B1308

What Does Code B1308 Mean?

Diagnostic Trouble Code (DTC) B1308 indicates that the Engine Control Module (ECM), also commonly referred to as the Powertrain Control Module (PCM), has detected a short circuit to ground within the engine oil level switch circuit. The oil level switch is a crucial component of the engine management system, designed to monitor the quantity of lubricant within the oil sump to prevent engine damage due due to insufficient oil. This sensor typically operates on a simple principle, often utilizing a reed switch or thermistor type mechanism that changes its electrical resistance or closes/opens a circuit based on the oil level. The ECM supplies a reference voltage to this circuit and monitors the returning signal voltage. When a B1308 code is set, it means the ECM has observed the signal voltage dropping below a predetermined threshold, or exhibiting a continuous low-resistance path to chassis ground, indicating an unintended electrical connection to the vehicle’s ground plane. This condition prevents the ECM from accurately reading the actual oil level and often triggers the Malfunction Indicator Lamp (MIL) or a dedicated oil level warning on the instrument cluster. The subsystem affected is the engine lubrication monitoring system, which plays a vital role in protecting critical internal engine components.

Common Symptoms

• Malfunction Indicator Lamp (MIL) illumination: The “Check Engine” light will be activated on the dashboard.
• Oil Level Warning Light/Message: An oil can icon or a “Low Oil Level” message may appear on the instrument cluster, potentially even if the actual oil level is adequate.
• Incorrect Oil Level Gauge Reading: On vehicles equipped with an oil level gauge, it might display an errant reading or no reading at all.
• No immediate drivability concerns: As the code pertains to a sensor circuit rather than a direct engine performance issue, there are typically no noticeable changes in engine operation or performance, aside from the warning indicators.

What Causes the Code B1308?

• Damaged Wiring Harness: Frayed, pinched, or chafed wires in the oil level switch circuit, leading to a direct short circuit to the vehicle’s chassis ground or another ground source. This is a very common cause due to the harsh engine bay environment.
• Faulty Oil Level Switch/Sensor: Internal short circuit within the oil level sensor itself, where the internal components or wiring have failed, causing the signal line to perpetually short to the sensor’s body or an internal ground.
• Corroded Electrical Connectors: Water ingress, oil contamination, or general environmental exposure can lead to corrosion within the oil level switch connector or the wiring harness connectors leading to the ECM, creating an unintended low-resistance path to ground.
• Internal ECM/PCM Fault: Although less common, an internal short circuit within the ECM’s input monitoring circuit for the oil level switch can cause this code. This is usually considered only after all other external possibilities have been thoroughly ruled out.

How to Diagnose and Troubleshoot

Diagnosis of DTC B1308 requires a systematic approach, utilizing a digital multimeter (DMM) and an OBD-II scan tool capable of reading live data.

1. Verify Actual Oil Level: Before any electrical diagnosis, visually inspect the engine oil level using the dipstick. While the code indicates a circuit fault, it is always prudent to ensure the engine has an adequate oil supply.
2. Visual Inspection:
   • Locate the engine oil level switch, typically mounted on the oil pan.
   • Inspect the wiring harness leading to the switch for any visible signs of damage, such as chafing, cuts, pinches, or rodent damage. Pay close attention to areas where the harness passes near sharp edges or high-heat components.
   • Examine the electrical connector at the oil level switch for corrosion, bent pins, or signs of water or oil ingress. Disconnect and inspect both male and female terminals.
3. Scan Tool Live Data Analysis:
   • Connect an OBD-II scan tool and access live data parameters.
   • Look for the engine oil level sensor/switch input. A short to ground will likely cause the ECM to report a constant ‘low’ oil level signal or an implausibly low voltage (e.g., 0V or very near 0V) regardless of the actual oil level.
   • Clear the B1308 code. If the code immediately returns, the short is likely continuous and present.
4. Digital Multimeter (DMM) Testing:
   • Isolate the Circuit: Disconnect the electrical connector from the oil level switch. This separates the sensor from the wiring harness and the ECM.
   • Test Harness for Short to Ground: With the ignition OFF and the oil level switch disconnected, use your DMM set to ohms (Ω) to test for continuity between the signal wire terminal at the harness connector (the side going towards the ECM) and a known good chassis ground point. There should be infinite resistance (open circuit). If you read low resistance (e.g., less than 50Ω), it indicates a short to ground in the wiring harness between the connector and the ECM. Systematically trace the harness to locate the damage.
   • Test Sensor for Internal Short: With the oil level switch disconnected from the harness, measure the resistance between the signal terminal(s) of the sensor itself and its metal housing or ground terminal (if applicable). Consult service information for expected resistance values for your specific sensor type. An internal short would typically show very low resistance where high resistance or an open circuit is expected under normal conditions.
   • Verify Reference Voltage: With the ignition ON and the oil level switch disconnected, use your DMM set to DC volts (V) to measure the voltage between the reference voltage terminal at the harness connector and chassis ground. You should typically see a 5V or 12V reference voltage (consult wiring diagram for specific vehicle). Also, check for proper ground at the ground terminal of the harness connector.
5. Wiggle Test: If the short is intermittent, gently wiggle sections of the wiring harness and connectors while monitoring the DMM for changes in resistance or voltage, or the scan tool for changes in live data.

Recommended Repairs and Solutions

Once the source of the short to ground has been positively identified through diagnosis, the appropriate repair can be made:

• Repair or Replace Damaged Wiring Harness: If the short is found in the wiring, the damaged section should be repaired using appropriate automotive-grade wire, soldering techniques, and heat shrink tubing to ensure a durable and waterproof connection. For extensive damage, harness replacement might be more practical. Ensure new wiring is routed correctly to prevent future chafing or exposure to heat.
• Replace the Oil Level Switch/Sensor: If DMM testing confirms an internal fault or short within the sensor itself, replace the oil level switch. Always use an OEM equivalent or a high-quality aftermarket part. When replacing, ensure the sealing O-ring or gasket is also replaced to prevent oil leaks from the oil pan.
• Clean and Repair Corroded Connectors: If corrosion is the issue, carefully clean the connector terminals using an electrical contact cleaner and a small brush. If pins are bent, damaged, or severely corroded, the connector body or individual terminals may need to be replaced. Ensure good pin tension for a secure electrical connection.
• ECM/PCM Replacement: This is a last resort. Only consider ECM replacement if all other possibilities (wiring, sensor, connectors) have been meticulously ruled out, and tests indicate an internal fault within the ECM’s oil level sensor monitoring circuit. ECM replacement often requires programming or “flashing” to the specific vehicle, which should be performed by a qualified technician with specialized equipment.

Important Mechanics’ Tips: After any repair, clear the DTCs with a scan tool. It is highly recommended to perform a comprehensive drive cycle to allow the ECM to run all system monitors and confirm that the B1308 code does not return, ensuring the repair was successful.