U350B

What it means

U350B means a control module has seen its input power supply on circuit 'C' drop out, fluctuate or behave erratically, rather than stay steady. Which feed counts as circuit 'C' (for example a switched, ignition or sensor-supply feed) depends on the module and the vehicle, so check the wiring diagram for the model. The fault is intermittent, so it may not be present when you test the car.

Symptoms

• Warning lamps on the dash that come and go, depending on which module is affected
• Intermittent loss of a function, such as driver aids, infotainment, lighting or comfort systems
• Other modules logging communication (U-series) codes at the same time
• Occasional stalling, hesitation or limp mode if the affected module is engine or transmission related
• Problems that appear over bumps or when the car is started, or when the battery is low

Possible causes

• Loose, corroded or damaged connector pins at the affected module's power or earth terminalslikely
• Weak or failing battery, or poor battery terminal connections causing voltage dipslikely
• Chafed, stretched or partly broken wiring in the power feed, often at flex points or where the harness has been repaired beforelikely
• Poor earth connection or high resistance in the module's ground pathpossible
• Intermittent fuse, fuse holder or relay contact feeding the circuitpossible
• Charging system fault causing unstable supply voltagepossible
• Internal fault in the control module or outdated module software where a manufacturer bulletin identifies itrare

How to diagnose

1. Read all stored and pending codes with freeze frame data, and note which module set U350B and any other low-voltage or communication codes
2. Check battery condition and state of charge, then check charging voltage with the engine running
3. Inspect battery terminals, main earth straps and the affected module's connector for corrosion, looseness, pushed-back pins or water ingress
4. Look up the wiring diagram for the model to identify which feed is circuit 'C' and which fuses, relays and earths serve it
5. Check the relevant fuses and relays and their sockets for heat marks, poor contact or fretting
6. With the circuit loaded, wiggle the harness and connector while monitoring the supply voltage to catch the dropout
7. Carry out voltage drop tests on the power and earth paths to the module
8. Check for any previous repairs or aftermarket accessories tapped into the same supply, and check for manufacturer bulletins on the module

How to fix

• Clean, tighten or replace corroded or loose battery terminals and earth connections
• Repair or replace damaged wiring or terminals in the power feed, using proper crimps or the correct repair method rather than twisted joints
• Replace the faulty fuse, fuse holder or relay found to be causing the dropout
• Replace the battery or repair the charging system if supply voltage is unstable
• Remove or correctly rewire any faulty aftermarket accessory sharing the supply
• Update the module software where a manufacturer bulletin lists it as a cause, or replace the module only once the supply and earth are proven good

Can I keep driving?

Nearby codes

• U350AGenericControl Module Input Power C Circuit High
• U350CGenericControl Module Cooling Fan Circuit/Open