B1791 — Autolamp Sensor Input Circuit Open

B1791 — Autolamp Sensor Input Circuit Open. A fault in the body electrical system. The car is usually drivable while you plan the repair. Typical causes: blown fuse or failed relay, damaged wiring in door jambs or wet areas, faulty switch, motor or actuator of the affected system.

body electrical

Is it safe to drive?

Short-term driving is OK Usually safe to drive — body codes rarely affect drivability. Exceptions: exterior lighting and wipers in bad weather, and central locking faults that can leave the car unsecured.

How to diagnose with an OBD scanner

Read the code and freeze frame Connect an OBD2 scanner, confirm B1791 is stored, and save the freeze frame data — it shows engine conditions when the fault was detected.
Check for companion codes and TSBs Related codes often reveal the real root cause. Also check manufacturer technical service bulletins (TSB) for your make and model — many B0xxx issues have factory fixes or software updates. Search by VIN on the manufacturer site or nhtsa.gov.
Inspect wiring and connectors Visually inspect the wiring harness and connectors related to the body electrical system for damage, corrosion, chafing or poor contact.
Subsystem check Do a continuity test end-to-end on the autolamp sensor input circuit and wiggle the harness while measuring — intermittent opens show up as fluctuating readings.
Subsystem check Operate the affected function while watching its switch and motor PIDs in live data.
Subsystem check Inspect connectors and wiring at flex points (door jambs, tailgate hinges) — wires break there first.
Verify the repair Clear the code, complete a full drive cycle including the conditions from the freeze frame, and confirm B1791 does not return.

Approximate repair cost

Repair cost depends on which component actually failed. Typical scenarios for B1791 (parts and labor, USD, independent shop rates): Repair wiring or connectors: $80–$250; Replace the switch or actuator: $120–$450; BCM repair or replacement: $300–$900. Overall, fixing B1791 usually costs between $80 and $900. An accurate diagnosis before buying parts saves money — the most expensive scenario is rarely the first one to check.

Common questions

Will B1791 clear itself?

Sometimes B1791 clears after a few drive cycles if the fault was intermittent — for example a loose connector or a tank of bad fuel. The underlying cause usually remains, so the code tends to return. Clear it once; if it comes back, diagnose rather than erase it repeatedly.

Is it safe to drive with B1791?

Usually safe to drive — body codes rarely affect drivability. Exceptions: exterior lighting and wipers in bad weather, and central locking faults that can leave the car unsecured.

What are the symptoms of B1791?

Drivers most often notice: no impact on engine operation in most cases, warning light on the dashboard, the affected function (locks, windows, lighting, wipers) works intermittently or not at all.

What does OBD2 code B1791 mean?

How much does it cost to fix B1791?

Repair wiring or connectors — $80–$250; Replace the switch or actuator — $120–$450; BCM repair or replacement — $300–$900. Exact cost depends on the root cause, your vehicle and local labor rates — a proper diagnosis is the cheapest first step.

When your OBD2 scanner pulls code B1791, the vehicle's control unit has detected a problem within the body electrical system's autolamp circuit. Specifically, the module registers an "open" condition, meaning the electrical pathway that carries the autolamp sensor's signal has been interrupted somewhere between the sensor and the control module. Without a complete circuit, the module cannot read the sensor's input, and it logs this failure.

The autolamp system uses a light sensor—typically mounted on the dashboard or near the windshield—to automatically control exterior lighting based on ambient light conditions. When ambient light drops below a set threshold, the module expects a corresponding voltage signal from the autolamp sensor. If that signal never arrives because the circuit is broken, the module logs B1791 and illuminates the check engine or body system warning light. This code falls under the B-series (body) category, which handles comfort, convenience, and lighting functions rather than engine or transmission controls.

Setting B1791 requires the control module to compare the autolamp sensor's actual input against its expected range, usually across multiple drive cycles to confirm a genuine fault rather than a temporary glitch. The module also captures a freeze frame—a snapshot of engine RPM, load, coolant temperature, and vehicle speed recorded the instant the fault was detected. That data becomes invaluable during diagnosis because it tells you exactly what conditions triggered the code.

Common Symptoms of B1791

Illuminated body system or check engine warning light
Autolamp function fails to operate or behaves erratically
Exterior headlights may not turn on automatically in low light
Manual headlight operation typically remains functional
Affected function (lighting, locks, windows, wipers depending on integration) works intermittently or stops completely
No impact on engine performance, transmission operation, or fuel economy in most cases

What Causes B1791?

The underlying cause is almost always a break in the electrical pathway feeding the autolamp sensor input circuit:

Corroded or moisture-damaged electrical connectors at the sensor or module
A connector that has backed out or lost terminal tension, creating a poor connection
Broken or chafed wires, especially in areas prone to flexing like door jambs and hatch hinges
A blown fuse that supplies power to the autolamp circuit
The autolamp sensor itself has failed internally, presenting an open circuit
Wiring insulation deteriorated by heat, age, or chemical exposure in engine bays or wheel wells
In rare cases, the body control module has developed an internal fault

How to Diagnose B1791

Connect an OBD2 scanner and verify B1791 is present. Record the freeze frame data before clearing anything—this snapshot reveals engine conditions when the failure occurred.
Pull all stored codes from the body control module. Related codes often point to the root cause or a second failing component in the same circuit.
Visually inspect the autolamp sensor connector and wiring for damage, bent pins, corrosion buildup, or evidence of moisture intrusion. Pay special attention to the sensor itself and its ground connection.
Perform a continuity test on the entire autolamp sensor input circuit from the sensor harness to the body control module. Gently wiggle the wiring during testing—intermittent opens only reveal themselves under movement.
Test the autolamp sensor independently if possible, or monitor its voltage signal in live data while the circuit is powered. Compare readings to known-good specifications for your vehicle.
Inspect wiring at flex points such as door hinges, tailgate hinges, and any area where wires travel through moving joints. These locations suffer the most strain over time.
Clear the code, complete a drive cycle that recreates the conditions from the freeze frame, and verify B1791 does not reappear.

Repair Options and Costs

B1791 repair costs vary widely depending on which component failed:

Wiring repair or connector replacement: $80–$250
Autolamp sensor replacement: $120–$450
Body control module repair or replacement: $300–$900

Always address the cheapest and most common failures first—wiring issues and poor connections account for the majority of B-series code cases. Spending time on thorough diagnosis prevents replacing expensive parts unnecessarily.

Driving Safety Verdict

B1791 carries a short severity rating. This code affects comfort and convenience features rather than critical engine or safety systems, so most drivers can continue operating the vehicle without immediate concern. The headlight automation may fail, which becomes a safety issue only in low-visibility conditions like darkness or heavy rain. Central locking malfunctions linked to the same module could leave the vehicle unsecured. Address B1791 at your earliest convenience, but there is no urgent need to stop driving or call for a tow.