U0099Vehicle Communication Bus H (-) High
- Generic OBD-II code · Network
- Can trigger limp mode
- How the car detects it
- 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.
- What you may notice
- On high-speed CAN: Bus H goes silent
- On fault-tolerant CAN: degraded but working communication
- Likely causes
- Likely(-) wire shorted to a battery feed
- PossibleCorrosion or water in a connector between (-) and a supply pin
- RareFaulty transceiver driving the (-) line high
- Repair
- Hard · Usually a job for a workshop.Typical cost: €60 – €800
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.