B1318 B0xxx · Body (B) Medium severity
System Voltage Low (Body Module)
The body module has seen supply voltage below its operating threshold. Always find the cause — a weak battery sets this long before it fails to start.
Severity guidance: Medium — drivability or emissions affected, book it in this week
Symptoms you will notice
- Intermittent electrical faults
- Central locking or windows slow
- Warning lamps on start-up
Common causes
Ordered from most to least likely — start at the top.
- 1 Weak or failing battery
- 2 Loose or corroded earth
- 3 High resistance in the main supply
- 4 Parasitic drain
How to diagnose it
Cheapest and least invasive first — most of these codes are not a parts problem.
- ✓ Load-test the battery — do not trust an open-circuit voltage reading.
- ✓ Measure voltage drop across the main earth and supply while cranking.
- ✓ Check for a parasitic drain if the fault appears after parking.
- ✓ Inspect the earth points under the dash and in the engine bay.
Repair options
- Replace the battery
- Clean and torque the earth points
- Find and remove the parasitic drain
B1318 — frequently asked questions
The battery reads 12.4 V — is that fine? +
Only a load test tells you. A battery can read healthy at rest and collapse under starter load.
Will a new battery clear the code? +
It will stop the cause, but you still need to clear stored codes and re-check after a few cold starts.
Confirm the diagnosis before you buy parts
Send us the code, freeze-frame data and the vehicle — our engineers will tell you what to test first.