OBD-II trouble code
P2045: Reductant Temperature Sensor Circuit High
The symptom that gives this code away is seasonal nonsense: a vehicle that behaves as though its DEF were frozen solid in the middle of August, because with no credible temperature the module assumes the worst and waits.
Quick facts
- System
- Powertrain
- Category
- Exhaust / Aftertreatment
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $90 – $1,300
- DIY difficulty
- Advanced DIY
Browse every code in P2000–P20E8, or start from the full code library.
What does P2045 mean?
On a thermistor circuit, the reading pinned at the top of the scale is the classic signature of an open. The module holds the signal line up through a pull-up resistor, and if the sensor, its ground return or its signal wire is no longer completing the path, the line simply floats to reference voltage. Maximum resistance reads as maximum voltage, and maximum voltage is what P2045 is reporting.
What makes this code recognisable in the workshop is not the electrical theory but the behaviour it produces. Faced with an input it cannot believe, a control module does not guess optimistically — it takes the conservative option, and on this system the conservative option is to assume the fluid may be frozen. Dosing is therefore held off until a fallback timer has run, sometimes with the tank heater commanded on as a precaution. The result is a vehicle that acts out a cold-start thawing routine on a warm afternoon, refuses to dose for a period that has nothing to do with conditions, and then behaves normally. A DEF system that thinks it is winter in August is the pattern worth remembering, because it points at this code before anyone has connected a scan tool.
The second thing to know is where these faults come from, and it is more often people than age. The DEF header is the service point for that whole end of the system: the pump, the filter, the level sender and the pickup all converge there, and a great many of them cannot be reached without disturbing the sensor's connector or the harness serving it. A P2045 that appears within a few weeks of DEF pump, filter or level sender work is far more likely to be a terminal pushed out of its housing, a seal left unseated, or a wire trapped when the assembly went back down than a sensor that happened to fail at the same moment. What work has been done under this vehicle recently is a fair question to ask before any measurement is taken.
The third is the failure mode of the sensor itself, and it is gradual rather than sudden. This thermistor lives immersed in urea solution, and immersed sensors rarely die cleanly. What fails first is the seal where the leads pass out of the element, and as it degrades the resistance in the path climbs — not to infinity, but steadily upward. So a failing sensor reads high but finite, and a meter used only to check for continuity will call it good right up until it opens completely. The test that catches it is resistance measured at a known fluid temperature and compared against the manufacturer's table. A sensor reading several times its table value at that temperature has already failed, whatever a continuity beep says.
One practical note on priority. Because the module's fallback here is to withhold dosing rather than to dose blindly, this fault does not lay down deposits the way an air-side or injector fault can. Nothing is being sprayed badly; nothing is being sprayed at all during the fallback period. That makes it a genuinely low-damage code. What it is not is a code with no deadline, because the inducement sequence counts down on time and distance no matter how well the vehicle drives.
Common causes
- Signal or ground conductor open in the harness run to the DEF tank
- Terminal backed out, unseated or damaged at the DEF header connector
- Connector left unlatched or a seal displaced after DEF pump, filter or level sender work
- Sensor element seal degraded by long immersion, with resistance climbing out of specification
- Sensor failed fully open
- Corroded ground stud or eyelet in the DEF sender circuit
- Harness broken where it crosses the tank or passes a fixed bracket
- Signal wire shorted to reference voltage or another higher-voltage circuit
- Header assembly damaged internally, with a broken conductor inside the potting
- Reductant control module fault or a supply and ground problem at the module
Symptoms
- Warning lamp with the vehicle driving normally
- Scan tool showing DEF temperature at the top of the scale, or a default value, regardless of conditions
- System declining to dose for a period after start-up in warm weather
- Tank heater commanded on as a precaution when no cold weather is present
- DEF or SCR system message on the dashboard
- Fault appearing shortly after DEF pump, filter or level sender work
- DEF level or quality faults at the same time, since those senders share the header connector
- Reduced NOx conversion efficiency or a companion SCR efficiency code
- Staged power reduction as the inducement sequence advances
- Fault intermittently present before becoming permanent
Diagnostic steps
- 1.Ask what work has been done at the back of the vehicle lately. The DEF header is the service point for the pump, the filter and the level sender, and a fault appearing soon after any of those jobs points at the connector before it points at the sensor.
- 2.Read the live value and note whether the system is withholding dosing in conditions that plainly do not call for it. A warm-weather thawing routine is the behavioural confirmation of this code.
- 3.Open the header connector and check every terminal for full seating and latch engagement. Backed-out pins and unlatched connectors are the single most common cause here and cost nothing to correct.
- 4.Confirm reference voltage and ground are present at the harness side, then measure voltage drop on the ground path under load rather than relying on a continuity check.
- 5.Bridge the signal pin to ground at the connector and watch the live value. If it swings to the opposite extreme, the harness back to the module is intact and the fault is at or beyond the connector.
- 6.Measure the sensor's resistance at a known fluid temperature and compare it against the manufacturer's temperature and resistance table. High but finite is still a failed sensor, and continuity alone will not show it.
- 7.Inspect the harness where it crosses the tank and passes fixed brackets, flexing each section while watching the value for a change.
- 8.Clean and re-torque the ground stud for the DEF sender circuits, since a corroded return produces a full-scale reading that looks exactly like an open sensor.
- 9.Check for DEF level or quality codes stored alongside. Several faults from the same assembly usually mean the connector or the header rather than one failed element.
- 10.After repair, verify the reading matches ambient following a cold soak, then confirm dosing begins promptly on a warm start rather than after a fallback delay.
Repair cost
$90 – $1,300
Diagnosis is $120 to $250. This code has the best chance in the family of a near-free outcome, because a backed-out terminal or an unlatched connector left over from previous DEF work is one of its most common causes and takes minutes to correct once found. Connector or harness repair runs $100 to $400. Where the sensor has genuinely gone open, it is usually not sold separately: a DEF header or heater and sender assembly is $250 to $900 in parts with 1 to 4 hours of labour, often including draining or dropping the tank. Light-duty vehicles sit at the lower end and commercial saddle-tank installations at the top. Unlike the air-side faults in this neighbourhood, this one does not normally bring deposit-removal costs with it, because the system withholds dosing rather than dosing badly.
Estimate your repair
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Open the Repair Cost Estimator with def / scr reductant system service preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an advanced DIY job. It typically requires specialty tools, scan-tool access, lifting equipment, or careful sequencing to avoid causing new failures. Plan for extended downtime and have a backup vehicle. Most owners are better served by a shop that has done this repair before.