Drivers who encounter a U0668 code most often notice an oil‑level warning that does not correspond to the actual oil quantity. The instrument cluster may illuminate a yellow oil‑level lamp, flash an “Oil Level” alert, or, on some vehicles, temporarily limit engine power or prevent start‑up until the fault clears. The warning can appear intermittently or remain steady, and it may be accompanied by a loss of power‑train warning lights if the PCM decides to protect the engine.
Because the code signals a loss of communication rather than a low‑oil condition, the vehicle’s oil pressure and level are usually normal. However, ignoring the warning can lead to repeated PCM resets, unnecessary service visits, and, in rare cases, the PCM entering a fail‑safe mode that limits performance.
The oil‑level sensor is typically mounted on the oil pan and connects to the PCM through a dedicated harness. Exposure to heat, moisture, or road debris can degrade insulation, cause short circuits, or corrode pins. A broken wire or high‑resistance connection prevents the PCM from receiving the sensor’s voltage signal, triggering U0668.
The sensor circuit draws a small reference voltage from the PCM’s 5 V supply. A blown fuse, a failing PCM voltage regulator, or a grounding issue can drop the supply below the sensor’s operating threshold. The PCM then registers a loss of communication.
Modern PCMs contain multiple microcontrollers and communication buses (CAN, LIN). A failed communication controller, damaged memory, or internal component fatigue can isolate the oil‑level sensor channel while leaving other functions intact. The PCM logs U0668 when it cannot poll the sensor.
Occasionally, a corrupted calibration file or outdated firmware can cause the PCM to misinterpret a valid sensor signal as “no communication.” Manufacturers release re‑flash updates to correct such logic errors.
Although the code points to communication loss, a completely failed sensor can also produce the same symptom because the PCM receives no valid data. In this scenario, the sensor’s internal resistance is open, mimicking a broken wire.
Connect a professional OBD‑II scanner, confirm U0668, and note any additional power‑train codes that may indicate broader PCM issues.
– Examine the oil‑level sensor wiring for frayed strands, chafing, or exposed conductors.
– Check connectors for corrosion, bent pins, or moisture. Clean with electrical contact cleaner if needed.
– With the ignition off, measure resistance between the sensor’s signal wire and ground. Manufacturer specifications typically range from 1 kΩ to 3 kΩ; values outside this range suggest a short or open circuit.
– Verify 5 V reference voltage at the PCM connector while the engine is running. Voltage below 4.5 V indicates a power‑supply problem.
Use a scan tool that can perform “bi‑directional” tests. Command the PCM to request sensor data; observe whether a valid voltage is returned. Failure to receive data confirms the communication loss.
Replace damaged harness sections, reseat connectors, and apply dielectric grease to prevent future corrosion. Re‑scan; if U0668 clears, the repair is complete.
If wiring is sound and the PCM still reports loss, check the manufacturer’s technical service bulletins for a PCM re‑flash. Update the PCM firmware using a dealer‑level tool or an authorized re‑programming service.
When the PCM fails the communication test after wiring and software checks, replacement is the logical next step. A new PCM must be programmed with the vehicle’s VIN, calibration data, and security keys. Flagship One offers VIN‑matched PCM units pre‑programmed to factory specifications; installation typically costs $150‑$250 for programming plus $600‑$900 for the module itself.
Clear all codes, perform a drive cycle, and re‑scan to ensure U0668 does not return. Verify that the oil‑level warning lamp remains off .
Service Recommendation: Most issues related to this fault are diagnosed and corrected through inspection, wiring repair, and calibration rather than module replacement. For modules not typically replaced through aftermarket suppliers, diagnosis and repair should be performed by a certified automotive technician with access to factory service information and tooling.