U301B code meaning
Control Module Wake-up Circuit A Low
What U301B means
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.
U301B at a glance
- Check engine lightUsually not
- Emissions testNot an emissions code
- Limp modeNot expected
- Repair difficultyMedium
- Typical labor0.5 to 3 hours
- Clears after repairWhen cleared with a scanner
Symptoms of U301B
- Module stays asleep, function or charging unavailable
- Lost-communication codes in other modules
Common causes of U301B
- Most likelyWake-up wire chafed through and touching ground
- PossibleCorrosion or moisture pulling the level down at a connector
- PossibleSending module output stuck low
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Wakeup circuit aWakeup wiring
What each part of U301B means
| U | Network system: communication between the control modules (CAN bus). |
| 3 | Reserved range, usually generic. |
| 1B | The exact fault in that area. |
What to do next
- Note U301B 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 U301B
What does U301B mean?
U301B means "Control Module Wake-up Circuit A Low". U301B means wake-up line A sits below its expected level, typically because it touches ground or is pulled down by a faulty sender. It is a generic code, so it means the same on every make.
Is it safe to drive with U301B?
U301B does not normally turn on the check engine light by itself. Watch for the symptoms above and have it checked soon.
Will U301B fail an emissions test?
U301B 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 U301B to fix?
Difficulty: medium. A typical repair takes 0.5 to 3 hours of work. This is a job for a workshop.
Will U301B clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.