Drivers first notice that the check‑engine illumination appears without a clear loss of power. In many cases the light flashes briefly during start‑up and then steadies. A secondary symptom can be a reduced flow of diesel‑exhaust‑fluid (DEF) when the vehicle is in emission‑control mode, sometimes accompanied by a brief limp‑mode that limits torque to protect the emission system. Because the fault is tied to the reductant‑temperature sensor circuit, the vehicle’s on‑board diagnostics will not permit full DEF‑pump operation until the code is cleared.
These signs appear because the engine control module (ECM) receives a voltage that exceeds the calibrated range for the reductant‑temperature sensor, prompting it to disable or limit the DEF system as a protective measure.
The reductant‑temperature sensor outputs a voltage proportional to fluid temperature. A short to the power rail or a broken internal element can drive the voltage above the ECM’s upper limit, triggering P2045.
Wire harnesses that run near the exhaust system are exposed to heat, vibration, and moisture. Cracked insulation or corrosion at the sensor connector can create a high‑resistance path, causing the ECM to read an artificially high voltage.
Oxidation or debris in the sensor’s connector pins can introduce intermittent high‑voltage spikes. The ECM interprets these spikes as a genuine sensor reading, setting the fault.
Even with a healthy sensor and wiring, the ECM’s input stage can develop an internal fault—such as a failed op‑amp or voltage regulator—that forces the circuit high. This is less common but requires module‑level troubleshooting.
Occasionally, a software version that does not match the vehicle’s production date will misinterpret normal sensor voltages as out‑of‑range, especially after a flash update. Re‑programming the ECM resolves the mismatch.
– Connect a dealer‑level scan tool and monitor the “Reductant Temp Sensor A” voltage. A normal reading is typically 0.5 V – 5 V, varying with temperature. Anything above ~5.5 V indicates a circuit‑high condition.
– With the ignition on, measure the reference voltage at the sensor connector (usually 5 V). Confirm it matches the service specification. A higher supply suggests a wiring short to a higher voltage source.
– Visually examine the harness for chafing, heat damage, or exposed conductors. Use a multimeter to check continuity from the sensor connector back to the ECM pin. Resistance should be near zero; any significant drop points to a break.
– Disconnect the sensor and clean the pins with electrical contact cleaner. Re‑torque the connector to the manufacturer’s torque specification (often 4–6 Nm). Re‑scan to see if the code clears.
– If a known‑good sensor is available, install it temporarily. If the voltage returns to normal and the code disappears, the original sensor is defective.
– Should wiring and sensor be verified as good, the next step is to perform an ECM input‑stage test using the scan tool’s “ECM diagnostics” routine. This checks internal voltage regulators and op‑amps.
– When a software calibration issue is identified, update the ECM to the latest factory‑approved calibration file. This is performed with a dealer‑level flashing tool and the vehicle’s VIN to ensure correct mapping.
– If the ECM input circuit fails internal testing, replacement of the ECM/PCM is required. Replacement units must be VIN‑matched and programmed with the correct software version before installation.
Typical costs
All repairs should be performed with a scan tool capable of reading live sensor data and performing module re‑programming.
By keeping the sensor circuit clean and the ECM software current, the likelihood of a P2045 fault diminishes dramatically.
Service Recommendation: Most issues related to this fault are diagnosed and corrected through inspection, wiring repair, and calibration rather than module replacement. For modules not typically replaced through aftermarket suppliers, diagnosis and repair should be performed by a certified automotive technician with access to factory service information and tooling.