P0685
System
Powertrain (Electrical)
Scanner required
Full-system scanner (engine, ABS, SRS, transmission, service resets)

ECM/PCM Power Relay Control Circuit Open

Understand P0685, common causes, and the right TOPDON scanner to diagnose it.

Main ECM power relay control circuit open — engine may not start or stall.

Common symptoms

  • No-start
  • Stalling
  • Intermittent crank no-start

Typical causes

  • Failed ECM relay
  • Open relay control wire
  • Blown fuse
  • ECM internal driver fault

What scanner do you need?

Full-system scanner (engine, ABS, SRS, transmission, service resets)

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 P0685 with a basic code reader?
P0685 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 P0685?
It depends on the code and symptoms. Flashing check engine lights and misfire codes should be addressed immediately. Consult a professional if unsure.