P1391
System
Powertrain (Diesel)
Scanner required
Enhanced OBD2 scanner (live data graphs, freeze frame, battery voltage test)

Glow Plug Circuit Low (Cylinder 1) — Diesel

Understand P1391, common causes, and the right TOPDON scanner to diagnose it.

Glow plug circuit low on cylinder 1 — diesel cold-start aid.

Common symptoms

  • Hard cold start
  • White smoke on cold start
  • Check engine light

Typical causes

  • Failed glow plug
  • Short in glow plug circuit
  • Glow plug module fault

What scanner do you need?

Enhanced OBD2 scanner (live data graphs, freeze frame, battery voltage test)

Use live data and freeze frame to confirm the fault before replacing parts. ABS, SRS, and network U-codes usually require a full-system scanner.

Diagnostic tips

  • Record freeze frame data when the code first sets
  • Clear the code only after fixing the root cause
  • Check for related pending codes that point to a common issue
  • Verify battery voltage is above 12.4V before chasing sensor codes

Recommended TOPDON scanners

Frequently asked questions

Can I diagnose P1391 with a basic code reader?
P1391 may set in the engine module, but related diagnostics often benefit from an enhanced or full-system scanner with live data and freeze frame.
Is it safe to drive with P1391?
It depends on the code and symptoms. Flashing check engine lights and misfire codes should be addressed immediately. Consult a professional if unsure.