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Home / DTC / P0685 — ECM/PCM Power Relay Control Circuit/Open

P0685 — ECM/PCM Power Relay Control Circuit/Open

Detailed page for trouble code P0685.

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22,491specific
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Code

P0685

Generic P — Powertrain

ECM/PCM Power Relay Control Circuit/Open

Brand: Generic
Type: P — Powertrain
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Blown fuse or fusible link feeding ECM/PCM power relay
  • Faulty ECM/PCM power relay (coil or contacts open)
  • Open or disconnected wiring between ECM/PCM and relay (broken wire, poor crimp)
  • Corroded or loose relay socket/terminals or battery/ground connections
  • Open coil circuit in relay (internal failure)
  • Faulty ECM/PCM driver (rare)

Symptoms

  • No-start or intermittent no-start condition
  • MIL (Check Engine Light) may be on
  • Loss of communication with ECM/PCM on scan tool
  • Engine stalls or shuts down unexpectedly
  • Accessories or engine controlled systems unpowered or intermittent

What to check

  • Read freeze frame and stored codes; note any related codes
  • Verify battery voltage is healthy (12.4–12.8 V at rest) and battery connections are clean/tight
  • Inspect fuses/fusible links related to ECM/PCM power and relay feed
  • Visually inspect relay, socket, and wiring for corrosion, damage or loose terminals
  • Swap relay with identical known-good relay (if available) to see if fault clears
  • Attempt communication with ECM using scan tool; check for loss of comm

Signal parameters

  • Battery feed to relay (constant) should be ~12 V with key off and on (subject to vehicle design)
  • Relay control (ECM driver) should switch between near 0 V (ground) and near battery voltage when commanded — verify expected on/off states with key/engine start
  • Relay coil DC resistance typically in the range of 50–200 ohms (varies by relay) — check against known-good
  • Open-circuit reading (infinite/open) on coil or control wire indicates a break
  • Short-to-ground or short-to-battery on control wire will show near 0 V or near battery voltage respectively with circuit inactive

Diagnostic algorithm

  1. Verify code P0685 and retrieve related codes/freeze frame using a scan tool.
  2. Check battery voltage and charge if low; ensure battery terminals are clean and tight.
  3. Inspect fuses/fusible links that feed the ECM/PCM power relay; replace any blown fuses.
  4. Visually inspect the ECM/PCM power relay and its socket for corrosion, melted plastic, or loose terminals. Replace relay if suspect.
  5. Swap the suspect relay with an identical known-good relay (from another circuit) and observe whether the code returns or symptoms change.
  6. With ignition ON (engine off), backprobe relay socket: verify constant 12 V feed to relay power terminal and measure the control terminal. Command the relay (if possible) and confirm the control terminal toggles to the expected voltage (ground or 12 V depending on design).
  7. Measure coil resistance across relay coil terminals (relay removed). Infinite resistance = open coil; very low = shorted coil. Compare to specification or known-good relay.
  8. If relay control does not toggle, disconnect battery, inspect and check continuity of the control wire between relay socket and ECM connector. Repair any open circuits or bad connectors.
  9. Check grounds related to ECM power. Poor ground can mimic open/erratic behavior.
  10. If wiring and relay check good, command ECM power output (if manufacturer procedure available) and verify driver operation. If ECM driver will not switch but wiring is confirmed good, suspect ECM/PCM fault and consult manufacturer guidance before replacement.
  11. After repairs, clear codes, perform key cycles and a road/test drive to verify the fault does not return.

Likely causes

  • Failed power relay
  • Open circuit in control wire (ECM to relay) or power feed to relay
  • Blown fuse supplying relay coil or relay feed
  • Corroded connector at relay or ECM connector pin
  • Intermittent connector/wiring fault due to vibration or heat

Fault status

⚠️ Status
ECM/PCM detected open in the power-relay control circuit. Relay coil or control wiring not responding to driver command; MIL stored. May result in no communication or intermittent power to ECM/PCM until circuit is repaired.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-2.0 hours

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Code

P0685

HUMMER P — Powertrain

Engine Controls Ignition Relay Control Circuit (PCM)

Brand: HUMMER
Type: P — Powertrain
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Blown fuse or fusible link feeding ECM/PCM power relay
  • Faulty ECM/PCM power relay (coil or contacts open)
  • Open or disconnected wiring between ECM/PCM and relay (broken wire, poor crimp)
  • Corroded or loose relay socket/terminals or battery/ground connections
  • Open coil circuit in relay (internal failure)
  • Faulty ECM/PCM driver (rare)

Symptoms

  • No-start or intermittent no-start condition
  • MIL (Check Engine Light) may be on
  • Loss of communication with ECM/PCM on scan tool
  • Engine stalls or shuts down unexpectedly
  • Accessories or engine controlled systems unpowered or intermittent

What to check

  • Read freeze frame and stored codes; note any related codes
  • Verify battery voltage is healthy (12.4–12.8 V at rest) and battery connections are clean/tight
  • Inspect fuses/fusible links related to ECM/PCM power and relay feed
  • Visually inspect relay, socket, and wiring for corrosion, damage or loose terminals
  • Swap relay with identical known-good relay (if available) to see if fault clears
  • Attempt communication with ECM using scan tool; check for loss of comm

Signal parameters

  • Battery feed to relay (constant) should be ~12 V with key off and on (subject to vehicle design)
  • Relay control (ECM driver) should switch between near 0 V (ground) and near battery voltage when commanded — verify expected on/off states with key/engine start
  • Relay coil DC resistance typically in the range of 50–200 ohms (varies by relay) — check against known-good
  • Open-circuit reading (infinite/open) on coil or control wire indicates a break
  • Short-to-ground or short-to-battery on control wire will show near 0 V or near battery voltage respectively with circuit inactive

Diagnostic algorithm

  1. Verify code P0685 and retrieve related codes/freeze frame using a scan tool.
  2. Check battery voltage and charge if low; ensure battery terminals are clean and tight.
  3. Inspect fuses/fusible links that feed the ECM/PCM power relay; replace any blown fuses.
  4. Visually inspect the ECM/PCM power relay and its socket for corrosion, melted plastic, or loose terminals. Replace relay if suspect.
  5. Swap the suspect relay with an identical known-good relay (from another circuit) and observe whether the code returns or symptoms change.
  6. With ignition ON (engine off), backprobe relay socket: verify constant 12 V feed to relay power terminal and measure the control terminal. Command the relay (if possible) and confirm the control terminal toggles to the expected voltage (ground or 12 V depending on design).
  7. Measure coil resistance across relay coil terminals (relay removed). Infinite resistance = open coil; very low = shorted coil. Compare to specification or known-good relay.
  8. If relay control does not toggle, disconnect battery, inspect and check continuity of the control wire between relay socket and ECM connector. Repair any open circuits or bad connectors.
  9. Check grounds related to ECM power. Poor ground can mimic open/erratic behavior.
  10. If wiring and relay check good, command ECM power output (if manufacturer procedure available) and verify driver operation. If ECM driver will not switch but wiring is confirmed good, suspect ECM/PCM fault and consult manufacturer guidance before replacement.
  11. After repairs, clear codes, perform key cycles and a road/test drive to verify the fault does not return.

Likely causes

  • Failed power relay
  • Open circuit in control wire (ECM to relay) or power feed to relay
  • Blown fuse supplying relay coil or relay feed
  • Corroded connector at relay or ECM connector pin
  • Intermittent connector/wiring fault due to vibration or heat

Fault status

⚠️ Status
ECM/PCM detected open in the power-relay control circuit. Relay coil or control wiring not responding to driver command; MIL stored. May result in no communication or intermittent power to ECM/PCM until circuit is repaired.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-2.0 hours

Similar codes

P0680 P0681 P0620 P0625 P1685
Repair manuals

Manual library for HUMMER

138

Browse 138 HUMMER manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.

HUMMER

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Code

P0685

ISUZU P — Powertrain

ECM/PCM Power Relay Control Circuit/Open

Brand: ISUZU
Type: P — Powertrain
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Blown fuse or fusible link feeding ECM/PCM power relay
  • Faulty ECM/PCM power relay (coil or contacts open)
  • Open or disconnected wiring between ECM/PCM and relay (broken wire, poor crimp)
  • Corroded or loose relay socket/terminals or battery/ground connections
  • Open coil circuit in relay (internal failure)
  • Faulty ECM/PCM driver (rare)

Symptoms

  • No-start or intermittent no-start condition
  • MIL (Check Engine Light) may be on
  • Loss of communication with ECM/PCM on scan tool
  • Engine stalls or shuts down unexpectedly
  • Accessories or engine controlled systems unpowered or intermittent

What to check

  • Read freeze frame and stored codes; note any related codes
  • Verify battery voltage is healthy (12.4–12.8 V at rest) and battery connections are clean/tight
  • Inspect fuses/fusible links related to ECM/PCM power and relay feed
  • Visually inspect relay, socket, and wiring for corrosion, damage or loose terminals
  • Swap relay with identical known-good relay (if available) to see if fault clears
  • Attempt communication with ECM using scan tool; check for loss of comm

Signal parameters

  • Battery feed to relay (constant) should be ~12 V with key off and on (subject to vehicle design)
  • Relay control (ECM driver) should switch between near 0 V (ground) and near battery voltage when commanded — verify expected on/off states with key/engine start
  • Relay coil DC resistance typically in the range of 50–200 ohms (varies by relay) — check against known-good
  • Open-circuit reading (infinite/open) on coil or control wire indicates a break
  • Short-to-ground or short-to-battery on control wire will show near 0 V or near battery voltage respectively with circuit inactive

Diagnostic algorithm

  1. Verify code P0685 and retrieve related codes/freeze frame using a scan tool.
  2. Check battery voltage and charge if low; ensure battery terminals are clean and tight.
  3. Inspect fuses/fusible links that feed the ECM/PCM power relay; replace any blown fuses.
  4. Visually inspect the ECM/PCM power relay and its socket for corrosion, melted plastic, or loose terminals. Replace relay if suspect.
  5. Swap the suspect relay with an identical known-good relay (from another circuit) and observe whether the code returns or symptoms change.
  6. With ignition ON (engine off), backprobe relay socket: verify constant 12 V feed to relay power terminal and measure the control terminal. Command the relay (if possible) and confirm the control terminal toggles to the expected voltage (ground or 12 V depending on design).
  7. Measure coil resistance across relay coil terminals (relay removed). Infinite resistance = open coil; very low = shorted coil. Compare to specification or known-good relay.
  8. If relay control does not toggle, disconnect battery, inspect and check continuity of the control wire between relay socket and ECM connector. Repair any open circuits or bad connectors.
  9. Check grounds related to ECM power. Poor ground can mimic open/erratic behavior.
  10. If wiring and relay check good, command ECM power output (if manufacturer procedure available) and verify driver operation. If ECM driver will not switch but wiring is confirmed good, suspect ECM/PCM fault and consult manufacturer guidance before replacement.
  11. After repairs, clear codes, perform key cycles and a road/test drive to verify the fault does not return.

Likely causes

  • Failed power relay
  • Open circuit in control wire (ECM to relay) or power feed to relay
  • Blown fuse supplying relay coil or relay feed
  • Corroded connector at relay or ECM connector pin
  • Intermittent connector/wiring fault due to vibration or heat

Fault status

⚠️ Status
ECM/PCM detected open in the power-relay control circuit. Relay coil or control wiring not responding to driver command; MIL stored. May result in no communication or intermittent power to ECM/PCM until circuit is repaired.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-2.0 hours

Similar codes

P0680 P0681 P0620 P0625 P1685
Repair manuals

Manual library for ISUZU

86

Browse 86 ISUZU manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.

ISUZU

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+100 karma for a short comment :)
Keep it short and practical. Up to 500 characters.
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Code

P0685

LAND ROVER P — Powertrain

main relay of the module control of the short circuit force train to ground or battery voltage or circuit open

Brand: LAND ROVER
Type: P — Powertrain
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Blown fuse or fusible link feeding ECM/PCM power relay
  • Faulty ECM/PCM power relay (coil or contacts open)
  • Open or disconnected wiring between ECM/PCM and relay (broken wire, poor crimp)
  • Corroded or loose relay socket/terminals or battery/ground connections
  • Open coil circuit in relay (internal failure)
  • Faulty ECM/PCM driver (rare)

Symptoms

  • No-start or intermittent no-start condition
  • MIL (Check Engine Light) may be on
  • Loss of communication with ECM/PCM on scan tool
  • Engine stalls or shuts down unexpectedly
  • Accessories or engine controlled systems unpowered or intermittent

What to check

  • Read freeze frame and stored codes; note any related codes
  • Verify battery voltage is healthy (12.4–12.8 V at rest) and battery connections are clean/tight
  • Inspect fuses/fusible links related to ECM/PCM power and relay feed
  • Visually inspect relay, socket, and wiring for corrosion, damage or loose terminals
  • Swap relay with identical known-good relay (if available) to see if fault clears
  • Attempt communication with ECM using scan tool; check for loss of comm

Signal parameters

  • Battery feed to relay (constant) should be ~12 V with key off and on (subject to vehicle design)
  • Relay control (ECM driver) should switch between near 0 V (ground) and near battery voltage when commanded — verify expected on/off states with key/engine start
  • Relay coil DC resistance typically in the range of 50–200 ohms (varies by relay) — check against known-good
  • Open-circuit reading (infinite/open) on coil or control wire indicates a break
  • Short-to-ground or short-to-battery on control wire will show near 0 V or near battery voltage respectively with circuit inactive

Diagnostic algorithm

  1. Verify code P0685 and retrieve related codes/freeze frame using a scan tool.
  2. Check battery voltage and charge if low; ensure battery terminals are clean and tight.
  3. Inspect fuses/fusible links that feed the ECM/PCM power relay; replace any blown fuses.
  4. Visually inspect the ECM/PCM power relay and its socket for corrosion, melted plastic, or loose terminals. Replace relay if suspect.
  5. Swap the suspect relay with an identical known-good relay (from another circuit) and observe whether the code returns or symptoms change.
  6. With ignition ON (engine off), backprobe relay socket: verify constant 12 V feed to relay power terminal and measure the control terminal. Command the relay (if possible) and confirm the control terminal toggles to the expected voltage (ground or 12 V depending on design).
  7. Measure coil resistance across relay coil terminals (relay removed). Infinite resistance = open coil; very low = shorted coil. Compare to specification or known-good relay.
  8. If relay control does not toggle, disconnect battery, inspect and check continuity of the control wire between relay socket and ECM connector. Repair any open circuits or bad connectors.
  9. Check grounds related to ECM power. Poor ground can mimic open/erratic behavior.
  10. If wiring and relay check good, command ECM power output (if manufacturer procedure available) and verify driver operation. If ECM driver will not switch but wiring is confirmed good, suspect ECM/PCM fault and consult manufacturer guidance before replacement.
  11. After repairs, clear codes, perform key cycles and a road/test drive to verify the fault does not return.

Likely causes

  • Failed power relay
  • Open circuit in control wire (ECM to relay) or power feed to relay
  • Blown fuse supplying relay coil or relay feed
  • Corroded connector at relay or ECM connector pin
  • Intermittent connector/wiring fault due to vibration or heat

Fault status

⚠️ Status
ECM/PCM detected open in the power-relay control circuit. Relay coil or control wiring not responding to driver command; MIL stored. May result in no communication or intermittent power to ECM/PCM until circuit is repaired.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-2.0 hours

Similar codes

P0680 P0681 P0620 P0625 P1685
320

Browse 320 LAND ROVER manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.

LAND ROVER

Your experience will help others
+100 karma for a short comment :)
Keep it short and practical. Up to 500 characters.
Send to email

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