Code
B0285
HUMMER
B — Body
Electric Rear Defrost Circuit Low (BCM)
AI status
Completed
Completed
100%
Causes
- Blown or high-resistance fuse protecting the rear defogger circuit
- Failed or stuck defogger relay or relay socket corrosion
- Poor or corroded connectors or grounds in the rear defogger circuit
- Open, high-resistance or shorted wiring between the BCM/relay and the defrost grid
- Defrost grid damage, loss of continuity, or high resistance in the rear window heating element
- Faulty rear defogger switch or HVAC control input to the BCM
Symptoms
- Rear window defroster does not heat or heats weakly
- Defogger indicator lamp may illuminate but grid not warming
- DTC B0285 stored in BCM when defogger is commanded ON
- Intermittent defroster operation (works sometimes)
- Fuses blow repeatedly or thermal cutouts trip
- Possible battery drain if circuit is shorted
What to check
- Read and record stored codes and any freeze-frame data with a scan tool
- Visually inspect fuse(s) and relay for the rear defogger circuit; check for blown fuse or melted relay socket
- Inspect wiring harness and connectors at the rear window, relay, fuse box and BCM for corrosion, damage, or loose pins
- Verify rear window grid visually for broken traces or repair tabs
- Actuate rear defogger using a scan tool (bi-directional control) and observe behavior
- Measure voltage at defogger power feed and at the defogger element/ground when commanded ON
Signal parameters
- Expected supply voltage to defogger (when ON): approximately battery voltage ~11–14 V at the defogger power feed
- Expected control output from BCM/relay: either battery voltage to the element when closed, or ~0 V (open) when off; relay coil control may be a switched ground or +12V depending on vehicle design
- Expected current draw when active: typically several amps (commonly ~5–20 A) depending on vehicle and grid design — verify with service data before high-current tests
- Typical defogger element resistance: low-ohm range; open circuit indicates a failed trace. Exact ohm value varies by vehicle — consult wiring/service data
- Voltage drop limits: minimal voltage drop (
Diagnostic algorithm
- Connect a diagnostic scan tool, read BCM codes and freeze frame. Confirm B0285 presence and note any accompanying codes.
- Attempt to command the rear defogger ON from the scan tool. Observe instrument panel indicator and whether the BCM attempts to switch the circuit (watch relay click or command status).
- Visually inspect condition of the rear defogger fuse, relay, relay socket, wiring harness, rear window harness connector and body grounds. Repair any obvious damage or corrosion.
- With defogger commanded ON, measure battery voltage at the power feed to the defogger and at the BCM/relay output. If no voltage present at the element but relay coil is being commanded, suspect relay or power feed; if BCM output not commanding, suspect BCM or control input.
- Measure voltage drop across the fuse and relay contacts while the defogger is ON. Excessive drop indicates high resistance connection — clean/replace contacts or repair wiring.
- Check continuity/resistance of the defogger grid. Use an ohmmeter to measure overall resistance and inspect for open traces or high-resistance sections. Repair broken traces or replace rear glass if necessary.
- Check for short to ground or power by measuring resistance from circuit to ground/power with power removed. Repair wiring shorts before replacing modules.
- Test or swap known-good relay if available. Re-test operation and scan tool command. If relay swap cures the issue, replace faulty relay.
- If wiring, relay and defogger element check good and the BCM still does not supply expected output while commanded, review service data for BCM outputs, update/reflash BCM software if applicable, and consider BCM replacement only after exhaustive wiring and component checks.
- Clear codes and verify repair by commanding the system multiple times and road or environmental test to ensure stable normal operation.
Likely causes
- Fuse blown or corroded fuse terminals
- Corroded/loose ground or power connector at the rear window harness or body ground
- Open/high-resistance wiring to the defrost grid or relay coil
- Failed relay or relay control by BCM (stuck or not making good contact)
- Damaged rear defrost grid with localized open or high resistance
Fault status
Status
BCM detected low voltage/current on the electric rear defrost circuit while the system was commanded ON; circuit is not providing expected power to the rear defogger.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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Repair manuals
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138
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