Code
P1804
HUMMER
P — Powertrain
Ground Control Relay
AI status
Completed
Completed
100%
Causes
- Failed (stuck/open/shorted) Ground Control Relay
- Blown or corroded fuse for the relay circuit
- Poor connector contact, corrosion, or water intrusion at the relay socket
- Open, shorted or damaged wiring between relay, power source, ground, and control module
- Poor chassis ground at relay or control module
- Faulty control module (open driver or incorrect output)
Symptoms
- MIL (Check Engine) or relevant warning lamp illuminated
- Associated subsystem operation intermittent or inoperative (depends on what Ground Control Relay powers)
- Blown fuses or repeated fuse failures
- Possible parasitic battery drain if relay contacts are stuck closed
- Relay may click intermittently or not at all when commanded
What to check
- Read stored freeze frame and related codes with a scan tool; note conditions when code set
- Visual inspection of relay, socket, wiring harness, and fuses for corrosion, water, damage or loose terminals
- Check fuse continuity and proper fuse seating
- Verify battery voltage (11–14.5 V) at relay power terminal with key on
- Backprobe relay coil/control terminal while commanding the circuit to observe control voltage/ground
- Bench-test or swap relay with a known-good same-type relay to confirm function
Signal parameters
- Battery supply at relay common: ~11–14.5 V (key ON, engine off) - should be same as battery
- Relay coil/control: 0 V (inactive) and ~12 V or chassis ground depending on design when commanded
- Expected coil resistance (typical 12 V automotive relay): ~50–200 ohms (refer to relay spec for exact value)
- Switched contact (load) voltage: near battery voltage when relay energized, open/infinite when not energized
- No voltage drop >1 V across relay contacts when energized under normal load
Diagnostic algorithm
- Connect a scan tool, confirm P1804 is current or stored, and record freeze-frame/related codes and conditions.
- Perform a visual inspection: relay, socket, wiring harness, fuses, and grounds for corrosion, loose pins, water, or damage.
- Check and verify the fuse(s) protecting the relay circuit — replace any blown fuses and inspect why they blew before proceeding.
- With ignition ON, measure battery voltage at the relay power terminal. Expect ~11–14.5 V. No voltage indicates upstream power fault.
- Command the circuit using the scan tool (if supported) or by operating the affected function while backprobing the relay coil terminals to monitor control signal. Verify the module is commanding the relay (voltage or ground present).
- If the module commands the relay but the relay does not operate, bench-test or swap the relay with a known-good unit. If swapped relay works, replace defective relay.
- If the relay clicks but the load does not receive power, check continuity and resistance of the switched contact and wiring to the load. Repair any open/shorts or high-resistance connections.
- If the module does not command the relay, check wiring between module and relay for open/short/poor ground. Verify module driver output and ground reference. Replace or repair wiring as needed.
- If wiring and relay check good but fault persists, suspect control module fault. Verify software/calibration and consider module testing/replacement per manufacturer procedures.
- After repairs, clear codes, verify proper operation of the affected subsystem, and perform a road test or functional cycle to confirm the issue is resolved.
Likely causes
- Defective relay (most common)
- Loose/corroded relay connector or socket
- Blown fuse or poor fuse connection
- Broken or shorted wiring to relay coil or load
- Faulty module driver or module software fault (least common)
Fault status
Status
Ground Control Relay circuit fault detected — control module reports open/short/intermittent operation of the relay circuit. Related subsystem may be disabled or operate erratically.
Repair difficulty: Medium
Diagnostic time: 0.5-1.5 hours
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Repair manuals
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138
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