OBD-II Diagnostic Trouble Code
P0684 — Glow Plug Control Module to PCM Communication Range/Performance
P0684 means the communication signal between the glow plug control module and the ECM is present but out of the expected range or not plausible. Examples are wrong signal frequency, duty cycle or message content. It points to an intermittent connection, electrical noise, or a failing module rather than a fully open circuit.
Severity 2/5
Truck-relevant
Service soon
Severity
Service soon
Glow Plug Control Module to PCM Communication Range/Performance
Code
P0684
Category
Powertrain
Subsystem
Cold Start
Truck-relevant
Yes
What this means for truckers
Service this week on a Cummins X15, Detroit DD15, PACCAR MX-13. Not an immediate stop, but ignoring it usually leads to a more expensive fix downstream.
Before you replace parts — try this first
P0684 on the Cold Start system is most often intermittent or high-resistance connection in the communication wire or connectors — not a dead part. On commercial diesels especially, work these in order before spending money on parts:
- Confirm the symptoms. Match what your truck is doing to the Symptoms section below. If the code is set but the symptoms are not there, clear the code and re-test after a drive cycle — it may not return.
- Clean and dry the sensor connector. Water, road salt, or oil seeping into a connector is the #1 fake sensor failure. Disconnect, look for green corrosion or moisture, spray with electrical contact cleaner, dry, reseat. Many faults clear here for $0.
- Inspect the wiring harness. Trace the wire from the sensor toward the ECM — look for chafing against the engine block, melted insulation near the turbo or exhaust, or pinch points where a clamp has rubbed through.
- Test the sensor before swapping it. Most Common Causes (below) list the expected voltage or resistance range — a $20 multimeter saves a $300 sensor swap that does not fix the problem.
- Only then, replace the part. Use the OEM or a tier-1 aftermarket. Cheap sensors throw the same code in two weeks.
Symptoms
Intermittent hard cold starts, a glow plug lamp that behaves oddly, and the check-engine light. Glow plug fault reporting may be unreliable, so a weak plug can go unreported.
Common causes
- CommonIntermittent or high-resistance connection in the communication wire or connectors ($80–$350)
- CommonFailing glow plug control module sending a bad signal ($150–$600)
- OccasionalPoor module ground or low supply voltage during glow (heavy current draw) ($60–$300)
- RareElectrical interference from damaged harness routing near high-current glow plug leads ($100–$400)
How to fix
Inspect and clean the module and ECM connectors and check terminal tension
When to use: First step
Voltage-drop-test the module ground and power during a commanded glow cycle
When to use: When connectors look fine
Check the communication signal on a scope against the manufacturer's spec
When to use: To confirm a bad signal from the module
Replace the glow plug module
When to use: When the wiring and supply are good and the signal is still out of range
Affects most
No specific vehicle pattern found in our complaint database. This code can appear on any vehicle that supports OBD-II diagnostics.
Related
- → Browse all DTC trouble codes
- → Decode a VIN to check codes for your truck
- → Owner-operator maintenance guides
- → P0670 — Glow Plug Control Module Circuit
- → P0671 — Cylinder 1 Glow Plug Circuit
- → P0672 — Cylinder 2 Glow Plug Circuit
- → P0673 — Cylinder 3 Glow Plug Circuit
- → P0674 — Cylinder 4 Glow Plug Circuit
Frequently Asked Questions
Can I drive with code P0684?
P0684 is severity 2 — service this week. Not an immediate stop, but ignoring it usually compounds the problem. Most common cause for this code is a worn sensor or minor performance fault that won't strand you today but will get more expensive the longer you wait.
How much does it cost to fix P0684 on a commercial truck?
Total repair cost ranges $60–$600 depending on the root cause. The cheapest fix is usually the first one listed below — Intermittent or high-resistance connection in the communication wire or connectors — which is also the most common. On commercial trucks the cost is usually higher than passenger vehicles due to part availability and shop labor rates ($120–185/hour at most truck shops, vs $90–130 at general auto shops). DIY-capable fixes (sensors, hoses, filters) are at the low end; major component replacements (turbo actuator, high-pressure fuel pump, transmission solenoid pack) are at the high end. Always diagnose with a scan tool BEFORE replacing parts — guessing wrong on a $1,500 turbo actuator when the actual issue was a $30 boost hose is a common expensive mistake.
Will P0684 fail a DOT inspection?
P0684 is generally not a direct DOT inspection fail item by itself. Inspectors focus on safety-critical systems (brakes, steering, tires, lights, hours-of-service). However, the check engine light alone can trigger additional inspection scrutiny. Best practice: fix the code and clear it before a planned inspection.
What's the difference between P0684 and related codes?
P0684 is closely related to: P0670 (Glow Plug Control Module Circuit), P0671 (Cylinder 1 Glow Plug Circuit), P0672 (Cylinder 2 Glow Plug Circuit), P0673 (Cylinder 3 Glow Plug Circuit). These codes share the same subsystem (cold-start) and often appear together. If you see this code, scan for the full code list — fixing the root cause usually clears multiple companion codes at once. If only P0684 is set without the companions, the issue is more isolated.
How do I prevent P0684 from coming back after a repair?
For P0684 (Glow Plug Control Module to PCM Communication Range/Performance), prevention centers on the cold start system. Test glow plug system annually before winter. Use a battery in good condition — weak batteries make glow plugs work harder. Park indoors below -10°F if possible.