Code
B1846
Other
B — Body
Ignition Tamper Circuit Open
AI status
Completed
Completed
100%
Causes
- Broken or disconnected wire in the ignition tamper/immobilizer circuit
- Corroded, bent or poorly seated connector(s)
- Blown fuse supplying the tamper/immobilizer circuit
- Faulty ignition switch, tamper/immobilizer sensor or antenna ring
- Aftermarket alarm/immobilizer incorrectly wired or interfering
- Body or security control module internal fault
Symptoms
- Security/immobilizer warning lamp illuminated or flashing
- Engine will not crank or start intermittently
- No response from ignition in START/ON positions or keyless start fails
- Erratic operation of security system or false tamper alerts
- Possible loss of communication with related control modules
What to check
- Use a scan tool to read freeze-frame data and confirm B1846 and any related codes
- Visually inspect wiring harnesses and connectors for damage, corrosion, or loose pins at ignition switch, antenna, and security module
- Check related fuses and power/ground circuits for the security/immobilizer system
- Confirm whether any aftermarket alarms or immobilizers are installed and inspect their wiring
- Measure continuity and voltage at suspected circuit points with a digital multimeter
Signal parameters
- Ignition tamper/immobilizer input: should switch between ~0 V (ground) and battery voltage (~12 V) depending on key/antenna state
- Continuity: expected near 0 ohms through harness sections when connectors mated; open/infinite ohms indicates break
- Resistance to ground for shield/ground conductors: low (single-digit ohms)
- No-load voltage at module input: battery voltage with ignition ON (unless designed as pulsed signal)
Diagnostic algorithm
- Confirm the code with a scan tool and record any related DTCs and freeze-frame data.
- Inspect visually: follow the wiring from the ignition switch/key antenna to the security/body control module. Look for damaged insulation, chafing, or disconnected connectors.
- Check fuses and relays related to the security/immobilizer circuit; replace any blown fuses and retest.
- With key/ignition in required positions, backprobing connectors measure voltage at the tamper input of the control module. Compare to expected battery voltage or switching behavior.
- Perform continuity checks between the ignition/antenna connector and the module input with ignition off. Repair any opens or high-resistance joints.
- If wiring and connectors are good, inspect/replace the ignition switch, key/antenna coil, or tamper sensor as per manufacturer procedure.
- If component replacement does not clear the fault, verify module power/ground and communication; consider module diagnostics or replacement following manufacturer guidance.
- After repairs, clear codes and perform multiple start cycles and road test to confirm the code does not return.
Likely causes
- Open (cut) harness or broken conductor between ignition switch/antenna and control module
- Corroded terminal or water ingress at connectors causing open
- Removed or disabled immobilizer antenna or module (aftermarket work)
- Faulty tamper/immobilizer switch or sensor not returning expected signal
- Failed body control/security module input stage
Fault status
Status
Ignition Tamper Circuit Open — open circuit detected in the vehicle's ignition tamper/immobilizer wiring or input.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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