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P2478 — Exhaust Gas Temperature Out Range Bank 1 Sensor 1

Manufacturer-specificPowertrain (engine & transmission)

This code indicates that the exhaust gas temperature sensor located on bank 1, sensor 1 (closest to the engine) is reporting a reading outside the expected operating range. This sensor is used primarily on diesel and some turbocharged gasoline vehicles to monitor exhaust temperature for emission system management. An out-of-range signal typically means the sensor, its wiring, or its connector has a fault rather than an actual temperature problem.

Symptoms

What causes it — most likely first

1. Failed exhaust gas temperature sensor

The sensor itself is the most common failure point. These sensors operate in extreme heat and are prone to internal degradation, open circuits, or calibration drift over time.

2. Wiring harness damage near the exhaust

Heat soak, chafing, or melted insulation in the wiring between the sensor and the engine computer can cause an out-of-range or erratic voltage signal.

3. Corroded or loose sensor connector

The sensor connector is exposed to road debris, moisture, and heat. Corrosion or a poor terminal connection can alter the resistance and trigger this code.

4. Exhaust leak upstream of the sensor

A cracked manifold or leaking exhaust gasket before the sensor can introduce ambient air, causing the sensor to read lower than expected and flag an out-of-range condition.

5. PCM/ECM internal fault

Rarely, a failure in the engine computer's temperature-sensor input circuit can cause an erroneous out-of-range reading. This should only be considered after all other causes are eliminated.

How it's diagnosed

  1. Read and record all stored codes, then clear the code and see if it returns after a drive cycle.
  2. Visually inspect the sensor connector and wiring harness for heat damage, corrosion, chafing, or loose pins.
  3. Compare the live data reading from bank 1 sensor 1 to bank 2 sensor 1 (if equipped) or to other EGT sensors on a cold start — they should read close to ambient temperature before the engine warms up.
  4. Perform a resistance or voltage check on the sensor per the manufacturer's specification at known temperatures to verify calibration.
  5. Check for exhaust leaks upstream of the sensor location.
  6. If the sensor, wiring, and connector all test good, verify PCM power and ground supplies before considering control-module replacement.

Code facts & related codes

SystemPowertrain (engine & transmission)
StandardizationManufacturer-specific (P1/B1/C1/U1)
Related codesP2470 — Exhaust Gas Temperature Sensor CircuitP2471 — Exhaust Gas Temperature Sensor CircuitP2472 — Exhaust Gas Temperature Sensor CircuitP2473 — Exhaust Gas Temperature Sensor CircuitP2474 — Exhaust Gas Temperature Sensor CircuitP2475 — Exhaust Gas Temperature Sensor Circuit

Frequently asked

Can I drive with code P2478?

In most cases the vehicle will drive normally, but on diesel vehicles with a DPF system, regeneration may be disabled, which can lead to a clogged filter over time. It is best to address the code promptly.

Is the PCM/ECM the problem?

Almost never. The exhaust gas temperature sensor and its wiring are by far the most likely culprits. Some shops or sources may suggest replacing the PCM, but this is usually incorrect and should only be a last resort after thorough testing.

Does this sensor affect fuel economy?

Indirectly, yes. If the sensor fault prevents proper DPF regeneration or causes the engine to run in a protective mode, fuel consumption may increase.

Is bank 1 sensor 1 the same as an oxygen sensor?

No. This is an exhaust gas temperature sensor, not an O2 sensor. On many vehicles both are present in the exhaust stream, but they serve different purposes and are not interchangeable.