P066F

3 Glow Plug Control Circuit High

P066F is a generic OBD-II powertrain diagnostic trouble code: 3 Glow Plug Control Circuit High. It is logged by the engine control unit when the glow monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
P066F
Group
Powertrain
System
Glow
Severity
Warning (MIL on)
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What P066F means

P066F is a generic diesel powertrain code indicating the engine control module has detected an abnormally high voltage in the glow plug control circuit for cylinder 3. Diesel engines rely on compression heat rather than a spark to ignite fuel, and glow plugs preheat each combustion chamber to aid cold starting and stabilize combustion during warm-up. The glow plug control module (GPCM) drives each plug and reports circuit status back to the ECM.

When the ECM commands the cylinder 3 glow plug and instead measures a voltage higher than the expected range, or sees the circuit remain electrically high when it should be pulled low, it sets P066F and typically illuminates the malfunction indicator lamp. A high reading usually points to an open circuit, a break in the wiring, a failed glow plug, or a control module output that is stuck high.

The most common effects are hard cold starting, rough running until the engine warms, white exhaust smoke, and occasionally reduced power. Because a single dead glow plug can be the culprit, diagnosis should confirm the individual plug and its wiring before condemning the control module or ECM. Prompt repair is advised to protect drivability and reduce cold-start emissions.

Common causes

Most-frequently reported root causes when P066F is logged.

  • 1
    Open or high-resistance wiring in the cylinder 3 glow plug control circuit.
  • 2
    Failed or open-circuit cylinder 3 glow plug.
  • 3
    Corroded, loose, or damaged connector at the glow plug or control module.
  • 4
    Faulty glow plug control module output driver for cylinder 3.
  • 5
    Chafed or broken harness between the control module and cylinder 3 glow plug.
  • 6
    Poor ground or supply connection at the glow plug control module.
  • 7
    ECM fault or software issue misreporting the circuit state (rare).

Symptoms drivers notice

Malfunction indicator lamp illuminated.
Hard starting when the engine is cold.
Rough idle or misfire-like running until the engine warms up.
White or grey exhaust smoke during cold start-up.
Glow plug warning lamp may flash or stay on.
Possible reduced power or hesitation in cold conditions.

How to diagnose P066F

A typical diagnostic flow when this code is present.

  1. 1
    Connect a scan tool and record all stored DTCs and freeze frame data before clearing anything.
  2. 2
    Note whether other cylinder glow plug codes are present, which can indicate a control module or supply fault rather than a single plug.
  3. 3
    Inspect the cylinder 3 glow plug connector and harness for corrosion, damage, or loose terminals.
  4. 4
    Measure the resistance of the cylinder 3 glow plug and compare it to specification; an open plug reads infinite resistance.
  5. 5
    Check continuity of the control wire between the glow plug control module and the cylinder 3 glow plug.
  6. 6
    Verify supply voltage and ground at the glow plug control module during the preheat cycle.
  7. 7
    If the plug, wiring, and supply are good, evaluate the glow plug control module output for cylinder 3.

Vehicles where we've handled P066F

Platforms in our catalogue with confirmed P066F coverage.

BMW 320D
2016

Related powertrain codes

Frequently asked questions

Can I drive with P066F active?

Short trips may be possible, but a faulty glow plug causes hard cold starts, rough running, and increased emissions, so the fault should be repaired promptly.

Does P066F always mean the glow plug is bad?

No. An open or damaged control wire, a bad connector, or a failed control module output can produce the same high-circuit reading, so the wiring and plug must both be tested.

Why does the code point specifically to cylinder 3?

Each glow plug has its own control circuit, and the ECM monitors them individually, so P066F identifies the cylinder 3 circuit as the one reading high.

Will one bad glow plug affect the whole engine?

A single dead glow plug mainly hurts cold starting and warm-up on that cylinder, causing rough running and smoke until the engine reaches operating temperature.

Disabling P066F in software

RaceTune can permanently disable P066F — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.

Permanent
The monitor is disabled in the ECU itself — not just cleared. It cannot return.
Tailored to your file
Each patch is matched to your specific software version — never a one-size-fits-all file.
Reversible
The original file is always preserved. Reflash the stock to return the ECU to factory state.

ECU families we can disable P066F on

We hold the DaVinci A2L disable definitions for these families, so the exact P066F path and mask addresses are mapped. verified marks a confirmed disable definition. We support many more — upload your file and our identifier will match it automatically.

  • Bosch EDC17C50 verified
  • Bosch EDC17C56 verified
  • Bosch EDC17C60 verified
  • Bosch EDC17CP44 verified
  • Bosch MD1CP002 verified
  • Bosch MD1CP004 verified
  • Bosch MD1CS001 verified

Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.

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