P2478: Ford exhaust-temperature sensor 1 exceeds engineering range

P2478 identifies exhaust-temperature sensor 1 outside its engineering range in Ford's 2015 diesel summary. Ford explicitly describes this out-of-range test as high-side only, with a typical temperature threshold of 1,150°C. It is distinct from simply detecting signal voltage outside electrical limits.

Temperature range and circuit range differ

P0545 and P0546 are the neighboring sensor 1 low- and high-circuit checks. The summary explains that the sensor transfer function does not allow unique electrical and engineering low-side thresholds. This is why P2478 should not be described as a symmetric too-hot-or-too-cold monitor.

The check runs with the key on, battery voltage in the documented 9–16.25 volt window and a typical duration of two seconds. The separate cold-soak rationality tests use different enabling conditions.

What a high result means

The code establishes that the interpreted temperature exceeded the monitor's range; it does not alone prove either genuine exhaust overheating or a defective sensor. Compare the temperature information with the electrical and rationality results using the correct exhaust layout. Do not use the threshold as a permissible operating temperature or touch hot exhaust parts during diagnosis.

Source and applicability

Ford 2015 model-year diesel OBD system operation summary, revision December 22, 2014; the cited monitor only, without blanket model or engine applicability. Ford source, physical PDF pages 128, 129. The documented monitor explains the fault decision; vehicle-specific service information is still needed for connector identification and a complete repair procedure.