P0678 — Cylinder 8 Glow Plug Circuit/Open #

The code description may differ depending on the manufacturer. Below are known variations:

• Cylinder 8 Glow Plug Circuit/Open (Generic OBD-II)
• Glow Plug/Heater Circuit “H” (Cylinder 8) (Ford)
• Glow Plug Circuit Open, Cylinder 8 (GM/Chevrolet)
• Glow Plug Control Circuit Low – Cylinder 8 (Dodge/RAM)

Summary of code P0678 #

P0678 is a diagnostic trouble code (DTC) that indicates the Engine Control Module (ECM)—the computer that manages engine functions—has detected an issue with the glow plug circuit for cylinder 8. Glow plugs are electrical heating elements used in diesel engines to help ignite the fuel-air mixture when the engine is cold, especially during startup. This code usually means the circuit is open or has a higher resistance than expected, which prevents the glow plug from functioning correctly. The problem could arise from the glow plug itself, the associated wiring, the relay, or the ECM’s control side. Addressing this code promptly is essential for reliable cold starts and optimal engine performance.

Frequent symptoms for code P0678 #

When P0678 appears, you may notice one or more of the following symptoms. Some can be subtle, while others, especially during cold weather, can be quite pronounced. Watch for these signs:

• Hard starting when cold
• Extended cranking time before the engine starts
• Rough idle after cold start
• Increased white smoke from the exhaust during startup
• Illuminated check engine light (MIL)
• Possible reduction in power until the engine warms up

Probable causes for P0678 #

The most common reasons behind P0678 relate to electrical faults or failures within the glow plug system, particularly in cylinder 8. Here’s what typically goes wrong, ranked in order of likelihood:

1. Failed glow plug (cylinder 8)
2. Open or damaged wiring/connectors leading to the glow plug
3. Defective glow plug relay or control module
4. Corroded or loose connections at the glow plug or relay
5. ECM (Engine Control Module) malfunction (rare)

Step-by-step diagnosis for P0678 #

A systematic approach makes diagnosing P0678 much less daunting. Here’s a proven process you can follow, whether you’re a DIYer or an experienced tech:

• 1. Confirm the code: Use a scan tool to verify P0678 and check for related codes, especially those for other glow plug circuits.
• 2. Visual inspection: Examine the glow plug, wiring, and connectors at cylinder 8 for signs of damage, corrosion, or disconnection.
• 3. Test the glow plug: Remove and check resistance with a digital multimeter (should typically be 0.5–2.0 ohms; verify OEM specs).
• 4. Check circuit voltage: With the ignition on, measure voltage supplied to the glow plug connector during the pre-glow phase.
• 5. Inspect relay and control: Test glow plug relay function; ensure it delivers power to the circuit during startup.
• 6. Check ECM signal: Confirm the ECM is providing the control signal to the relay (as per OEM wiring diagrams).
• 7. Clear codes and retest: After repairs, clear codes and verify the fix with at least one cold start cycle.

Possible repairs for P0678 #

Once you’ve pinpointed the cause, the appropriate repair will address the specific fault. Here are typical fixes aligned with the causes above:

• Replace the glow plug in cylinder 8 if it tests faulty or out of OEM spec.
• Repair or replace damaged wiring or connectors to the glow plug circuit.
• Replace the glow plug relay or control module if it fails to deliver power as commanded.
• Clean and secure all connections at the glow plug and relay to eliminate corrosion or looseness.
• Replace or reprogram the ECM only if all other causes are ruled out and manufacturer diagnostics confirm a fault.

Can you drive with code P0678? #

Yes, but with limitations. The vehicle will usually run, but cold starting can be difficult, especially in colder weather. Ignoring P0678 may lead to rough running, excessive white smoke, and increased wear on the starter and battery. If you live in a cold climate or rely on your car for daily use, address this code as soon as possible to avoid being stranded on a frosty morning.

Vehicles most commonly affected by P0678 #

P0678 is seen most often in diesel engines, particularly V8 configurations where cylinder 8 is present. Models from the following manufacturers are frequently affected:

• Ford Super Duty trucks (Power Stroke diesel engines)
• General Motors/Chevrolet Silverado and Sierra (Duramax diesel)
• Dodge/RAM trucks (Cummins diesel)
• Some Mercedes-Benz and Sprinter vans (V8 diesels)

Common mistakes & best practices with P0678 #

Rushing the diagnostic process can lead to unnecessary part replacements and wasted time. Here’s how to avoid common pitfalls:

• Assuming the glow plug is always at fault—wiring issues are just as likely.
• Not verifying the glow plug resistance against OEM specs.
• Overlooking corroded or loose connectors, especially in harsh climates.
• Skipping a scan for additional related codes.
• Failing to clear the code and confirm a successful repair after service.

Quick FAQ for code P0678 #

Is it OK to replace just one glow plug?
Yes, you can replace only the faulty glow plug, but if others have high mileage, consider replacing all for even performance, especially in cold areas.

Will this code cause engine damage if ignored?
Not directly, but repeated hard starting can strain the battery and starter motor. Unburned fuel during cold starts may dilute engine oil over time if the problem persists.