B1491

Code

Other B — Body

Ignition Cylinder Sensor Circuit Failure

Brand: Other

Type: B — Body

Views: UK: 22 EN: 39 RU: 51

AI status

Completed

ready

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Page language: EN

Causes

Open or short in ignition cylinder sensor wiring or connector
Corroded or loose connector pins at the sensor or module
Failed ignition cylinder position sensor (Hall effect, reed, or switch type)
Blown fuse or poor ground for the sensor circuit
Faulty body control module / immobilizer module or software error
Aftermarket immobilizer, alarm or starter-interrupt device interfering with circuit

Symptoms

Key position not recognized (No Start or intermittent starting issues)
Key cylinder feel may be abnormal or loose
Remote start/immobilizer or keyless-entry functions may not work
Warning lamp or message related to key/immobilizer or start system
Intermittent loss of accessory or start power when key is turned

What to check

Read stored freeze-frame and pending codes and note when the fault occurred
Visual inspection of ignition cylinder sensor connector and wiring for damage, corrosion, or pin push-out
Verify fuses and grounds related to body/immobilizer and ignition circuits
Check for aftermarket devices (alarms, immobilizers) that may tap the ignition circuit
Use a scan tool to view live data: key position or sensor status while turning the key

Signal parameters

Sensor type varies by vehicle: may be a digital (on/off) switch, Hall-effect (0–5 V) or low-voltage signal to module
Typical reference voltage: 5 V supply to sensor (check OEM wiring diagram for exact value)
Expected signal: changes between ~0 V and ~5 V or a discrete on/off state corresponding to key positions (OFF/ACC/ON/START)
Continuity: wiring to sensor should not be open; expected continuity to ground where applicable
Current draw: sensor circuits are low current (tens of mA); shorted condition may blow related fuse

Diagnostic algorithm

Retrieve and record all related codes and freeze-frame data with a capable scan tool. Confirm B1491 is current or stored.
Inspect ignition cylinder area, connector and harness for physical damage, corrosion, or rodent chewing. Repair any visible damage.
Check relevant fuses and ground points. Repair poor grounds or replace blown fuses and retest.
Backprobe the sensor connector with the key in different positions and observe live data (key-position parameter) or measure voltage across the signal pin and ground. Expect defined changes when rotating key; consult OEM wiring diagram for pinout and expected voltages.
If no signal or abnormal signal, perform continuity check between the sensor connector and the control module connector. Repair open or shorted wiring as needed.
Wiggle-testing: with the system energized and a scan tool monitoring the signal, gently move the harness and cylinder to attempt to reproduce the fault (intermittent connector/contact issue).
If wiring and connectors are good but the signal is incorrect, replace the ignition cylinder sensor. Follow vehicle-specific procedures for removing/replacing the sensor and any immobilizer pairing steps.
After repair, clear codes and perform functional test: verify correct key position reporting, starting, and immobilizer behavior. Re-scan to confirm B1491 does not return.
If code persists after sensor replacement and wiring confirmed good, consider module communication faults or module replacement — consult OEM diagnostic procedures for module bench tests or reprogramming.

Likely causes

Damaged wiring or poor connector pin contact at the ignition cylinder sensor
Failed ignition cylinder sensor
Poor ground or blown fuse feeding the sensor circuit

Fault status

⚠️ Status

Ignition cylinder sensor circuit fault detected. The body/immobilizer module reports an abnormal or missing signal from the ignition cylinder sensor; this can prevent correct key position detection or immobilizer authorization.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2.0 hours