse or faulty glow/relay fuse
Faulty glow plug relay or power switching device
Poor, corroded or loose wiring/connectors to the candle control module
Faulty or intermittent ground connection at the module

Symptoms

Long crank or hard starting in cold conditions
Glow plug / preheat warning lamp illuminated or flashing
Increased white/black smoke on cold start
Engine may run rough until fully warmed
Reduced preheat performance and extended start times
Related diagnostic trouble lamp (MIL) illuminated

What to check

Read freeze frame and other stored codes and note operating conditions when code set
Check battery state of charge and surface charge voltage (resting and while cranking)
Inspect fuses and glow/auxiliary relays related to the candle control module
Visually inspect wiring, connectors and pins at the candle control module for corrosion, damage or loose terminals
Measure supply voltage at the candle control module supply pin with key ON and during preheat
Check continuity to ground at the module housing and ground strap connections

Signal parameters

Supply voltage to candle control module: approximately battery voltage (nominally 11–14.5 V depending on charge state)
Allowable voltage drop from battery to module under preheat load: typically
Control signal from ECM: switching or PWM enable/disable (verify presence when preheat requested)
Ground continuity: near 0 Ω between module ground and chassis ground
Circuit current draw during preheat: measurable in amps consistent with number and rating of glow plugs (compare to spec)

Diagnostic algorithm

Verify the code and note freeze frame/capture conditions (battery voltage, engine temperature, key position).
Check battery voltage at rest. If battery below recommended level, charge or replace battery and retest.
Inspect and secure battery terminals, main fuses, glow relay fuse and main power distribution for corrosion or damage.
With key ON (no start), measure voltage at the candle control module supply pin and at the battery positive. Confirm voltage is within expected range.
Command preheat via scan tool (if supported) and observe module supply voltage and control signal from ECM. Note any large voltage drop or intermittent loss.
Check ground at the module: measure resistance between module ground and chassis (should be near 0 Ω). Repair any high resistance grounds.
Inspect wiring harness and connector for signs of overheating, corrosion, exposed conductors or bent pins. Repair as necessary.
Measure individual glow plug resistances and measure total circuit current during preheat (follow safety precautions). Excessively low/high values suggest shorted/failed plugs or wiring problem.
If wiring, grounds and supply are good but voltage issue persists, consider swapping or bench-testing the candle control module or glow relay per OEM procedure.
After repairs, clear codes and perform several cold start cycles to confirm the fault does not return and that preheat operation is normal.

Likely causes

Weak battery or poor battery terminals causing voltage sag during preheat
Corroded connector or damaged harness supplying the candle control module
Faulty glow relay or control module internal electronics
Charging system overvoltage or regulator fault
Open or high-resistance ground for the candle control module

Fault status

⚠️ Status

Candle control module supply voltage abnormal — voltage out of expected range or intermittent. May result in reduced or failed glow preheating.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2.5 hours

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