OBD-II Diagnostic Trouble Code
P0608 — Control Module VSS Output A Malfunction
P0608 means the control module detected a fault on its vehicle speed signal output A. That's the speed signal the ECM or PCM sends to other devices such as the instrument cluster, cruise, or a body or trailer system. The ECM saw the output circuit shorted, open, or not following its command.
Severity 3/5
Truck-relevant
Service this week
Severity
Service this week
Control Module VSS Output A Malfunction
Code
P0608
Category
Powertrain
Subsystem
Network
Truck-relevant
Yes
What this means for truckers
P0608 (Control Module VSS Output A) — ECM's VSS output to other modules has a fault.
Before you replace parts — try this first
P0608 on the Network system is most often output wire shorted to ground or voltage between the ecm and the receiving device — 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
The speedometer or odometer may be erratic or dead, cruise control may not work, and other systems that use vehicle speed (speed-sensitive features, some body controls) may misbehave. The check-engine light comes on. The engine itself runs normally.
Common causes
- CommonOutput wire shorted to ground or voltage between the ECM and the receiving device ($120–$450)
- OccasionalFaulty receiving device (cluster or other module) loading the circuit ($300–$1200)
- OccasionalCorroded or loose connector on the speed output circuit ($60–$250)
- RareFailed ECM output driver ($1200–$3500)
How to fix
Find which devices receive the speed output on the wiring diagram and check whether they're getting the signal
When to use: First step
Unplug the receiving devices one at a time to see if the output fault clears
When to use: To find a device loading the circuit
Check the output circuit for shorts to ground or voltage and for opens
When to use: When the receiving devices check out
Replace the ECM
When to use: Only when the circuit and the receiving devices are confirmed good
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 P0608?
P0608 is severity 3 — plan service within a few days. Performance is likely degraded already (you may feel reduced power, harsher shifts, or slightly worse fuel economy). Continuing to drive risks additional damage to adjacent components.
How much does it cost to fix P0608 on a commercial truck?
Total repair cost ranges $60–$3500 depending on the root cause. The cheapest fix is usually the first one listed below — Output wire shorted to ground or voltage between the ECM and the receiving device — 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 P0608 fail a DOT inspection?
P0608 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 P0608 and related codes?
P0608 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 (network) 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 P0608 is set without the companions, the issue is more isolated.
How do I prevent P0608 from coming back after a repair?
For P0608 (Control Module VSS Output A Malfunction), prevention centers on the network system. Pay attention to ground connections at every PM — bad grounds are 70% of communication codes. Inspect harness routing for chafe points. Don't disconnect modules with the key on.