OBD-II Diagnostic Trouble Code
U0120 — Lost Communication With Starter/Generator Control Module
U0120 means a module lost communication with the starter/generator control module, used on vehicles with an integrated starter-generator or mild hybrid system. The module manages engine start and charging through the starter-generator. Usage varies by OEM.
Severity 3/5
Truck-relevant
Service this week
Severity
Service this week
Lost Communication With Starter/Generator Control Module
Code
U0120
Category
Network
Subsystem
Network
Truck-relevant
Yes
What this means for truckers
U0120 (Lost Communication With Starter/Generator Control Module) — on hybrid or stop-start systems (newer commercial trucks), this module manages the integrated starter/generator. Without it, stop-start won't function.
Before you replace parts — try this first
U0120 on the Network system is most often power or ground fault at the starter/generator module — 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
No-start or slow start, auto stop-start disabled, charging warning or low voltage, and hybrid system warning lamps.
Common causes
- CommonPower or ground fault at the starter/generator module ($50–$400)
- CommonNetwork wiring or connector damage ($150–$700)
- OccasionalWeak 12-volt battery ($150–$500)
- OccasionalFailed starter/generator control module ($800–$3000)
How to fix
Test 12-volt battery and check module power and ground
When to use: First step
Inspect network wiring and connectors
When to use: When power is good
Replace the control module
When to use: When power and wiring are good but it will not communicate
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
- → U0100 — Lost Communication With ECM/PCM A
- → U0101 — Lost Communication With TCM
- → U0102 — Lost Communication With Transfer Case Control Module
- → U0103 — Lost Communication With Gear Shift Module
- → U0104 — Lost Communication With Cruise Control Module
Frequently Asked Questions
Can I drive with code U0120?
U0120 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 U0120 on a commercial truck?
Total repair cost ranges $50–$3000 depending on the root cause. The cheapest fix is usually the first one listed below — Power or ground fault at the starter/generator module — 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 U0120 fail a DOT inspection?
U0120 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 U0120 and related codes?
U0120 is closely related to: U0100 (Lost Communication With ECM/PCM A), U0101 (Lost Communication With TCM), U0102 (Lost Communication With Transfer Case Control Module), U0103 (Lost Communication With Gear Shift Module). 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 U0120 is set without the companions, the issue is more isolated.
How do I prevent U0120 from coming back after a repair?
For U0120 (Lost Communication With Starter/Generator Control Module), 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.