Diesel Trouble Codes

P128C — Cylinder 1 Glow Plug Control Circuit High (L5P Duramax)

Cylinder 1 Glow Plug Control Circuit High

Updated September 2, 2026

What it means

The ECM has detected an abnormally high voltage or resistance in the control circuit for cylinder 1 glow plug.1

Most likely causes

  1. Glow plug control module circuit fault affecting cylinder 1 control line — The PCM has detected a fault in the glow-plug control module circuit.2
  2. Glow plug heater circuit malfunction on cylinder 1 — The engine control module has detected an electrical fault in the glow plug heater circuit.3
  3. Cylinder 1 glow plug failure — DTC P133X (Cylinder X Glow Plug Failure) was retrieved; the contact was informed that the Glow Plug needed to be replaced and reprogrammed.4

How to diagnose

  1. Verify code and freeze frame; confirm P128C is present and check for related glow plug codes such as P0670, P0380, or P067x.
  2. Inspect glow plug control module power/ground and harness for damage, corrosion, or chafing; check wiring to cylinder 1 glow plug.
  3. Measure control circuit voltage/resistance at the glow plug connector and compare to specification; look for open/high resistance.
  4. Check glow plug control module operation and communication; verify module output to cylinder 1.
  5. If circuit tests normal, evaluate cylinder 1 glow plug resistance and condition; replace as needed and reprogram if required.

Parts and tools you may need

Rough cost and difficulty

Parts and labor vary by region and shop; dealer diagnosis may include reprogramming.

When to see a pro

Glow plug control circuits are low-voltage but involve ECM/ module interfaces; incorrect testing can damage the control module. Have a qualified diesel technician confirm circuit integrity before replacing parts.

Sources

  1. Curated DTC seed — L5P Duramax (curated) ↩︎

  2. Curated DTC seed — L5P Duramax (curated) ↩︎

  3. Curated DTC seed — L5P Duramax (curated) ↩︎

  4. NHTSA owner report 11718543 ↩︎