OBD-II Diagnostic Trouble Code
P2042 — Reductant Temperature Sensor Circuit
P2042 is a general circuit fault for the reductant (DEF) temperature sensor. The ECM saw a missing or implausible signal from the sensor it uses to know DEF temperature, which controls tank and line heating and decides when dosing is allowed. On many trucks this sensor is built into the DEF tank header or sender unit, while some OEMs place it in the DEF pump or supply module.
Severity 3/5
Truck-relevant
Service this week
Severity
Service this week
Reductant Temperature Sensor Circuit
Code
P2042
Category
Powertrain
Subsystem
Aftertreatment
Truck-relevant
Yes
What this means for truckers
P2042 (Reductant Temperature Sensor Circuit) — DEF tank temperature sensor circuit fault. DEF freezes at 12°F; the system needs accurate temp to know when to heat. Replacement: $80-200.
Before you replace parts — try this first
P2042 on the Aftertreatment system is most often corroded def tank header or supply module connector from def creeping into the pins — not a dead part. On commercial diesels especially, work these in order before spending money on parts:
- Confirm the symptoms. Match what your truck is doing to the Symptoms section below. If the code is set but the symptoms are not there, clear the code and re-test after a drive cycle — it may not return.
- Clean and dry the sensor connector. Water, road salt, or oil seeping into a connector is the #1 fake sensor failure. Disconnect, look for green corrosion or moisture, spray with electrical contact cleaner, dry, reseat. Many faults clear here for $0.
- Inspect the wiring harness. Trace the wire from the sensor toward the ECM — look for chafing against the engine block, melted insulation near the turbo or exhaust, or pinch points where a clamp has rubbed through.
- Test the sensor before swapping it. Most Common Causes (below) list the expected voltage or resistance range — a $20 multimeter saves a $300 sensor swap that does not fix the problem.
- Only then, replace the part. Use the OEM or a tier-1 aftermarket. Cheap sensors throw the same code in two weeks.
Symptoms
DEF/SCR warning and check-engine light. The ECM may not heat the DEF correctly in cold weather, and dosing can be limited or disabled, causing high NOx and often an emissions derate countdown. In warm weather the truck may drive normally until the derate timer runs.
Common causes
- CommonCorroded DEF tank header or supply module connector from DEF creeping into the pins ($100–$350)
- CommonFailed temperature sensor inside the DEF tank header or supply module ($700–$2000)
- OccasionalDamaged harness between the tank and ECM or aftertreatment module ($100–$400)
- RareAftertreatment control module or ECM fault ($1000–$3000)
How to fix
Compare the DEF temperature reading to ambient after the truck has sat overnight
When to use: First check to see if the sensor reading is believable
Inspect the connector for white crystal deposits and green corrosion, clean and reseal
When to use: When the connector shows DEF contamination
Measure sensor resistance at the connector and compare to the OEM chart
When to use: To confirm whether the sensor or the wiring is at fault
Replace the DEF tank header, sender or supply module assembly that contains the sensor
When to use: When the sensor is out of specification, since it is usually not serviced separately
Affects most
No specific vehicle pattern found in our complaint database. This code can appear on any vehicle that supports OBD-II diagnostics.
Related
- → Browse all DTC trouble codes
- → Decode a VIN to check codes for your truck
- → Owner-operator maintenance guides
- → P2002 — Diesel Particulate Filter Efficiency Below Threshold (Bank 1)
- → P2003 — Diesel Particulate Filter Efficiency Below Threshold (Bank 2)
- → P2031 — Exhaust Gas Temperature Sensor Bank 1 Sensor 2
- → P2032 — Exhaust Gas Temperature Sensor Bank 1 Sensor 2 Low
- → P2033 — Exhaust Gas Temperature Sensor Bank 1 Sensor 2 High
Frequently Asked Questions
Can I drive with code P2042?
P2042 is severity 3 — plan service within a few days. Performance is likely degraded already (you may feel reduced power, harsher shifts, or slightly worse fuel economy). Continuing to drive risks additional damage to adjacent components.
How much does it cost to fix P2042 on a commercial truck?
Total repair cost ranges $100–$3000 depending on the root cause. The cheapest fix is usually the first one listed below — Corroded DEF tank header or supply module connector from DEF creeping into the pins — which is also the most common. On commercial trucks the cost is usually higher than passenger vehicles due to part availability and shop labor rates ($120–185/hour at most truck shops, vs $90–130 at general auto shops). DIY-capable fixes (sensors, hoses, filters) are at the low end; major component replacements (turbo actuator, high-pressure fuel pump, transmission solenoid pack) are at the high end. Always diagnose with a scan tool BEFORE replacing parts — guessing wrong on a $1,500 turbo actuator when the actual issue was a $30 boost hose is a common expensive mistake.
Will P2042 fail a DOT inspection?
Yes — P2042 is an emissions-related code, and DOT inspectors check emissions compliance. An active emissions code with a check engine light can result in an out-of-service violation under FMCSA Part 393. The truck typically can't be moved until the code is cleared. Repeated emissions violations escalate to CSA scores.
What's the difference between P2042 and related codes?
P2042 is closely related to: P2002 (Diesel Particulate Filter Efficiency Below Threshold (Bank 1)), P2003 (Diesel Particulate Filter Efficiency Below Threshold (Bank 2)), P2031 (Exhaust Gas Temperature Sensor Bank 1 Sensor 2), P2032 (Exhaust Gas Temperature Sensor Bank 1 Sensor 2 Low). These codes share the same subsystem (aftertreatment) and often appear together. If you see this code, scan for the full code list — fixing the root cause usually clears multiple companion codes at once. If only P2042 is set without the companions, the issue is more isolated.
How do I prevent P2042 from coming back after a repair?
For P2042 (Reductant Temperature Sensor Circuit), prevention centers on the aftertreatment system. Run quality DEF (verify supplier — BlueDEF, PEAK, Yara Air1). Don't skip parked regens — when the dash asks for one, do it. Avoid extended idle time which fouls the DPF. Replace NOx sensors at the OEM interval (typically 400K miles).