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Medium Severity — Powertrain OBD-II DTC

P0678: Computer and Auxiliary Outputs Malfunction (P0678)

P0678 is a generic (SAE standard) powertrain diagnostic trouble code (DTC) indicating: Computer and Auxiliary Outputs Malfunction (P0678). This code is set when the vehicle's onboard computer (ECM/PCM) detects a fault in the computer and auxiliary outputs system.

⚡ Quick Summary

Severity
Medium
DIY Level
Moderate
Repair Cost
$80–$300
Urgency
Fix within weeks

🚨 Symptoms of P0678

▸ Check engine light on
▸ Various drivability issues depending on the affected output
▸ Component not functioning (fan, relay, solenoid)

🔍 Common Causes of P0678

HIGH
Faulty computer and auxiliary outputs component
MED
Wiring issue — open, short, or corroded connector
LOW
ECM/PCM software issue or internal failure

🛠️ How to Fix P0678

⭐ Most Common Fix

Diagnose and repair computer and auxiliary outputs circuit

💰 $80–$300 🔧 Moderate

Repair or replace damaged wiring/connectors

💰 $50–$200 🔧 Easy

Replace faulty computer and auxiliary outputs component

💰 $150–$500 🔧 Moderate

🔬 Step-by-Step Diagnosis

  1. 1 Check the affected output circuit with a test light or multimeter
  2. 2 Verify power and ground at the component connector
  3. 3 Test the ECU output pin with a noid light or oscilloscope
  4. 4 Check for short circuits in the wiring harness

⚠️ Common Mistakes to Avoid

  • ✗ Replacing the ECU/PCM without testing the output circuit first
  • ✗ Not checking for corroded connectors or ground points

💡 Pro Tips

  • ★ Always check for Technical Service Bulletins (TSBs) for your specific vehicle — manufacturers often release updated repair procedures for common P0678 issues.
  • ★ Before replacing expensive parts, verify power, ground, and signal at the component connector with a multimeter.

❓ Frequently Asked Questions

What does P0678 mean? ▾
P0678 means Computer and Auxiliary Outputs Malfunction (P0678). The vehicle's computer has detected a fault in this system that needs diagnosis.
Can I drive with code P0678? ▾
This is a moderate issue. You can usually drive with this code, but schedule a diagnosis soon to prevent potential problems.
How much does it cost to fix P0678? ▾
Repair costs vary widely depending on the root cause. Diagnosis typically costs $80-$150. Repairs can range from $50 for simple fixes to $500+ for major component replacements. Always get a diagnosis before authorizing repairs.

🏥 When to See a Mechanic

If you're not comfortable with electrical diagnosis or don't have a multimeter and OBD-II scanner, have a qualified mechanic diagnose P0678. This code involves the computer and auxiliary outputs system which may require specialized knowledge.

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Disclaimer: This information is for educational purposes only. It is not intended as repair advice and we are not responsible for any actions you take on any vehicle. Always consult a qualified mechanic for diagnosis and repair. Repair costs shown are estimates and may vary by location, vehicle, and shop.