01 How Adaptive Cruise Control Actually Works • 02 Milling Machine Basics: A Complete Guide for Beginners • 03 This Is How Burning Odors Mean An Electrical Short Under Th… • 04 Does Your Car Hesitate When Accelerating? Here's What The P… • 05 This Is How Your Turbocharger Works And Why A Small Whistle… • 06 This Is Why Your Car’s Battery Terminals Are Corroded • 07 Engineers Share the Five Minute Secret That Keeps Vehicles … • 08 How to Clean Piston Ring Grooves: A Complete Guide • 09 Transmission Rebuild or Replacement: Making the Right Choice • 10 How Mobile Mechanics Are Changing the Industry (And Making … •

DTC codes / P06xx · Computer and output circuits / P066A

P066ACylinder 1 Glow Plug Control Circuit Low

P066A means the engine control module (ECM) commanded the glow plug for cylinder 1 on and the feedback circuit reported voltage below the expected threshold, indicating a short to ground or an open circuit in the control side. The ECM monitors the glow plug control circuit for expected voltage behavior and sets this code when the circuit stays low even though the glow plug should be energized.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaComputer and output circuits

Symptoms you may notice

  • Illuminated check engine light or glow plug warning lamp, often with no other noticeable symptoms
  • Hard cold starting or extended cranking before the engine fires, especially in low ambient temperatures
  • Rough idle and white smoke from the exhaust immediately after a cold start
  • Misfire or uneven running during the warm-up period that clears as the engine reaches operating temperature
  • Increased fuel consumption and reduced cold-weather driveability
  • In some vehicles, a reduced-power or limp mode strategy that limits engine performance until the fault is repaired

Common causes, most likely first

  1. Failed glow plug on cylinder 1 with an internal short to ground or open element
  2. Damaged or chafed glow plug harness wiring shorted to ground near the cylinder head or exhaust manifold
  3. Corroded, loose, or backed-out electrical connector at the cylinder 1 glow plug
  4. Faulty glow plug control module (GPCM) or relay driver internal to the ECM that cannot supply or monitor the circuit correctly
  5. Open circuit in the glow plug feed wire between the control module and the glow plug
  6. Water intrusion or oil contamination in the glow plug connector cavity causing a low-resistance path to ground
  7. Aftermarket or incorrect glow plug installed with resistance outside the range the ECM expects

How to diagnose and fix it

  1. Scan all modules and record freeze-frame data for P066A, then clear the code and verify whether it returns immediately on a cold key-on cycle
  2. Inspect the cylinder 1 glow plug connector and harness for corrosion, chafing, pin tension loss, and signs of shorting to the cylinder head or block
  3. Measure glow plug resistance at the plug terminal and compare it to the manufacturer's specification; a reading near zero ohms indicates a shorted plug
  4. Check the control circuit for a short to ground with a multimeter or test lamp while the glow plug is disconnected, and repair any damaged wire or connector
  5. Command the glow plug on with a bidirectional scan tool and verify voltage and current at the cylinder 1 plug to isolate whether the fault is in the plug, harness, or control module
  6. If the circuit and plug test good, substitute a known-good glow plug control module or perform the manufacturer's ECM driver test before condemning the ECM
  7. After repair, clear codes, perform a cold-start readiness cycle, and confirm no pending or stored P066A returns

Glow plug circuit low does not always mean a bad plug

  • A shorted glow plug harness rubbing against the cylinder head or exhaust manifold is a common cause that technicians miss when they replace the plug first and the code returns immediately.
  • The ECM may set P066A based on current feedback rather than simple voltage, so a glow plug with correct resistance can still trigger the code if the control module's current sense circuit is faulty.
  • On many diesel platforms the glow plug control module is a separate smart module that communicates over a data bus, and a module fault can set P066A for cylinder 1 even when the plug and wiring test perfectly.
  • A loose or corroded ground connection shared by the glow plug control module can pull the entire control circuit low and set this code along with other glow plug circuit codes.
  • P066A does not always mean the engine will fail to start; in mild weather the engine may start normally because the ECM can compensate with pilot injection, so the code can be present with no driveability complaint.

Can I keep driving with P066A?

The vehicle is generally safe to drive to a repair shop, but avoid repeated cold starts and heavy load until the fault is repaired because a shorted glow plug circuit can stress the control module. In freezing temperatures, expect hard starting and possible white smoke, and do not continue driving if the engine runs roughly or enters limp mode.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

Running on fumes Free guides aren't free to make. Top up the tank. Any amount, one time. Fuel the Garage