B1790 — Autolamp Sensor Input Circuit Failure

B1790 — Autolamp Sensor Input Circuit Failure. 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, corroded or loose connectors.

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 B1790 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 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 B1790 does not return.

Approximate repair cost

Repair cost depends on which component actually failed. Typical scenarios for B1790 (parts and labor, USD, independent shop rates): BCM repair or replacement: $300–$900; Repair wiring or connectors: $80–$250; Replace the switch or actuator: $120–$450. Overall, fixing B1790 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

Is it safe to drive with B1790?

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.

Will B1790 clear itself?

Sometimes B1790 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.

What does OBD2 code B1790 mean?

What is the most common cause of B1790?

The most common causes are: blown fuse or failed relay, damaged wiring in door jambs or wet areas, corroded or loose connectors.

How much does it cost to fix B1790?

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

When you pull code B1790 with a scan tool, the powertrain or body control module has detected that a sensor in the automatic lighting circuit is sending signals outside its expected range. Officially this is called "Autolamp Sensor Input Circuit Failure," and it falls under the broad category of body electrical faults rather than engine or drivetrain problems. The module constantly monitors voltage and resistance at the autolamp sensor, and when the signal drifts too far from factory specifications, it logs this code.

The autolamp system uses a photoelectric sensor—usually mounted near the top of the instrument panel or behind the windshield—to detect ambient light levels. When it gets dark enough, the module turns on the headlights automatically without driver input. If the sensor circuit opens, shorts, or experiences excessive resistance, the module can no longer trust the signal and triggers B1790. Most manufacturers require the fault to appear during multiple drive cycles before lighting the check engine or body control light, which is why the code sometimes seems to come and go.

Common Symptoms of B1790

Dashboard warning or body control light may illuminate
Automatic headlights fail to turn on or off at the correct light levels
Headlights may stay stuck in one position (always on or always off)
Manual headlight operation still functions in most cases
No effect on engine performance, transmission, or fuel economy
The defect may be intermittent, appearing only under certain temperature or humidity conditions

Most Likely Causes

The fault almost always originates in one of these areas, listed from most to least common:

Corrosion or moisture intrusion at the autolamp sensor connector
Broken or chafed wires in the door jamb hinge area where wiring passes through
Loose electrical connections at the headlight switch assembly or body control module
Failed autolamp sensor itself (photocell degradation over time)
Body control module software glitch or hardware failure
Short or open circuit in the wiring harness due to age or physical damage

How to Diagnose B1790

Follow this step-by-step approach to pinpoint the root cause before spending money on parts:

Connect an OBD2 scanner and confirm B1790 is present in the body control module. Record any related codes and capture the freeze frame data—this snapshot shows engine speed, load, coolant temperature, and vehicle speed at the exact moment the fault was recorded.
Research whether your specific make and model has any factory bulletins addressing B1790. Many manufacturers have released software updates or harness modifications that resolve recurring autolamp failures.
Perform a thorough visual inspection of the autolamp sensor and its wiring. Look for cracked insulation, greenish corrosion deposits, and wires pulled tight at the connector. A damaged harness in the driver's door hinge is a common failure point on many vehicles.
Test the sensor circuit with a multimeter while monitoring live data on your scan tool. Check for proper reference voltage at the sensor and verify that ground connections are solid. Many intermittent failures show up only when the wiring is flexed.
Disconnect the affected connectors and inspect the pins for corrosion or spread terminals. Clean contacts with electrical contact cleaner and apply dielectric grease before reassembling.
Clear the code and run a complete drive cycle that includes the conditions from your freeze frame data—typically night driving or entering a dark garage. If B1790 returns, you have narrowed the problem to a hard failure rather than a loose connection.

Repair Options and Parts Costs

Depending on what your diagnosis reveals, here are the typical repairs and their price ranges at independent shops:

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

Total Cost to Fix B1790

The wide cost range reflects how much investigation is needed before turning a wrench. In most cases, the problem is a simple connector or wiring issue that costs under $150 to fix. BCM replacement is the most expensive scenario and should only be considered after you have verified the module itself is faulty and ruled out all wiring problems. Spending $150–$300 on a proper diagnosis before buying a $600 control module almost always saves money in the long run.

Can You Drive with B1790?

B1790 carries a short-term safety rating because it does not affect engine operation or braking. However, relying on manual headlight control when the automatic function is broken creates a visibility risk, especially at night or in poor weather. If the autolamp system also controls other exterior lights, you may be driving with reduced visibility to other motorists. Address this code within a week or two to maintain proper lighting function and avoid a potential traffic violation in jurisdictions that require functioning automatic lights.