U301BControl Module Wake-up Circuit A Low
- Generic OBD-II code · Network
- How the car detects it
- U301B means wake-up line A sits below its expected level, typically because it touches ground or is pulled down by a faulty sender. What happens next depends on how the receiving chip works. NXP's CAN transceiver TJA1043, for example, wakes on a change of the WAKE pin in either direction once the new level has stayed stable for a short filter time, and its switching threshold lies a few volts below battery voltage (typically battery minus 2.5 V). A line clamped to ground therefore produces no further edge and acts like an unused pin, so the module stays asleep when it should start. A wire that only sags, for example through corrosion, can drop under the threshold too. No manufacturer documents U301B publicly; compared with U301A, look first for chafing against body metal.
- What you may notice
- Module stays asleep, function or charging unavailable
- Lost-communication codes in other modules
- Likely causes
- LikelyWake-up wire chafed through and touching ground
- PossibleCorrosion or moisture pulling the level down at a connector
- PossibleSending module output stuck low
- Repair
- Medium · Usually a job for a workshop.Typical cost: €50 – €600
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.