U0099 code meaning
Vehicle Communication Bus H (-) High
What U0099 means
U0099 describes a (-) wire on Vehicle Communication Bus H held at too high a voltage, most often through contact with battery power; on a CAN that wire is CAN-Low. Nothing public from a manufacturer covers U0099. NXP's data sheet for the fault-tolerant low-speed TJA1055 separates two cases: CAN-Low at battery voltage is recognised once the line stays above a comparator threshold for a set time, after which the chip turns off its CANL driver and listens over CANH alone; CAN-Low at the 5 V supply is tolerated without leaving differential mode. Recovery happens automatically after a run of recessive bits. On a high-speed CAN, by contrast, onsemi found that CAN-Low at 12 V blocks all transmission, so U007A for bus off may follow.
U0099 at a glance
- Check engine lightUsually not
- Emissions testNot an emissions code
- Limp modePossible
- Repair difficultyHard
- Typical labor1 to 5 hours
- Clears after repairWhen cleared with a scanner
Symptoms of U0099
- On high-speed CAN: Bus H goes silent
- On fault-tolerant CAN: degraded but working communication
Common causes of U0099
- Most likely(-) wire shorted to a battery feed
- PossibleCorrosion or water in a connector between (-) and a supply pin
- Less likelyFaulty transceiver driving the (-) line high
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Bus h wiringCAN low wire
What each part of U0099 means
| U | Network system: communication between the control modules (CAN bus). |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 99 | The exact fault in that area. |
What to do next
- Note U0099 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about U0099
What does U0099 mean?
U0099 means "Vehicle Communication Bus H (-) High". U0099 describes a (-) wire on Vehicle Communication Bus H held at too high a voltage, most often through contact with battery power; on a CAN that wire is CAN-Low. It is a generic code, so it means the same on every make.
Is it safe to drive with U0099?
Drive gently and only to a workshop. With U0099 the car can go into limp mode (reduced power) to protect itself.
Will U0099 fail an emissions test?
U0099 is not an emissions code, so on its own it usually does not fail an emissions test. Other stored codes or a lit check engine light still can.
How hard is U0099 to fix?
Difficulty: hard. A typical repair takes 1 to 5 hours of work. This is a job for a workshop.
Will U0099 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.