A/C Refrigerant Temperature Circuit Low
The A/C system's refrigerant temperature sensor is reading colder than expected, like a thermometer stuck on a low reading when the actual temperature is normal. The ECU thinks the refrigerant is too cold, which can prevent proper A/C operation.
The ECU monitors voltage from the A/C refrigerant temperature sensor to determine system conditions. When sensor voltage exceeds the upper threshold (indicating low temperature), the ECU disables the compressor clutch as a protection mechanism to prevent freeze-up and damage. The sensor uses a thermistor that changes resistance with temperature.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Refrigerant Temperature Sensor Voltage | 2.0–3.5 V (approximately 40–110°F) | >3.8 V (below 32°F / 0°C) |
| Refrigerant Temperature | 40–110°F operating range | Below 32°F triggers compressor disable |
Code P1438 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P1438 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.