Code
P1631
GM
P — Powertrain
Theft Deterrent Start Enable Signal Not Correct
AI status
Completed
Completed
100%
Causes
- Faulty or unrecognized transponder key or key reader (antenna coil)
- Open/shorted wiring or poor connector between theft module/BCM and PCM
- Faulty Body Control Module (BCM) or Theft Deterrent Module
- Blown fuse or low battery voltage affecting theft or PCM circuits
- PCM not receiving or misinterpreting Class 2/CAN data message
- Aftermarket alarm/remote starter or bypass device interfering
Symptoms
- No-crank or no-start condition when using key
- Engine cranks but will not run or stalls shortly after start
- Security/antitheft indicator lamp illumination or flashing
- Intermittent starting problems — may start sometimes
- Other modules reporting communication or theft-related codes
What to check
- Read all stored DTCs and freeze frame data with a capable scan tool and record theft-related flags
- Observe security/antitheft lamp behavior during key-on and cranking
- Verify battery voltage is within normal range (12.4–12.8V at rest) and charging system healthy
- Check fuses and power/ground at BCM, theft module and PCM
- Inspect ignition key and try known-good/programmed spare key
- Visually inspect wiring/connectors for damage, corrosion, or loose pins at BCM, PCM and steering column
Signal parameters
- Start-enable status is typically provided as a logic/serial flag to the PCM — expected state: 'enabled' during crank if key recognized
- When monitored with a scan tool, theft/immobilizer status parameter should read 'disarmed' or 'start enable = ON' for starting
- Voltage checks on dedicated start-enable lines (if present) should show logic-level change during key-on/crank (commonly 0–5V)
- No unexpected loss of Class 2/CAN messages between BCM and PCM during key-on/crank
Diagnostic algorithm
- Connect a factory-level scan tool and read all DTCs and live data related to theft, BCM, and PCM. Note freeze frame.
- Confirm battery state of charge and inspect/replace any low/weak batteries or suspect connections.
- Observe the security lamp: key-on should follow specified lamp sequence (lamp out after key recognized). Record behavior.
- Try a known-good programmed key. If vehicle starts, suspect key/transponder or learn data.
- Inspect wiring and connectors at the theft antenna coil (steering column), BCM and PCM. Repair any damage, poor contacts or corrosion.
- Check fuses and power/ground circuits for theft and PCM modules. Repair poor grounds and supply faults.
- Using the scan tool, monitor the start-enable/immobilizer status and serial data messages during key-on/crank. Look for missing or incorrect messages.
- Remove or disable aftermarket alarm/bypass devices and retry; these devices commonly disrupt the start-enable signal.
- If wiring and keys are good and communication faults persist, consider BCM/module fault. Verify software levels and reprogram or replace modules only after confirming diagnostics.
- Clear codes and retest operation after repairs. If intermittent, road-test and monitor for reappearance.
Likely causes
- Damaged/loose connector at BCM or immobilizer antenna coil
- Poor ground or low battery causing intermittent module operation
- Key transponder failure or using non-programmed/spare key
- Corroded wiring in steering column (ignition switch area)
- Module communication fault on serial data bus (Class 2/CAN)
Fault status
Status
Theft Deterrent Start Enable Signal Not Correct — PCM/BCM did not receive a valid start-enable from the immobilizer system.
Repair difficulty: Medium
Diagnostic time: 1.0-3.0 hours
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