CAT 12 Fault Code: Coolant Level voltage low
Coolant Level voltage low · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT code 12 (SPN 111 / FMI 4) means the ECM sees a short or low-voltage condition in the coolant level sensor circuit, and it only sets when the 'Coolant Level Sensor' parameter is programmed to '2-wire float sensor'. It applies to C11, C13, C15, and C18 engines.
Medium severity. Coolant level monitoring is a safety feature but the code itself is a sensor circuit fault, not proof of an actual low coolant condition. Treat it as a diagnose-soon item, but verify actual coolant level right away since a real low-coolant condition can cause engine damage if ignored.
What does CAT error code 12 mean?
CAT fault code 12, SPN 111 FMI 4, means the ECM has detected a low-voltage or shorted condition on the coolant level sensor circuit. This code only exists when the engine is configured with a coolant level sensor and the customer programmable parameter 'Coolant Level Sensor' is set to '2-wire float sensor'. If that parameter is set to '2-pin switch' or '4-pin', this specific code cannot occur.
The coolant level sensor itself is installed by the vehicle's Original Equipment Manufacturer, not by CAT. It feeds into the ECM's engine monitoring system, a programmable feature that watches coolant level as part of overall engine protection. Some OEM-installed coolant level sensors are wired independently and are not connected to engine monitoring at all, so it's important not to confuse a standalone OEM sensor with the one used by the ECM for this diagnostic.
In plain terms, the ECM is telling you it is receiving a signal it interprets as too low to be a real, valid float sensor reading, which usually points to a wiring short, a bad connector, or a failed sensor rather than necessarily an actual coolant loss.
What triggers a CAT 12 code?
The ECM detects that the signal voltage from the coolant level sensor is less than 0.5 VDC for more than ten seconds, while the 'Coolant Level Sensor' parameter is programmed to '2-wire float sensor', and after the ECM has been powered for at least two seconds.
Common causes of 12
- Coolant level sensor has failed internally
- Short in the coolant level sensor harness or connector
- Damaged, corroded, or loose wiring between the ECM and the coolant level sensor
- Faulty or corroded ECM connection
- Low actual coolant level (listed as a possible cause alongside circuit faults)
- ECM failure (less common, but listed as a possible cause)
How to troubleshoot CAT 12: first checks
- Confirm the actual physical coolant level in the system before assuming it's purely electrical
- Check that the 'Coolant Level Sensor' parameter is indeed programmed to '2-wire float sensor', since this code cannot occur under other settings
- Inspect the coolant level sensor connector and harness for corrosion, moisture intrusion, chafing, or a pinched/shorted wire
- Check the ECM connector for the coolant level circuit for corrosion or a loose pin
- Test the sensor signal voltage at the connector and compare against the 0.5 VDC low threshold and the 4.5 VDC high threshold used by the related high-voltage version of this diagnostic
- If wiring and sensor check out, suspect the ECM connection or, less likely, an ECM failure
How the code clears
No separate clearing step is listed. Once the short or low-voltage condition on the coolant level sensor circuit is repaired and the signal voltage returns to a valid range, the code should clear on its own after the ECM re-evaluates the circuit. Always verify actual coolant level and top off if needed before closing out the repair.
Affected models and serial ranges
12 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 code 12 SPN 111 FMI 4 mean?
It means the ECM sees the coolant level sensor signal voltage below 0.5 VDC for more than ten seconds, which it interprets as a short or low-voltage fault in the 2-wire float sensor circuit.
Does this code mean my engine is actually low on coolant?
Not necessarily. This is a sensor circuit fault. It can be triggered by wiring or connector problems even when coolant level is fine, but low coolant level is also listed as a possible cause, so always physically check the coolant level.
Why does this code only show up on some CAT engines and not others with the same coolant sensor installed?
This code only sets when the customer programmable parameter 'Coolant Level Sensor' is programmed to '2-wire float sensor'. If it's set to '2-pin switch', '4-pin', or 'NO', this diagnostic is disabled.
Who installs the coolant level sensor on CAT C11, C13, C15, and C18 engines?
The coolant level sensor is installed by the vehicle's Original Equipment Manufacturer, not by Caterpillar. Some OEM-installed sensors are wired independently of the ECM's engine monitoring system.
What is the difference between this code and the high-voltage version of the coolant level fault?
This code (FMI 4) triggers when signal voltage drops below 0.5 VDC for more than ten seconds. A related condition triggers instead when voltage rises above 4.5 VDC for more than ten seconds, indicating an open circuit rather than a short.
What should I check first if I get this code?
Verify the actual coolant level, confirm the sensor parameter programming, and inspect the sensor harness, connector, and ECM connection for corrosion or damage before replacing parts.