Network, generic

U0099 Vehicle Communication Bus H (-) High

Check engine lightUsually off

EmissionsNot relevant

Drive cycleNot required

Limp modePossible

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.

Likely causes

(-) wire shorted to a battery feed

Corrosion or water in a connector between (-) and a supply pin

Faulty transceiver driving the (-) line high

Symptoms

On high-speed CAN: Bus H goes silent
On fault-tolerant CAN: degraded but working communication

Repair estimate

DifficultyHardWorkshop job

Parts and labour€60–800

Time1–5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

bus h wiringcan low wire

Related codes

U007A U0090 U0096 U0098

References: nxp.com, onsemi.com, law.cornell.edu.