CAN network

U3003 — Control Module Supply Voltage Out of Range

Severity: medium Drive with care, to a shop

What U3003 means

U3003 means a control module on the network measured its supply voltage outside the allowed range, most often too low. Modules need roughly 12 to 14.5 volts to run and communicate properly. When voltage sags, modules can reset, lose communication and log extra network codes.

Symptoms

  • Several U-codes stored right after startup
  • Random faults in different modules
  • Hard or slow engine start
  • Warning lights flickering or coming on
  • Power steering or other systems temporarily shutting off
  • Dim lights and erratic electronics

Possible causes

  1. Weak or discharged 12V battery
  2. Corroded or loose battery terminals
  3. Alternator not charging or unstable output
  4. Parasitic drain with the ignition off
  5. Poor ground or damaged wiring harness
  6. Power supply problem at one module (fuse, connector)

How to diagnose

  1. Scan all modules and note which one reported U3003 and any extended failure type (such as 16).
  2. Measure battery voltage with the engine off: a healthy, charged battery reads about 12.4 to 12.7 V.
  3. Measure voltage with the engine running: typically 13.5 to 14.5 V and steady when loads are switched on.
  4. Inspect and tighten battery terminals and check ground points for corrosion.
  5. Load-test the battery to check capacity and cold cranking amps.
  6. Measure parasitic draw after the modules sleep and trace any excess draw by pulling fuses.
  7. Check power and ground at the connector of the reporting module and inspect its wiring.
  8. Clear codes and confirm they don't return after the repair.

Common fixes

  • Charge or replace the battery
  • Clean and tighten terminals and ground connections
  • Repair or replace the alternator
  • Find and fix the source of parasitic drain
  • Repair the wiring or connector of the affected module

U3003 on different brands

Ford · manuals
[U3003:16] Circuit voltage below threshold: on the Ford Explorer, the power steering control module (PSCM) saw low supply voltage; this applies to the documented EPAS controller and model years.
Land Rover
[U3003:16] Circuit voltage below threshold: on the L405, the deployable side step ECU (DSS) detected low voltage; this applies only to that controller.

Questions and answers

Can a weak battery cause U3003?

Yes, it's the most common cause. Low voltage makes modules drop off the network and log faults. Test and charge the battery first, then clear the codes.

Do I need to replace a module for U3003?

Usually not. Check the battery, alternator, terminals and grounds first. A module is rarely at fault unless power problems have been ruled out.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.