B0840 code meaning
Left Rear Turn Signal Circuit
What B0840 means
Neither J2012 extracts nor any OEM or NHTSA publication name B0840; its use is unproven. The title concerns the left rear indicator. In the 2003–2012 Saab 9-3 the rear electrical centre supervises this lamp under the number B0530: an open lamp (04) or a short to ground (02) shows up as a warning message and the remaining indicators flashing at double speed, which is exactly the pilot-indicator behaviour FMVSS 108 permits for a failed turn lamp. HELLA notes that modern rear lamps are driven by body or lighting control units and may consist of LED segments, so diagnosis should include fault codes, connectors and the supply to the lamp unit. A failed rear indicator hides lane changes from following traffic. Right rear: B0841.
B0840 at a glance
- Check engine lightUsually not
- Emissions testNot an emissions code
- Limp modeNot expected
- Repair difficultyEasy, DIY possible
- Typical labor0.2 to 1.5 hours
- Clears after repairWhen cleared with a scanner
Symptoms of B0840
- Indicators flash at double speed when signalling left
- Left rear indicator dark, warning message shown
- Lane changes not visible to following traffic; repair promptly
Common causes of B0840
- Most likelyRear indicator bulb or LED segment failed
- PossibleCorroded plug or bulb carrier in the rear lamp
- PossibleWire break in the trunk lid or tailgate harness
- Less likelyRear electrical centre output faulty
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Left rear turn signalBody control module
What each part of B0840 means
| B | Body system: airbags, seat belts, power seats, lights and climate control. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 40 | The exact fault in that area. |
What to do next
- Note B0840 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 B0840
What does B0840 mean?
B0840 means "Left Rear Turn Signal Circuit". Neither J2012 extracts nor any OEM or NHTSA publication name B0840; its use is unproven. It is a generic code, so it means the same on every make.
Is it safe to drive with B0840?
B0840 does not normally turn on the check engine light by itself. Watch for the symptoms above and have it checked soon.
Will B0840 fail an emissions test?
B0840 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 B0840 to fix?
Difficulty: easy. A typical repair takes 0.2 to 1.5 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will B0840 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.