P066B

Code

Generic P — Powertrain

Cylinder 1 Glow Plug Control Circuit High

Brand: Generic

Type: P — Powertrain

Views: UK: 19 EN: 39 RU: 26

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Short to battery voltage (B+) on the Cylinder 1 glow plug control circuit
Failed glow plug (open or internally shorted)
Stuck or welded glow plug relay / heater relay contacts
Faulty glow plug driver or controller in the ECU/module
Corroded or damaged connector/pin at glow plug or harness
Poor ground or harness chafing causing intermittent shorts

Symptoms

Glow plug / preheat warning lamp or MIL illuminated
Hard starting or extended cranking in cold conditions
Rough cold idle or difficulty starting until engine reaches operating temperature
Reduced glow plug system operation for cylinder 1
Possible stored or intermittent code depending on conditions

What to check

Retrieve freeze frame and full DTC list with a scan tool; note freeze-frame conditions (battery voltage, engine temp)
Visually inspect wiring harness, connectors, and pins at cylinder 1 glow plug and relay/module for damage or corrosion
Check glow plug relay operation (listen for click or verify coil voltage & switching)
Measure voltage at glow plug control connector with ignition ON and during glow command
Measure resistance of the cylinder 1 glow plug (engine cold, with plug disconnected)
Perform continuity checks from ECM/module pin to glow plug connector and to battery + and ground

Signal parameters

Control line expected: ≈ 0 V when off, approximately battery voltage (≈12–14 V) or PWM switching when commanded ON
Glow plug resistance: typically low (single-digit ohms); very high or open indicates failed plug
Current draw: glow plug circuits draw significant current when active (verify with clamp ammeter on harness feed)
If PWM-controlled, the duty cycle should vary with temperature/command; an always-high voltage when OFF is abnormal

Diagnostic algorithm

Safety: work in a well-ventilated area, take appropriate precautions for high-current circuits; disconnect battery if required for repairs.
Read and record freeze-frame data and all stored DTCs with scan tool. Clear codes and attempt to re-run to reproduce P066B.
Visually inspect connectors, wiring, and routing to cylinder 1 glow plug and the glow plug relay/module for damage, corrosion, melted insulation or evidence of shorting.
With key ON (engine OFF) have an assistant command the glow system via scan tool while measuring voltage at the glow plug control pin and at the glow plug terminal. Observe if voltage is high when it should be low or if it remains high when command is OFF.
Remove the cylinder 1 glow plug electrical connector and measure glow plug resistance to engine ground. Compare to manufacturer spec; an open or very high reading suggests a failed plug.
Measure current draw of the glow plug circuit during a glow command using a clamp ammeter on the supply feed. Low or zero current with high circuit voltage can indicate open or wiring issue; excessive current can indicate a shorted plug or short to ground.
Perform continuity/resistance checks from the ECM/driver module pin to the glow plug connector and to battery + to locate shorts to B+ or open circuits. Inspect grounds for corrosion and continuity to chassis.
If the relay is suspected, bench-test or swap with a known-good relay of same type. If the driver module/ECM is suspected after verifying wiring and components, consult manufacturer service procedures before replacement.
Repair or replace damaged wiring, connectors, glow plug, relay or module as indicated by tests. Reconnect, clear codes, and verify repair by repeating the tests and confirming codes do not return under same conditions.

Likely causes

Wiring short to B+ at harness near cylinder 1 glow plug
Defective glow plug relay that does not open when commanded
Failed glow plug driver inside the glow plug control module/ECM
Open or high-resistance connector at the glow plug causing abnormal voltage reading
Single glow plug that has failed open or changed characteristics

Fault status

⚠️ Status

Cylinder 1 glow plug control circuit voltage higher than expected. ECM stored P066B and may illuminate MIL; cold-start performance for cylinder 1 may be compromised.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2.0 hours