Code
B1372
HUMMER
B — Body
Device Ignition 1 Circuit Low
AI status
Completed
Completed
100%
Causes
- Blown or corroded fuse for Ignition 1 circuit
- Faulty ignition switch or key cylinder electrical contacts
- Open, short to ground, or high-resistance wiring/connector in Ignition 1 feed
- Faulty ignition relay or its control circuit
- Poor battery condition, weak charging system or low battery voltage
- Corroded or loose battery terminals or ground connections
Symptoms
- No or intermittent power to accessories when key in RUN/START (radio, HVAC, instrument cluster)
- Crank but no start or no crank depending on affected circuits
- Malfunction indicator lamp or body warning messages
- Intermittent electrical faults that clear with key cycling or jiggling harness
- Stored or active communication errors between modules
What to check
- Check battery state of charge and terminal condition; ensure 12.4–12.8 V at rest
- Visually inspect fuses and relays for the Ignition 1 circuit; replace if blown
- Check battery positive and ground connections for tightness and corrosion
- Backprobe the Ignition 1 fuse or connector and verify voltage with key OFF, ON (RUN), and START
- Inspect wiring and connectors from ignition switch to fuse/relay/BCM for damage or corrosion
- Swap or test the ignition relay if applicable
Signal parameters
- Ignition 1 voltage with key OFF: ~0 V (or battery voltage through expected paths depending on design)
- Ignition 1 voltage with key ON (RUN): battery voltage ~11–14.5 V
- Ignition 1 voltage during START (cranking): typically ≥9 V (may droop depending on battery/starting load)
- Fault threshold: voltage significantly below battery (commonly
- Typical expected continuity: low ohms between ignition feed and fuses/relay inputs
Diagnostic algorithm
- Retrieve freeze frame and related DTCs with a scanner. Note when the fault occurred (key position, engine state).
- Verify battery voltage and charging system health. Charge or replace if battery is weak (below ~12.4 V at rest).
- Inspect and test all fuses and the ignition relay for the Ignition 1 circuit. Replace any blown items and retest.
- With a DVOM, backprobe the Ignition 1 fuse or connector. Verify voltage in OFF, ON (RUN), and START positions. Document readings.
- If voltage is low or missing with key ON, trace wiring back to ignition switch and BCM: check for open circuits, shorts to ground, and high resistance using continuity and voltage drop tests.
- Test the ignition switch output terminal for proper switching to battery voltage in RUN/START positions. Repair or replace switch if contacts fail to provide voltage.
- Inspect and test ground connections and main battery positive feed to the fuse/relay box. Clean/tighten as required and remeasure.
- If wiring and switch check good, test operation or replace the ignition relay and retest circuit voltage.
- If module supply still low and wiring/relays/fuse/switch are good, suspect BCM/module internal fault. Verify communication and power pins at the module; consult manufacturer procedure before replacement.
- After repair, clear codes, perform functional tests and a road test to confirm the fault does not return.
Likely causes
- Blown fuse or failed ignition relay
- Loose/corroded battery or ground connection
- Damaged or disconnected Ignition 1 feed at a connector or splice
- Faulty ignition switch
Fault status
Status
Ignition 1 Circuit voltage below expected threshold. Stored when the body control or related module detects low or missing Ignition 1 supply during key ON/START conditions.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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Repair manuals
Manual library for HUMMER
138
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