P2042 — Reductant Temperature Sensor Circuit #

The wording of this code may vary slightly depending on the OEM (Original Equipment Manufacturer).

• P2042: Reductant Temp Sensor Circuit (General)—Generic OBD-II
• P2042: Reductant Tank Temperature Sensor Circuit—Ford
• P2042: SCR Urea Temp Sensor Circuit—Toyota, Lexus
• P2042: DEF Temp Sensor Circuit High—Chevrolet, GMC, RAM

Summary of code P2042 #

The P2042 diagnostic trouble code (DTC) indicates a malfunction detected in the reductant temperature sensor circuit by the vehicle’s engine control module (ECM). This sensor is responsible for monitoring the temperature of the reductant fluid—commonly known as Diesel Exhaust Fluid (DEF) or AdBlue—which is crucial for proper operation of the Selective Catalytic Reduction (SCR) system. The SCR system reduces harmful nitrogen oxide (NOx) emissions in modern diesel engines. When this code is triggered, it means the sensor is reporting implausible readings, or the circuit is open or shorted, potentially compromising emission control. Prompt diagnosis and repair are important to avoid drivability issues and regulatory compliance problems.

Common symptoms for code P2042 #

When the P2042 code appears, several symptoms can surface. Some of these may subtly impact performance, while others can be more obvious, especially if the emission system enters a failsafe or “limp” mode. Watch for these signs:

• Illuminated Check Engine Light (CEL)
• Diesel Exhaust Fluid warning light or message
• Reduced engine power or limp mode
• SCR system deactivation (possible inability to start after a number of cycles)
• Increased exhaust emissions
• Difficulty passing an emissions test

Probable causes for code P2042 (by frequency) #

Statistically, certain issues are more likely than others when P2042 appears. The most frequent culprits usually relate to the sensor and its circuit, but external factors can play a role. Here’s what technicians commonly find:

1. Faulty reductant temperature sensor (sensor internal failure or corrosion)
2. Damaged wiring or poor electrical connections (open, shorted, or corroded wires/connectors)
3. Connector contamination (DEF/urea is corrosive and may damage connectors)
4. DEF tank contamination or freezing (extreme cold can affect readings, especially if the tank heater fails)
5. Failed SCR control module or ECM (rare, but possible)

Step-by-step diagnosis for code P2042 #

Proper diagnosis is essential to avoid unnecessary parts replacement. Follow this structured approach for reliable results:

• 1. Verify the code—Use a scan tool to confirm P2042 is present and check for related codes (such as P2043, P2047).
• 2. Inspect the sensor and harness—Visually examine the reductant temperature sensor, wiring, and connector for signs of damage, corrosion, or contamination.
• 3. Check for DEF contamination—Look for signs of DEF leakage around the sensor and connector.
• 4. Test the sensor—With a multimeter, measure sensor resistance and compare to OEM specifications (refer to service manual).
• 5. Check wiring continuity—Test for open, short, or high resistance in the sensor circuit between the ECM and the sensor.
• 6. Inspect for tank freezing—If in cold climates, ensure DEF is not frozen and that the tank heater (if equipped) is operational.
• 7. Clear the code and test drive—After making any repairs or corrections, clear the code and perform a drive cycle to see if the code returns.

Possible repairs for code P2042 #

Once the root cause is identified, apply the appropriate repair. These solutions address the most common underlying issues:

• Replace the reductant temperature sensor if faulty or out of OEM specs.
• Repair or replace damaged wiring and connectors, ensuring all contacts are clean and secure.
• Clean or replace contaminated connectors affected by DEF fluid.
• Address DEF tank contamination or freezing by cleaning/replacing the tank or fixing the tank heater.
• ECM or SCR module replacement—only if all other options have been exhausted and verified faulty with advanced diagnostics.

Is it safe to drive with code P2042? #

Limited: Driving with a P2042 code is not immediately dangerous to the driver or passengers, but it is not recommended for extended periods. The SCR system may become inoperative, which can eventually trigger a “no start” condition after several drive cycles or cause the vehicle to enter limp mode with reduced power. Emissions will increase, and you may fail a state inspection. Address this issue as soon as possible to avoid inconvenience and potential legal compliance problems.

Vehicles most commonly affected by code P2042 #

While any diesel-equipped vehicle using a Selective Catalytic Reduction system can develop this code, it is most prevalent among certain makes and models:

• Ford Super Duty trucks (F-250, F-350, F-450) – Power Stroke Diesel
• GM/Chevrolet Silverado and GMC Sierra HD (Duramax Diesel)
• RAM 2500/3500/4500 (Cummins Diesel)
• Mercedes-Benz Sprinter vans
• Toyota Land Cruiser, Hilux (where SCR is equipped)
• Volkswagen and Audi TDI models

Common mistakes & best practices for code P2042 #

Misdiagnosing P2042 can lead to wasted time and expense. Keep these tips in mind to avoid common pitfalls:

• Overlooking wiring and connectors—Don’t replace the sensor before checking the harness and connections.
• Ignoring DEF contamination—DEF fluid is corrosive; always inspect for leaks or residue near the sensor.
• Skipping freeze diagnostics—In cold climates, frozen DEF can mimic sensor failure.
• Not checking for related codes—Associated DTCs may provide valuable clues.
• Using non-OEM sensors—Aftermarket sensors can have compatibility issues; OEM parts are often preferable.

Quick FAQ for code P2042 #

Need answers fast? Here are some popular questions about P2042:

• Q: Can a bad reductant temperature sensor cause my truck not to start?
  A: Yes, if the code is ignored for too long, many OEMs will eventually trigger a no-start condition to protect emission controls.
• Q: Do I have to use OEM sensors, or are aftermarket options okay?
  A: While some quality aftermarket sensors exist, OEM sensors are generally more reliable for this application due to strict calibration requirements.
• Q: Will clearing the code fix the problem?
  A: Clearing the code resets the warning, but if the root cause is not fixed, the code will quickly return.