B1390 — Oil Temperature Sensor Circuit Short To Ground

What is the most common cause of B1390?

The most common causes are: faulty switch, motor or actuator of the affected system, damaged wiring in door jambs or wet areas, blown fuse or failed relay.

Will B1390 clear itself?

Sometimes B1390 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.

How do I diagnose B1390 myself?

Start with a scan: confirm B1390, save the freeze frame and note any companion codes. Then inspect the wiring and connectors related to the body electrical system, and verify the suspect component against live data before buying parts.

What does OBD2 code B1390 mean?

B1390 — Oil Temperature Sensor Circuit Short To Ground. 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, damaged wiring in door jambs or wet areas, blown fuse or failed relay.

Is it safe to drive with B1390?

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.

When your vehicle's control unit detects that the oil temperature sensor circuit has shorted to ground, it logs trouble code B1390. This diagnostic code lives in the body electrical category, meaning it relates to modules and sensors outside the powertrain control system. The oil temperature sensor itself provides data to multiple systems—the engine computer uses it for fuel mapping and protection strategies, while body modules may use it for dashboard displays or ancillary functions. When that sensor's wiring develops a direct short to ground, the circuit cannot function correctly, and the monitoring system flags the condition.

The engine control module does not set B1390 after a single bad reading. Regulatory requirements and sound engineering practice demand that intermittent faults be confirmed before triggering a warning light. The module performs repeated checks under specific operating conditions, and only when the short-to-ground condition persists across multiple cycles does it store the code and illuminate the malfunction indicator lamp. This is why clearing the code without fixing the underlying problem results in the light returning within minutes or miles. The freeze frame data captured alongside B1390 records engine load, RPM, coolant temperature, vehicle speed, and other parameters at the exact moment the fault was detected. That snapshot is your roadmap—when you replicate those conditions during diagnosis, you dramatically increase your chances of catching the problem.

Understanding why the ECU cares about an oil temperature sensor short requires knowing what that circuit does. Oil temperature affects viscosity, which in turn affects lubrication efficiency and combustion characteristics. The powertrain control module adjusts injector pulse width, ignition timing, and idle speed based partly on oil temperature data. When the sensor circuit fails, the module loses that input and typically substitutes a default value, allowing the engine to continue running. This is why B1390 rarely causes a stall or no-start condition—it is a non-critical sensor failure from the engine management perspective, though it represents a genuine electrical fault that should be addressed.

Symptoms

• The check engine light or body system warning lamp illuminates on the instrument cluster
• Oil temperature gauge reads incorrectly—either stuck high, stuck low, or showing dashes
• Engine runs without drivability symptoms in most cases due to fallback strategies
• The affected circuit function operates intermittently or ceases entirely
• Other codes may accompany B1390, especially in the B0xxx range or related sensor circuits
• In some vehicles, transmission shift quality may feel slightly different without accurate oil temperature data
• Fuel economy may suffer marginally when the engine computer relies on estimated rather than actual oil temperature

Causes

• Blown fuse or failed relay supplying the oil temperature sensor circuit
• Corroded, damaged, or loose electrical connectors at the sensor or body control module
• Broken or chafed wiring, particularly in areas exposed to heat, moisture, or physical movement
• Faulty oil temperature sensor itself with internal short to ground
• Body control module software error or hardware failure affecting the sensor input
• Wiring harness damage in door jambs, engine compartments, or undercarriage areas where flex and moisture combine
• Aftermarket accessories incorrectly wired into the sensor circuit, causing shorts

How to Diagnose

1. Connect a compatible OBD2 scanner to the vehicle's data port, retrieve all stored codes, and record the freeze frame data associated with B1390. Note the conditions under which the code was set—you will need to recreate them later.
2. Perform a thorough visual inspection of the oil temperature sensor connector and wiring. Look for melted insulation, corroded pins, bent terminals, or evidence of oil intrusion. Disconnect and reconnect each connector while watching for intermittent behavior.
3. Check all related fuses and relays in the body electrical fuse box. A blown fuse is often the root cause and the cheapest fix. Replace any suspect fuses with the correct amperage rating.
4. Using a multimeter, test for resistance and continuity in the sensor circuit. With the sensor disconnected, check for an unintended ground path between the signal wire and chassis. A reading below manufacturer specifications confirms a short-to-ground condition.
5. Inspect the wiring harness at flex points—door hinges, firewall passages, and engine mounts are common failure locations. Wires break internally while the outer jacket appears intact.
6. Clear the code after completing your inspection, then drive the vehicle through the freeze frame conditions. If B1390 returns, the fault is confirmed and present. If it does not return, you may have fixed the issue during inspection or encountered a now-resolved intermittent connection.

Repair Options

Repairing the underlying cause of B1390 depends entirely on what the diagnostic process reveals:

• Wiring repair or connector replacement: $80–$250. Often the culprit involves damaged insulation or corroded terminals that can be soldered, taped, or connector-pinned for a reliable fix.
• Oil temperature sensor replacement: $120–$450. If the sensor itself has failed internally, replacement restores proper circuit function.
• Body control module repair or replacement with software update: $300–$900. BCM failure is less common but possible, especially in older vehicles with moisture intrusion history.

Total Cost

The total expense to resolve B1390 varies significantly based on which component failed and whether labor rates are factored in. Parts alone might cost as little as $15 for a fuse, while full BCM replacement with programming at a dealer can reach $900 or more. Independent shop labor typically runs $75–$125 per hour. An accurate diagnosis before purchasing parts is the single most effective way to control costs—replacing a perfectly good BCM because of a simple shorted wire is an expensive mistake.

Can You Drive with B1390?

In most cases, yes—B1390 is classified as a non-critical body electrical fault. The engine will continue running, and you will not experience stalling, hesitation, or performance loss under normal conditions. However, driving without accurate oil temperature data means you lose an important monitoring safeguard. Extended operation with severely degraded oil—visible on the gauge or inferred from long aggressive drives—can lead to engine wear. Additionally, if the short is severe enough, it could drain the battery over time or cause unexpected behavior in other body functions. Address B1390 promptly, but there is no need to call a tow truck if the vehicle drives normally.

Related OBD2 codes

• B1300: Power Door Lock Circuit Failure
• B1301: Power Door Lock Circuit Open
• B1302: Accessory Delay Relay Coil Circuit Failure
• B1303: Accessory Delay Relay Coil Circuit Open
• B1304: Accessory Delay Relay Coil Circuit Short To Battery
• B1305: Accessory Delay Relay Coil Circuit Short To Ground