CAT 38 Fault Code: Intake Manifold Air Temperature voltage low
Also called Intake Manifold Air Temperature voltage high, Intake manifold air temperature sensor
Intake Manifold Air Temperature voltage low · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT code 38 (SPN 105 / FMI 3) flags a voltage problem on the intake manifold air temperature sensor circuit. It sets when the ECM reads the signal voltage above 4.9 VDC or below 0.2 VDC for more than 8 seconds while the ECM has been powered for at least 2 seconds. It affects C11, C13, C15, and C18 engines and points to wiring, connector, sensor, or ECM problems rather than actual overheated intake air.
Medium severity. This is a sensor circuit fault, not a direct overheat or overpressure event. The engine can usually keep running, but the ECM may lose accurate intake air temperature data used for fueling and emissions control, so it should be diagnosed the same shift rather than ignored.
What does CAT error code 38 mean?
CAT fault code 38 corresponds to SPN 105 with FMI 3, covering the intake manifold air temperature sensor circuit on C11, C13, C15, and C18 engines. FMI 3 means the ECM sees a voltage on the signal circuit that is out of the expected range: too high (open circuit condition) or too low (short to ground condition).
The intake manifold air temperature sensor is a passive, two-terminal sensor. It doesn't get power from the ECM. Instead, the ECM supplies a pull-up voltage on the signal wire (terminal 1) and uses the sensor's return path (terminal 2) to read resistance-based temperature. If that circuit opens or shorts, the voltage reading goes out of range and the ECM logs this code instead of a valid temperature.
Because this is a circuit-level fault, the ECM can't tell if intake air is actually hot or cold. It only knows the wiring or sensor isn't producing a believable voltage. This matters because intake air temperature feeds into fueling, boost, and emissions strategies, so bad data here can affect performance and how the engine protects itself from actual thermal events.
What triggers a CAT 38 code?
High-voltage version: the intake manifold air temperature sensor signal voltage is above 4.9 VDC for more than 8 seconds, engine coolant temperature is above -10°C (15.0°F), and the ECM has been powered for at least 2 seconds. Low-voltage version: the signal voltage is below 0.2 VDC for more than 8 seconds, with the ECM powered for at least 2 seconds. On some engine variants the low-voltage condition also requires that 168-01 (Low ECM Battery Power), 105-00 (High Intake Manifold Air Temperature Warning), and 105-11 (Very High Intake Manifold Air Temperature) are not currently active.
Common causes of 38
- ECM connection problem, including a loose, corroded, or damaged ECM connector
- Sensor failure: the intake manifold air temperature sensor itself has failed internally
- Sensor harness or connection failure between the sensor and the ECM, such as a broken wire, corroded pin, or bad connector seal
- ECM failure (listed as a possible but less common cause)
- On some variants, ECM harness or connector faults are called out separately from sensor harness faults
- If codes for the 5-volt supply voltage (232-3 or 232-4) are active at the same time, those should be diagnosed first since a bad 5-volt supply can cause this code to set as a side effect
How to troubleshoot CAT 38: first checks
- Check the ECM connector for corrosion, bent pins, or a loose fit. Reseat and clean if needed before condemning any parts.
- Disconnect the sensor at its connector and check for pull-up voltage at the connector. Presence of pull-up voltage means the wiring back to the ECM is not open or shorted to ground. Absence of pull-up voltage points to an open signal wire or a short to ground. A voltage reading that doesn't match expected pull-up voltage suggests the signal wire is shorted to another wire in the harness.
- Inspect the full length of the sensor harness for chafing, pinches, or corrosion at connectors, since this circuit shares a common wiring path style with other passive engine temperature sensors.
- Check for related active codes, especially 232-3 or 232-4 (5-volt supply codes), 168-01 (low ECM battery power), 105-00, and 105-11. If any of these are active, work those first since they can cause or mask this code.
- Test or swap the intake manifold air temperature sensor itself if wiring checks out clean, since sensor failure is a commonly listed cause.
- If wiring and sensor both check good, suspect the ECM connection or, less commonly, an ECM internal failure.
How the code clears
No separate clearing step is listed for this code beyond fixing the underlying wiring, connector, sensor, or ECM issue. Once the signal voltage returns to a normal range and stays there, the code should clear on its own after the ECM re-evaluates the circuit; a code clear with a diagnostic tool can be used to confirm the repair once the fault condition is gone.
Affected models and serial ranges
38 appears in our records across 4 CAT models. Match your machine by model and serial number.
| Model | Serial ranges |
|---|---|
| C11 | Serial range not listed in source records |
| C13 | Serial range not listed in source records |
| C15 | Serial range not listed in source records |
| C18 | Serial range not listed in source records |
Frequently asked questions
What does CAT fault code 38 mean?
It means the ECM detected an intake manifold air temperature sensor signal voltage that is out of the normal working range, either above 4.9 VDC or below 0.2 VDC, for more than 8 seconds. It's a wiring or sensor circuit fault, not a direct reading of actual intake air temperature.
Which engines use this code?
C11, C13, C15, and C18 engines list this fault as SPN 105 / FMI 3, covering the intake manifold air temperature sensor circuit.
Is it safe to keep running the engine with code 38 active?
The engine will typically keep running, but the ECM loses reliable intake air temperature data that feeds into fueling and emissions control. It's a medium-severity fault: get it diagnosed the same shift rather than letting it run indefinitely.
Why does this code sometimes point to other sensors like ambient air or coolant temperature?
The troubleshooting procedure is shared across passive temperature sensors on these engines, including the ambient air temperature sensor and engine coolant temperature sensor. Wiring problems, ECM connector issues, or 5-volt supply faults can affect more than one of these circuits, so related codes should be checked together.
What's the difference between the high-voltage and low-voltage versions of this fault?
High voltage (above 4.9 VDC for more than 8 seconds) usually points to an open circuit, such as a disconnected sensor or broken signal wire. Low voltage (below 0.2 VDC for more than 8 seconds) usually points to a short to ground in the signal wire or a failed sensor.
Should I check other codes before replacing the sensor?
Yes. If 232-3 or 232-4 (5-volt supply voltage codes) are active, troubleshoot those first, since a bad 5-volt reference can cause this code to set even if the temperature sensor itself is fine. Also check 168-01, 105-00, and 105-11 status, since some variants of this fault require those to be inactive before code 38 can set.
How do I test the sensor wiring myself?
Disconnect the sensor and check for pull-up voltage at the sensor connector. If pull-up voltage is present, the harness back to the ECM is intact. If it's absent, you likely have an open wire or a short to ground. If the voltage present doesn't match expected pull-up voltage, the signal wire is probably shorted to another wire in the harness.