Code
B1388
ALFA ROMEO
B — Body
Oil temperature sensor open or short to positive
Views:
UK: 4
EN: 8
RU: 3
AI status
Completed
Completed
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Causes
- Open circuit in oil temperature sensor wiring
- Short to positive (battery) on the sensor signal or power circuit
- Faulty oil temperature sensor (internally open/shorted)
- Corroded or damaged connector or pins
- Poor ground or ECU input circuit fault
Symptoms
- Malfunction indicator lamp (MIL) or service light illuminated
- Oil temperature gauge or message shows incorrect, fixed, or no reading
- Possible reduced engine control quality if oil temp used for fueling/strategy
- Stored diagnostic trouble code B1388
What to check
- Verify code present and freeze frame / live data for oil temperature sensor
- Visual inspection of sensor, connector, and wiring harness for damage or corrosion
- Back-probe sensor connector and measure signal voltage with ignition ON (engine OFF)
- Measure sensor resistance at ambient temperature with sensor disconnected
- Check for continuity to ECU and short to battery (12V) or ground using a multimeter
- Wiggle test harness while monitoring live data/voltage to find intermittent faults
Signal parameters
- Typical thermistor behavior: resistance decreases as temperature rises (NTC).
- Approximate operating voltage at ECU input: ~0.5–4.5 V depending on oil temperature and vehicle reference (varies by model).
- Open-circuit indication: very high resistance (>>100 kΩ) or signal voltage floating/unreliable.
- Short-to-positive indication: signal voltage close to battery voltage (~12 V) or above normal sensor voltage range.
- Short-to-ground indication (not this code): signal near 0 V.
Diagnostic algorithm
- Read and record freeze frame and live data for oil temperature. Confirm B1388 is current or historic.
- Perform a visual inspection of the sensor, connector, and wiring for obvious damage, corrosion, pin push-out, or melted insulation.
- With ignition OFF, disconnect sensor. Measure sensor resistance across its terminals at ambient temperature. Compare to service data; an open reading (OL/infinite) indicates internal break. Note: many oil temp sensors are NTC; resistance should be finite and decrease when warmed.
- With ignition ON (engine OFF) back-probe the sensor signal wire and measure voltage to ground. A voltage near battery voltage indicates a short to positive; a floating or no voltage may indicate open circuit or loss of pull-up/power.
- Check continuity between sensor connector and ECU pin for the signal and ground. Look for high resistance, opens, or shorts to battery/ignition feed on the harness.
- If wiring appears good, substitute a known-good sensor or bench-test the removed sensor (compare to specification).
- If sensor and wiring check good but fault persists, inspect ECU input for damage and consult wiring diagram; repair or replace ECU only after confirming wiring and sensor failure.
- After repairs, clear codes and test drive or cycle ignition to confirm the code does not return.
Likely causes
- Broken or disconnected wire between sensor and ECU (open circuit)
- Insulation abrasion contacting battery or ignition feed causing short to +12V
- Sensor element failed (internal short to supply or open thermistor)
- Connector corrosion that produces high resistance or intermittent connection
- ECU input damaged (rare) or poor chassis ground at sensor
Fault status
Status
B1388 - Oil temperature sensor open or short to positive
Repair difficulty: Medium
Diagnostic time: 0.5 - 1.5 hours
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