B1804 — Lamp Turn Signal Rear Output Circuit Short To Battery

B1804 — Lamp Turn Signal Rear Output Circuit Short To Battery. A fault in the body electrical system. The car is usually drivable while you plan the repair. Typical causes: faulty switch, motor or actuator of the affected system, corroded or loose connectors, body control module (BCM) fault.

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 B1804 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 B1804 does not return.

Approximate repair cost

Repair cost depends on which component actually failed. Typical scenarios for B1804 (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 B1804 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

What is the most common cause of B1804?

The most common causes are: faulty switch, motor or actuator of the affected system, corroded or loose connectors, body control module (BCM) fault.

Is it safe to drive with B1804?

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 B1804 clear itself?

Sometimes B1804 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 are the symptoms of B1804?

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

What does OBD2 code B1804 mean?

When your vehicle's computer detects an abnormal voltage condition in the rear turn signal circuit, it logs trouble code B1804. This specific fault points to a short to battery — meaning the wiring is feeding constant power where it shouldn't be. The rear turn signal lamp output circuit has a defined voltage range it operates within. When that range is violated, the powertrain control module (or body control module, depending on your vehicle) flags the issue and illuminates the check engine light.

The rear turn signal circuit falls under the body electrical category, not the powertrain. That means B1804 won't directly affect how your engine runs, idles, or accelerates. The module still monitors this circuit constantly while you drive. Once readings stray outside the acceptable window for a set number of drive cycles, the computer stores B1804 and records freeze frame data — a snapshot of engine speed, vehicle speed, coolant temperature, and load at the exact moment the fault triggered. That snapshot is your best starting point when you hook up a scanner.

Common Symptoms of B1804

The malfunction indicator lamp (MIL) illuminates on the dashboard
The affected function (rear turn signals) operates intermittently or stops working entirely
In most cases, engine performance and drivability remain unaffected
The turn signal indicator on the switch stalk may also behave abnormally
No impact on other powertrain systems under normal circumstances

What Causes B1804?

A blown fuse or failed relay feeding the rear turn signal circuit — the most frequent culprit
Corroded, damaged, or loose electrical connectors in the affected wiring harness
A faulty body control module (BCM) unable to regulate the circuit properly
Chafed or broken wiring creating an unintended path to battery voltage
A failed turn signal switch or actuator downstream in the circuit

How to Diagnose B1804

Connect an OBD2 scanner, retrieve code B1804, and save the freeze frame data. Note the engine conditions — rpm, speed, load, temperature — recorded when the fault appeared. This tells you exactly what the vehicle was doing when it failed.
Check for any related diagnostic codes stored alongside B1804. Secondary codes often point to the underlying cause rather than the symptom. Also look up technical service bulletins (TSBs) for your make and model — manufacturers sometimes publish known fixes or software updates for recurring body electrical faults.
Perform a hands-on inspection of the rear turn signal circuit wiring and connectors. Look for damaged insulation, corrosion on the terminals, frayed wires, or connectors that aren't fully seated. Pay special attention to flex points — wires at door jambs and trunk hinges break first from repeated motion.
Activate the turn signal and monitor the circuit using live data on your scanner. Watch switch state and motor/actuator PIDs while operating the stalk. Intermittent operation often shows up here before it becomes a permanent failure.
Clear the code, then drive the vehicle through a complete drive cycle that replicates the freeze frame conditions. If B1804 returns, the fault is confirmed and you'll need to isolate the affected component.

Repair Options and Costs

Fix the cheapest possibilities before replacing expensive modules. Here is what each repair typically costs at an independent shop (parts and labor, USD):

Body control module (BCM) repair or replacement: $300–$900. BCMs rarely fail outright — corrosion inside the unit or a poor connector often mimics a module fault. Have the existing module tested before buying a new one.
Wiring harness repair or connector replacement: $80–$250. Most shorts to battery develop from broken insulation or corroded terminals. Proper repair of the affected section costs far less than a module swap.
Turn signal switch or actuator replacement: $120–$450. The stalk assembly or the actual lamp driver inside the BCM can fail. Diagnosis determines which one.

Overall, expect to spend between $80 and $900 to resolve B1804. An accurate diagnosis before purchasing parts is the single best way to keep costs down — the most expensive scenario is rarely the first one to investigate.

Can You Drive with B1804?

Generally yes — body electrical faults like B1804 typically do not impair engine operation or transmission function. Your vehicle will start, drive, and stop normally. The practical concern is safety: non-operational rear turn signals reduce visibility to other drivers, particularly in rain, fog, or darkness. If the fault also affects your hazard flashers or the central locking system, those represent additional safety considerations. Get it diagnosed and repaired at your earliest convenience, but there is no urgent risk of breakdown or drivability loss.