U0077
Generic (SAE) Basic entry
Reserved by J2012
Severity
Medium
Typical repair cost
50 – 700 EUR
System
Network
What it means
U0077 is defined as "Reserved by J2012". a control module on the vehicle network stores it when the monitored condition for this component or system is outside the limits programmed by the manufacturer.
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Symptoms
- Several warning lights on at the same time
- Functions controlled by the affected module do not work
- Scan tool cannot communicate with one module
- Gauges dropping to zero or dashboard flickering
Common causes
- Blown fuse or no power supply to the Reserved by J2012
- Poor ground for the module
- Damaged CAN/data bus wiring or connector
- Failed module
- Low battery voltage during cranking or after a jump start
How to diagnose
- Check which modules respond to a full system scan and which do not
- Verify fuses, power and ground at the Reserved by J2012 connector
- Measure CAN bus resistance between the two bus wires (about 60 ohms with the battery disconnected) and inspect the wiring
- Check battery voltage and charging system; low voltage causes many communication codes
- Replace or reprogram the module only after power, ground and bus wiring are confirmed
Frequently asked questions
Can I keep driving with U0077?
Usually yes for a short time, but expect reduced performance or higher fuel use. Have it diagnosed within a few days.
Will clearing U0077 fix it?
No. Clearing only erases the stored code; it returns as soon as the monitor runs again unless the cause is repaired.