B1390

What Does Code B1390 Mean?

DTC B1390 signifies an “Oil Temperature Sensor Circuit Short To Ground.” This code is set when the Powertrain Control Module (PCM), also referred to as the Engine Control Module (ECM), detects an abnormally low voltage signal from the Engine Oil Temperature (EOT) sensor circuit. The EOT sensor is typically a Negative Temperature Coefficient (NTC) thermistor, meaning its electrical resistance decreases as the oil temperature increases, and vice versa. The PCM supplies a reference voltage (usually 5V) to the sensor through a resistor, and then monitors the voltage drop across the sensor. When the sensor’s signal wire experiences an unintentional contact with a ground source (e.g., chassis, engine block, or another grounded wire), the voltage returning to the PCM drops significantly, often to near 0 volts. This low voltage implies an extremely high oil temperature (or an open circuit/faulty sensor depending on the specific system’s logic), an implausible reading that triggers the B1390 code. The PCM uses oil temperature data for critical functions such as variable valve timing (VVT) adjustments, fuel trim calculations, engine cooling fan operation, and overall engine protection strategies. An inaccurate or missing oil temperature signal can force the PCM to use a default or calculated value, potentially leading to suboptimal engine performance and efficiency.

Common Symptoms

• Check Engine Light (CEL) Illumination: The most common and immediate symptom.
• Inaccurate or Erratic Oil Temperature Gauge Reading: If equipped, the dashboard gauge may display an extremely high, extremely low, or fluctuating temperature, or default to a fixed value.
• Engine Performance Issues: The engine may exhibit symptoms such as reduced power, rough idling, or decreased fuel efficiency due to the PCM using incorrect data for its calculations.
• Incorrect Engine Cooling Fan Operation: The cooling fans may run continuously at high speed, or conversely, not engage when required, leading to potential overheating.
• Variable Valve Timing (VVT) Malfunctions: If the VVT system relies on accurate oil temperature, its performance may be compromised.
• Failed Emissions Test: An illuminated CEL typically prevents a vehicle from passing an emissions inspection.

What Causes the Code B1390?

• Damaged Wiring Harness: The most frequent cause involves frayed, chafed, pinched, or melted insulation on the EOT sensor signal wire, leading to direct contact with a grounded surface (e.g., engine block, chassis, metal brackets, or exhaust components).
• Faulty Engine Oil Temperature Sensor (EOT): An internal short circuit within the EOT sensor itself, causing the signal output to be pulled to ground. While less common for a “short to ground” compared to an open circuit, it is a possibility.
• Corrosion or Contamination in Connectors: Water intrusion, oil contamination, or general corrosion within the EOT sensor electrical connector can create a conductive bridge between the signal terminal and a ground terminal or the connector housing, resulting in a short.
• Internal Powertrain Control Module (PCM) Fault: Extremely rare, but an internal short or component failure within the PCM’s input circuit for the EOT sensor could theoretically cause this code. This should be considered a last resort after extensive external circuit diagnosis.

How to Diagnose and Troubleshoot

Diagnosing B1390 requires careful inspection and precise electrical testing:

1. Visual Inspection:
   • Begin by thoroughly inspecting the EOT sensor, its electrical connector, and the entire wiring harness leading from the sensor back towards the PCM. Look for obvious signs of damage such as chafing, cuts, melting, or pinching on the wires. Pay close attention to areas where the harness passes near hot engine components, sharp edges, or moving parts.
   • Check the EOT sensor connector for proper seating, corrosion, bent pins, or signs of oil/water contamination.
2. OBD-II Scan Tool Data Analysis:
   • Connect an OBD-II scan tool and check for any other related diagnostic trouble codes (DTCs) that might offer additional clues.
   • Access the live data stream and monitor the “Engine Oil Temperature” (EOT) or “Oil Temperature Sensor” parameter. If a short to ground is present, the reading will typically be extremely high (e.g., maximum possible reading for the sensor, sometimes exceeding 300°F or 150°C) or, depending on the PCM’s logic, an implausibly low fixed value near ambient.
3. Digital Multimeter (DMM) Testing (with Battery Disconnected):
   • Disconnect Battery: Always disconnect the negative battery terminal before performing resistance tests to prevent accidental shorts or damage to electrical components.
   • Isolate the Sensor: Disconnect the electrical connector from the EOT sensor.
   • Wiring Harness Short to Ground Test: Refer to the vehicle’s specific wiring diagram to identify the signal wire for the EOT sensor within the harness connector. Set your DMM to measure resistance (ohms). Place one probe on the signal wire terminal inside the harness connector and the other probe on a known good chassis ground (e.g., clean metal on the engine block). A reading of very low resistance (typically less than 5 ohms) indicates a short to ground in the wiring harness. While observing the DMM, carefully wiggle, bend, and manipulate the wiring harness along its entire path to identify any intermittent shorts.
   • EOT Sensor Resistance Test: With the sensor disconnected, measure the resistance across the two terminals of the EOT sensor itself (or signal and ground, as per specific sensor design). Compare this reading to the manufacturer’s specified temperature-resistance chart for the current ambient temperature. An internal short within the sensor might show near-zero resistance or an abnormally low value compared to specifications.
   • Continuity Test (Signal Wire to PCM): To definitively rule out a harness issue, disconnect the PCM connector (refer to service manual for location and pinout). Perform a continuity test from the EOT signal wire terminal at the sensor harness connector to the corresponding EOT signal input pin at the PCM connector. This verifies the wire’s integrity (no open circuit). Then, perform a short-to-ground test again, from the signal wire at the sensor harness connector to chassis ground, ensuring no low resistance reading is present.
4. Reference Voltage Check (with Ignition ON, Engine OFF):
   • Reconnect the negative battery terminal. With the EOT sensor connector still disconnected, turn the ignition to the “ON” position (engine off).
   • Using the DMM set to DC volts, measure the voltage between the PCM’s reference voltage supply wire (usually 5V, identify via wiring diagram) at the EOT connector and a known good chassis ground.
   • Measure the voltage between the ground wire terminal at the EOT connector and chassis ground (should be approximately 0V).
   • Measure the voltage on the signal wire terminal (harness side, EOT sensor disconnected). If this wire is pulled to ground (near 0V) despite the sensor being disconnected and the reference voltage present, it strongly confirms a wiring short to ground between the connector and the PCM.

Recommended Repairs and Solutions

Once the root cause is identified through the diagnostic process, the following repairs are typically recommended:

• Repair or Replace Damaged Wiring Harness: If the wiring harness is found to be shorted to ground, the damaged section must be repaired or replaced. For localized damage, carefully cut out the compromised section, solder in new wire of the correct gauge, and seal the splices with heat-shrink tubing. Ensure the repaired section is properly routed and secured to prevent future chafing or contact with hot/moving parts. For extensive or inaccessible damage, replacement of the entire affected section of the engine wiring harness may be necessary.
• Replace Faulty Engine Oil Temperature Sensor (EOT): If testing confirms the EOT sensor itself is internally shorted, it must be replaced. Always use an OEM or high-quality aftermarket sensor specific to your vehicle’s make, model, and engine. Ensure the sealing O-ring or washer is correctly installed to prevent oil leaks.
• Clean or Replace Corroded Connectors: If corrosion or contamination is present in the EOT sensor connector, attempt to clean the terminals thoroughly using specialized electrical contact cleaner and a small brush. If terminals are severely corroded, bent, or damaged, the connector shell and/or individual terminals should be replaced. Apply dielectric grease to the clean terminals upon reassembly to prevent future corrosion and ensure a good electrical connection.
• Clear DTCs and Verify Repair: After any repair, use an OBD-II scanner to clear the B1390 code and any other related DTCs. Perform a test drive through multiple drive cycles under varying conditions to confirm that the code does not return and that all emissions readiness monitors set. Monitor the live data stream for the engine oil temperature to ensure it displays plausible and stable readings.
• PCM Replacement (Last Resort): Only if all wiring, connectors, and the EOT sensor have been thoroughly tested and confirmed to be in good working order, and the short to ground persists within the PCM’s input circuit, should a PCM replacement be considered. This is a very rare and costly repair, often requiring reprogramming or flashing of the new PCM.