U0044Vehicle Communication Bus B (-) High
- Generic OBD-II code · Network
- Can trigger limp mode
- How the car detects it
- U0044 means the (-) conductor of Vehicle Communication Bus B is held at an excessive voltage, mostly through contact with battery plus. Where Bus B is a high-speed-type CAN, the conductor is CAN-Low, and onsemi found that with CAN-Low at 12 V no frame gets out any more; the node's current even drops, to about 48 mA, because its transmit pin is driven low only some 2 % of the time while the controller retries and backs off. All Bus B members fall silent together. A fault-tolerant low-speed CAN handles the same fault differently: NXP's TJA1055 detects it after a filter delay, turns off the CAN-Low driver and listens on CAN-High only. No manufacturer documents U0044 publicly. Ignition on, CAN-Low against ground near battery voltage confirms it; look for chafing at supply wires and water in connectors.
- What you may notice
- All functions on Bus B fail together (high-speed type)
- Lost-communication codes in modules on other networks
- Fallback modes or warning lamps depending on the modules involved
- Likely causes
- Likely(-) wire chafed against a battery or ignition feed
- PossibleWater in a connector linking the (-) wire to a supply pin
- RareDamaged 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.