U3020

What it means

U3020 means a control module has detected a fault on its wake-up circuit B, with the signal reading higher than expected. The wake-up line tells a module to come out of sleep, for example when the ignition is switched on, a door opens or another module requests it. Which module and which wire counts as circuit B depends on the vehicle, so the wiring diagram is needed to identify them.

Symptoms

• Warning lights on the dashboard, or several other communication fault codes stored
• Battery going flat overnight because a module is not going to sleep
• Intermittent no-start or slow-start, or a system that does not wake up
• Electrical features such as locking, lights or infotainment working intermittently
• A system that stays awake or behaves oddly after the car is locked

Possible causes

• Wake-up wire shorted to a voltage supply or damaged insulation touching a live feedlikely
• Corroded, loose or water-damaged connector or terminals on the wake-up circuitlikely
• Low or unstable battery voltage, or poor earth or power connections, causing false high readingspossible
• Another module, switch or relay holding the wake-up line high when it should be releasedpossible
• Poorly fitted aftermarket accessory (alarm, tracker, audio) tapped into the wake-up or ignition feedpossible
• Internal fault in the reporting control modulerare

How to diagnose

1. Read and record all stored codes from every module, noting freeze frame data and whether U3020 is current or historic
2. Test battery condition and charging voltage, and check the main power and earth connections are clean and tight
3. Look for aftermarket accessories or recent repairs near the affected circuit and inspect wiring for chafing, water ingress and previous repair damage
4. Use the manufacturer wiring diagram to identify the module and the circuit B wake-up wire for this model
5. Inspect the connectors at the module and along the circuit for corrosion, bent or pushed-back pins and poor terminal tension
6. With the ignition off and then on, measure the voltage on the wake-up wire against the specified values and check for a short to supply
7. Unplug connected modules or accessories one at a time to see whether the high reading clears, and check for a parasitic battery drain
8. Carry out the manufacturer's module test only after wiring and supply faults are ruled out, and check for any applicable software updates

How to fix

• Repair or replace the wiring that is shorted to voltage, using proper soldered or crimped and sealed repairs
• Clean, repair or replace corroded or damaged connectors and terminals
• Replace a weak battery and repair charging or earth faults
• Replace the faulty switch, relay or module that is holding the wake-up line high
• Remove or correctly rewire poorly fitted aftermarket accessories
• Replace or reprogram the control module only when testing proves it is at fault and the manufacturer procedure calls for it

Can I keep driving?

Nearby codes

• U301FGenericControl Module Wake-up Circuit B Low
• U3021GenericControl Module Wake-up Circuit B Performance