Code
P1603
HUMMER
P — Powertrain
Loss of SDM Serial Data
AI status
Completed
Completed
100%
Causes
- Blown fuse or lost power to SDM
- Poor or corroded ground or power connection at SDM
- Damaged, shorted or intermittent CAN/serial data wiring or connector
- Open or shorted bus termination (missing/failed resistor)
- SDM module fault or failed internal circuitry
- Other module on same bus causing bus-off or heavy errors
Symptoms
- Airbag/SRS warning lamp illuminated
- ‘Service Airbag’ or ‘Check SRS’ message on dash
- Scan tool reports no response or intermittent response from SDM
- Loss of seatbelt pretensioner or airbag deployment readiness diagnostics
- Multiple communication-related DTCs present on vehicle network
What to check
- Verify stored DTCs and freeze-frame with a capable scan tool and capture any related U-codes
- Check battery voltage (engine off and cranking) and battery connections; ensure battery >= 12.4 V during diagnosis
- Inspect fuses and relay(s) for SDM/SIR and associated power circuits
- Visually inspect SDM connector, pins and harness for corrosion, bent pins, water intrusion or damage
- Check grounds for integrity and corrosion at SDM ground point(s)
- Use scan tool data list to confirm if SDM appears on the bus and whether messages are present or intermittent
Signal parameters
- Battery voltage: nominal 12–14.5 V (engine running/charging)
- CAN idle voltages: CANH ≈ 2.5–3.5 V, CANL ≈ 1.0–2.5 V (typical idle differential ≈ 0–0.5 V single-ended; differential ~0 V idle expected near 0 V? — check manufacturer spec)
- CAN differential voltage during activity: ~1.0–2.0 V (dependent on bus load and encoding)
- Termination resistance: ~60 Ω measured between CANH and CANL at the two ends combined
- Serial/SDM message rate: manufacturer-specific periodic SIR/SDM messages — verify presence with factory scan tool or data logger
Diagnostic algorithm
- Safety first: before disconnecting or working on SDM/SIR system, follow SIR safety procedure — disconnect battery negative and wait manufacturer-specified time (commonly ~2 minutes)
- Connect a diagnostic scan tool with SIR/airbag capability. Record P1603 and any related U- or B-codes and freeze-frame data
- Check battery voltage and charging system. Repair battery/charging issues before proceeding
- Inspect and verify fuses and power/ignition feed to SDM. Replace faulty fuses and retest communication
- Visually inspect SDM connector, pins and harness. Repair damaged pins, reseat connector, and apply dielectric grease if allowed
- With ignition on, use a scope or multimeter to check CANH/CANL voltages at the SDM connector and compare with a known-good location. Look for stuck-high/low or missing activity
- Measure termination resistance across CANH/CANL end-to-end; confirm near 60 Ω. If termination missing, locate and repair/replace
- If bus activity is missing at SDM but present elsewhere, trace wiring between SDM and next module; perform continuity and short-to-power/ground checks
- If multiple modules show communication loss, isolate by disconnecting modules one at a time to find bus offender (use service manual procedures)
- If wiring and power/ground checks are good and bus activity normal but SDM does not respond, consider SDM replacement following manufacturer programming/initialization procedures
- After repairs, clear codes, re-initialize modules as required, and perform full system scan and road test to confirm no return of P1603
- Refer to OEM service information / TSBs for model-specific test procedures and special tools
Likely causes
- Loose/corroded SDM connector or pin (common)
- Blown/failed fuse supplying SDM
- Damaged CAN/serial data wiring between SDM and network (chafing, pinched, rodent damage)
- Failed SDM (less common than wiring/connector faults)
- Another module driving the bus into error/passive state (bus-off)
Fault status
Status
Lost communication with SDM (Sensing and Diagnostic Module) — serial data messages not received.
Repair difficulty: Medium
Diagnostic time: 1.0-3.0 hours
Similar codes
Repair manuals
Manual library for HUMMER
138
Browse 138 HUMMER manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.
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