CAT 27 Fault Code: Coolant Temperature voltage high
Coolant Temperature voltage high · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT fault code 27, also listed as SPN 110 FMI 3, means the ECM sees the engine coolant temperature sensor signal voltage above 4.9 VDC (or 4.95 VDC depending on the specific diagnostic version) for more than eight seconds, while the engine has been running more than seven minutes and the ECM has been powered at least two seconds. This almost always points to an open circuit, bad connection, or failed sensor rather than an actual overheating condition.
Medium severity. This is a wiring/sensor signal fault, not confirmation of actual engine overheating. It should still be diagnosed promptly because the ECM may lose accurate coolant temperature input, which affects protection strategies and fan/derate logic on some setups.
What does CAT error code 27 mean?
Fault code 27 (SPN 110 FMI 3) on CAT C11, C13, C15, and C18 engines means the ECM has detected that the coolant temperature sensor's signal voltage is too high, which the ECM reads as an open circuit condition rather than a real temperature reading.
The coolant temperature sensor is a passive, two-terminal device. It does not receive power from the ECM. Instead, the ECM applies a small pull-up voltage on the signal wire and watches for changes as the sensor's resistance varies with temperature. If the circuit opens up (a broken wire, disconnected connector, or failed sensor), that pull-up voltage rises unchecked and crosses the ECM's threshold, which triggers this code.
Because this is a voltage-high, open-circuit type fault, it usually does NOT mean the engine is actually overheating. It means the ECM cannot trust the coolant temperature signal it's receiving. This can affect fan control, derate logic, and other coolant-temperature-dependent strategies until it is fixed.
What triggers a CAT 27 code?
The ECM sets this code when the coolant temperature sensor signal voltage exceeds 4.9 VDC (one version of this diagnostic) or 4.95 VDC (two other versions) for more than eight seconds, and only after the engine has been running for more than seven minutes and the ECM has been powered for at least two seconds.
Common causes of 27
- ECM connector connection problem (corrosion, loose pins, or poor seating)
- ECM wiring harness fault between the sensor and the ECM
- Failed or degraded coolant temperature sensor
- Sensor harness or connector failure (open wire, damaged pins, chafed insulation)
- ECM internal failure
- Active 5-volt supply voltage codes (232-3 or 232-4) can cause this code to set as a side effect; if either of those codes is active, troubleshoot the 5-volt supply issue first
How to troubleshoot CAT 27: first checks
- Check the coolant temperature sensor connector for corrosion, moisture, loose pins, or a poor seat. Unplug and reseat it, inspecting terminals closely.
- With the sensor disconnected, check for pull-up voltage at the sensor connector. Presence of pull-up voltage means the wiring back to the ECM is intact (not open or grounded). Absence of pull-up voltage points to an open signal wire or a short to ground. A voltage reading different from the expected pull-up value suggests the signal wire is shorted to another wire in the harness.
- Inspect the full length of the sensor harness for chafing, pinches, corrosion at splices, or rodent damage between the sensor and the ECM connector.
- Check for any active 5-volt supply codes (232-3 or 232-4). If either is present, diagnose and resolve that issue first since it can cause this coolant temperature code to set as a downstream symptom.
- Verify the ECM connector itself is clean, dry, and fully seated; inspect for bent or backed-out pins.
- If wiring and connections check good, test or swap the coolant temperature sensor itself to confirm whether it is the failed component.
How the code clears
No separate clearing step is listed. Once the underlying wiring, connector, or sensor issue is repaired and the signal voltage returns to a normal operating range, the code should stop being active. Confirm no other coolant, ambient air, or intake manifold temperature sensor codes are still present, since they share the same troubleshooting approach.
Affected models and serial ranges
27 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 27 mean?
It means the ECM sees a coolant temperature sensor signal voltage that is too high, above 4.9 VDC or 4.95 VDC for more than eight seconds, which the ECM interprets as an open circuit rather than a valid temperature reading.
Does fault code 27 mean my engine is overheating?
No. This code is about a voltage signal problem, not a confirmed high coolant temperature. It usually means the sensor circuit is open due to a bad connector, damaged wiring, or a failed sensor, not that the coolant is actually too hot.
Can I keep operating with this code active?
You can typically keep running short term, but the ECM may not have an accurate coolant temperature reading, which can affect fan control and other protective strategies. Get it diagnosed as soon as practical rather than ignoring it.
What should I check first if I get code 27?
Start with the coolant temperature sensor connector for corrosion or a loose pin, then check for pull-up voltage at the connector with the sensor disconnected to determine if the issue is in the sensor itself or in the wiring back to the ECM.
Is this the same as an ambient air or intake manifold temperature sensor code?
The troubleshooting steps are identical across the coolant, ambient air, and intake manifold temperature sensor circuits since they use the same type of passive sensor and pull-up voltage design, but code 27 specifically applies to the coolant temperature sensor circuit.
What if I also have a 5-volt supply code active?
If codes 232-3 or 232-4 (5-volt supply voltage codes) are active at the same time, diagnose and fix those first. A bad 5-volt supply can cause the coolant temperature code to set as a secondary symptom.
Which CAT engines use this fault code?
Fault code 27 (SPN 110 FMI 3) applies to the C11, C13, C15, and C18 engine models.