DURAMAX-P0685 serious Duramax ›

ECM/PCM Power Relay Control Circuit/Open

The DURAMAX-P0685 (Duramax) diesel fault code means: ECM/PCM Power Relay Control Circuit/Open. This is a serious severity code.

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Keep driving?
Yes, with caution
DIY difficulty
intermediate
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What Are the Symptoms of DURAMAX-P0685?

  • Check Engine Light on
  • Possible no start, stalling or the engine computer staying awake after shut-off

What Causes DURAMAX-P0685? (Most Likely First)

  • Open wiring or an unplugged connector High
  • Failed ECM/PCM power relay High
  • Corroded connector Moderate
  • Control module driver fault Low

Can I Still Drive with DURAMAX-P0685?

Yes, with caution. DURAMAX-P0685 is rated serious severity. Keep trips short and get it diagnosed soon rather than waiting for symptoms to worsen.

Frequently Asked Questions

What does P0685 mean on a Duramax?

P0685 means ECM/PCM Power Relay Control Circuit/Open. The control module has detected an open or faulty ECM/PCM power relay circuit.

Is P0685 a Duramax-specific code?

No. P0685 is a generic OBD-II code defined by SAE J2012, so it means the same thing on any vehicle that sets it. The exact conditions GM uses to set it on its diesels are in GM service information.

Mechanics Guide Step-by-step diagnosis

How Do You Diagnose DURAMAX-P0685?

  1. Inspect the ECM/PCM power relay connector and wiring.

  2. Check the ECM/PCM power relay resistance against the service specification.

  3. Check the circuit for an open, then command it with a scan tool.

Sources 1

This page is built from documented references. Verify against your own service info before repair work.

  • SAE J2012Diagnostic Trouble Code Definitions (defines P0685 as a generic OBD-II code)

P0685 on Other Platforms

The same code ID appears across other engines, vehicles, and equipment. Diagnostic flow varies by platform — see the matching breakdown:

Gas / Automotive (OBD-II) Generic OBD-II definition for P0685

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More Duramax codes → Diesel home All diesel codes Heavy-duty Guides

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