U0042 code meaning
Vehicle Communication Bus B (-) Open
What U0042 means
U0042 points to an interrupted (-) wire on Vehicle Communication Bus B; on a CAN-type bus that is CAN-Low. The network then splits along the break, and on a standard high-speed CAN neither side's transceiver can tell what happened, since NXP states that line interruptions escape even transceiver diagnosis. Detection rests on time-outs for messages from nodes on the far side. On a fault-tolerant low-speed CAN, the TJA1055 data sheet from NXP treats a CAN-Low interruption as a failure that frames in progress survive. A resistance check across the two bus wires at a central connector helps: with both terminations reachable it reads about 60 ohms, a break in one wire leaves only the near terminator at about 120 ohms. No manufacturer documents U0042 publicly; start with connectors that were opened recently.
U0042 at a glance
- Check engine lightUsually not
- Emissions testNot an emissions code
- Limp modeNot expected
- Repair difficultyHard
- Typical labor0.5 to 5 hours
- Clears after repairWhen cleared with a scanner
Symptoms of U0042
- Modules beyond the break no longer respond
- Lost-communication codes stored alongside
Common causes of U0042
- Most likely(-) wire of Bus B broken in the harness
- PossibleCorroded or pushed-back terminal in a connector
- PossibleConnector not reconnected after a repair
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Bus b wiringModule connector
What each part of U0042 means
| U | Network system: communication between the control modules (CAN bus). |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 42 | The exact fault in that area. |
What to do next
- Note U0042 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 U0042
What does U0042 mean?
U0042 means "Vehicle Communication Bus B (-) Open". U0042 points to an interrupted (-) wire on Vehicle Communication Bus B; on a CAN-type bus that is CAN-Low. It is a generic code, so it means the same on every make.
Is it safe to drive with U0042?
U0042 does not normally turn on the check engine light by itself. Watch for the symptoms above and have it checked soon.
Will U0042 fail an emissions test?
U0042 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 U0042 to fix?
Difficulty: hard. A typical repair takes 0.5 to 5 hours of work. This is a job for a workshop.
Will U0042 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.