SPN 1209 FMI 2 indicates the exhaust pressure signal is unstable or implausible — an erratic or data-validity class fault. Failing sensor, moisture or soot in the sense line, intermittent harness, leaking fittings, and intermittent restriction are the primary causes. Sense-line condition and connector fretting are equally important — both must be addressed before restriction conclusions are drawn.

Inspect the exhaust pressure sense hose and port for moisture, soot packing, and leaking fittings — sense-line condition is equally important as the electrical circuit for SPN 1209 FMI 2. Check connector fretting at the sensor connector. Graph exhaust pressure during steady load to distinguish electrical dropouts from mechanical restriction changes.

Graph exhaust pressure PID during steady load. Abrupt electrical dropouts differ from smoother mechanical restriction changes — abrupt spikes or dropouts to irrational values are electrical. Inspect the sense hose and port for soot packing and moisture. Perform electrical wiggle tests at the sensor connector and nearest harness clips while monitoring the PID. Stabilize sensing before any restriction conclusions are drawn.