P066B - Cylinder 1 Glow Plug Control Circuit High
System
Powertrain (Engine & Gearbox)
Technical Description
The control module has detected a fault in the Cylinder 1 Glow Plug Control Circuit High (P066B). The signal is above the upper threshold, typically from an open circuit, poor connection, or a short to battery voltage. The fault is isolated to cylinder 1, positioned closest to timing drive (reference cylinder). This directly impacts individual cylinder volumetric efficiency, firing synchronization, and dedicated harness branch loading.
Multimeter Test Points & Nominal Values
| Test Point / Component | Measurement | Nominal Target Range | Test Condition |
|---|---|---|---|
| Cylinder 1 Glow Plug Control Circuit High – Permanent B+ Supply | Voltage (V) | 12.4 - 12.7 V (ignition OFF) / 13.5 - 14.7 V (engine running) | At module supply pins |
| Cylinder 1 Glow Plug Control Circuit High – Ignition-Switched Supply | Voltage (V) | 11.8 - 14.4 V | Ignition ON |
| Cylinder 1 Glow Plug Control Circuit High – Ground Connections Voltage Drop | Voltage (V) | < 0.1 V per ground point | Under load to battery negative |
| Cylinder 1 Glow Plug Control Circuit High – 5 V Reference Outputs | Voltage (V) | 4.95 - 5.05 V per circuit | Ignition ON, sensors unplugged one at a time |
At module supply pins
Ignition ON
Under load to battery negative
Ignition ON, sensors unplugged one at a time
Oscilloscope Waveform Analysis
500 ms/div, 5 V/divPossible Causes
Open circuit on signal wire, spread terminal pin, or short to power feed at Cylinder 1 Glow Plug Control Circuit High
Timing drive harmonic zone (cylinder 1): Front reference cylinder exposed to cyclic vibrations from timing belt or chain drive and oil mist from front crank seal.
Plenum entry intake pulse:Cooler incoming charge air at the front manifold runner creates cold-start localized lean transients on cylinder 1.
Chafed or pinched wiring harness along the signal path to Cylinder 1 Glow Plug Control Circuit High
Poor ground connection or excessive voltage drop (> 0.1 V) at the ground point for Cylinder 1 Glow Plug Control Circuit High
Defective internal output driver or reference circuit in the control module for Cylinder 1 Glow Plug Control Circuit High
Symptoms
5
Recommended Solution
Swap component on cylinder 1 over to cylinder 2 and clear DTCs to confirm if the fault follows the part.
Inspect front timing cover wiring harness routing for mechanical rubbing against accessory belt tensioner pulleys.
Measure signal terminal voltage with ignition ON and sensor unplugged to determine if ECM pull-up is active or harness is shorted to 12V.
Verify loaded supply voltage and harness terminal pins at Cylinder 1 Glow Plug Control Circuit High.
Check circuit continuity and ground-side voltage drop (< 0.05 V) for Cylinder 1 Glow Plug Control Circuit High.
Inspect module enclosure and connector headers for water ingress, fretting corrosion, or PCB thermal damage.
Mechanic's Tip
Pro-tip for cylinder 1: Cylinder 1 is the primary sync reference. Always scope cam and crank phase correlation if the DTC recurs after replacing the component.