U3504

What it means

U3504 is a generic code meaning a control module has detected that the voltage on its 'B' power input circuit is outside the expected range or not behaving as it should. The module monitors its own supply voltage, and the reading was too high, too low or unstable for long enough to set the fault. Which module reports it, and which input counts as 'B', depends on the vehicle, so check the manufacturer's wiring information for that model.

Symptoms

• Warning lights on the dashboard, such as the battery, engine or other system warnings
• Intermittent loss of function in the system the affected module controls
• Hard starting or a slow crank, especially if the battery is weak
• Other modules logging communication or low-voltage codes at the same time
• Erratic gauges or electrical items that cut in and out

Possible causes

• Weak, discharged or ageing battery giving low or unstable supply voltagelikely
• Loose, corroded or dirty battery terminals, earth straps or power distribution connectionslikely
• Poor connection, corrosion or pin fretting at the affected module's power supply or earth connectorlikely
• Charging system fault, such as a failing alternator or regulator giving over- or under-voltagepossible
• Blown or high-resistance fuse or relay in the module's power feedpossible
• Damaged wiring, such as chafing, water ingress or a partial open circuit in the supply linepossible
• Internal fault in the control module's power supply circuitrare

How to diagnose

1. Scan all modules and record every stored code, plus freeze frame data and any voltage readings, noting which module sets U3504.
2. Check for other low-voltage, charging or communication codes, as these point to a wider supply problem rather than one module.
3. Inspect the battery terminals, earth straps and main power connections for looseness, corrosion or damage.
4. Test battery condition and measure resting voltage, then voltage while cranking and with the engine running to check charging output.
5. Look up the module's power and earth circuits in the model-specific wiring diagram and identify the 'B' supply feed, its fuse and any relay.
6. Check the relevant fuses and relays, and inspect the module connector for corrosion, moisture, bent pins or pin fretting.
7. With the circuit loaded, voltage-drop test the supply and earth wires to the module while wiggling the harness to find intermittent faults.
8. Clear the code and road test, then recheck for return of the fault to confirm the repair.

How to fix

• Charge and retest the battery, and replace it if it fails a load or condition test.
• Clean and tighten the battery terminals, earth points and power distribution connections.
• Repair or replace damaged wiring, and clean or replace corroded or damaged connector terminals at the module.
• Replace faulty fuses or relays after finding out why they failed.
• Repair the charging system, including the alternator or regulator, if output is out of specification.
• Replace or reprogram the control module only after the supply and earth circuits are proven good and the manufacturer's procedure calls for it.

Can I keep driving?

Nearby codes

• U3503GenericControl Module Input Power A Circuit Intermittent/Erratic
• U3505GenericControl Module Input Power B Circuit Low