12 April, 2025CAT 3406E (Prefix 5DS or 1LW) Fault Code: 12 PID: 111 SPN: FMI: 2 Coolant Level signal invalid The ECM detects the following conditions: •The coolant level sensor is sending an invalid signal for more than ten seconds.•The “Coolant Level Sensor” parameter is programmed to “Yes”. Probable Causes: 1) ECM Connection/Harness2) Coolant sensor failure3) Coolant sensor harness failure4) ECM Failure Share on Facebook Opens in a new window. Tweet on Twitter Opens in a new window. Pin on Pinterest Opens in a new window. Back to blog Leave a comment Name * Email * Comment * Please note, comments need to be approved before they are published.