OBD-II trouble code
P205C: Reductant Tank Temperature Sensor Circuit Low
Low signal voltage on a thermistor means the module is being told the fluid is hot. Believing that in January is the one member of this family that can cost you hardware, because a system convinced the tank is warm will ask its pump to move slush.
Quick facts
- System
- Powertrain
- Category
- Exhaust / Aftertreatment
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $150 – $1,600
- DIY difficulty
- Advanced DIY
Browse every code in P2000–P20E8, or start from the full code library.
What does P205C mean?
The sensing element here is a thermistor wired as one half of a voltage divider, and its resistance falls as it gets warmer. So a low voltage on the signal wire is not a neutral fact — it carries a specific meaning. It is the reading the circuit produces when the element is hot, and a short to ground produces the most extreme version of it. The module is therefore not merely confused. It has been handed a confident, specific and wrong answer: the fluid is warm.
That inversion is what separates this code from its high-side counterpart, and it is worth being concrete about the consequence. A module that believes the tank is warm sees no reason to run the tank heater, and no reason to hold dosing back after a cold start. On a mild day neither decision does any harm. On a genuinely cold morning both are wrong in the same direction. The heater stays off while the fluid is at least partly frozen, and the supply pump is asked to draw and pressurise a liquid that is still slush. Supply pumps in these systems are built to move a thin fluid, not to break ice, and asking one to prime against a partly frozen pickup is the realistic mechanism by which this fault turns into a bill for something other than a sensor. Pressure-side components and the pickup line share that exposure.
This is why, among five members of one sensor family that otherwise share a rating, this one deserves one piece of practical advice the others do not: if the weather is cold and the repair is not happening today, park the vehicle somewhere warmer — a garage, a shop bay, anywhere above freezing. That costs nothing and removes the only scenario in which waiting a few days can damage hardware. A caveat worth stating plainly: the system will also give up and fall back to a conservative strategy on many platforms, so this is a risk to manage rather than a certainty to panic about. But it is the real reason not to treat this code as purely cosmetic.
On the diagnosis, there is one test that deserves to come before everything else because of how it splits the problem in half. Disconnect the sensor at the tank and look at the signal again. With the element out of the circuit the signal should go high — that is what an open divider does. If it stays low with nothing connected, the element is irrelevant and the fault is a short somewhere in the harness or the connector. If it goes high as expected, the element or its immediate connection is the suspect. One minute, no special equipment, and it eliminates either the component or fifteen feet of wiring.
That test matters more here than it would elsewhere because of the commonest cause on the low side, which is not a classic short at all. It is water. A tank connector sitting under the vehicle collects spray, salt and washing water, and a wet connector does not short two pins together cleanly — it creates a resistive path between them. The module reads a partial pull-down, interprets it as a warmer-than-real temperature, and stores the code. Then the connector dries out and the fault goes away. A complaint that appears in wet weather, after a wash or a deep puddle, and clears in a dry spell is this mechanism rather than a broken wire, and it is the reason so many of these faults are declared intermittent and then blamed on a sensor that was never faulty.
Common causes
- Signal conductor shorted to ground in the run to the tank
- Water, salt slurry or washing water bridging terminals inside the tank connector
- Thermistor failed with an internally low resistance
- Sensing element shorted to the tank assembly body or its metal shell
- Chafed insulation where the harness passes a bracket, clip or body pass-through
- Connector terminal pushed out of position and touching the connector shell or a neighbouring pin
- Previous repair splice made without sealed joints and now wicking water
- Harness pinched during DEF pump, filter or level sender work
- Incorrect tank assembly fitted whose sensor curve sits far below the expected range
- Reductant control module internal fault on the sensor input
Symptoms
- Warning lamp on with the vehicle driving normally
- DEF or SCR system message on the dashboard
- Scan tool showing an implausibly high fluid temperature, often pegged at the top of the scale
- Tank heater never running, even on a freezing morning
- System attempting to dose immediately after a cold start instead of waiting
- Fault that appears in wet weather, after a wash or after a deep puddle, and clears when dry
- Low DEF pressure, pump or dosing faults stored alongside after cold-weather use
- Noisy or straining DEF supply pump on cold mornings
- Staged power reduction as the inducement sequence advances
- Fault appearing shortly after underbody, tank or exhaust work
Diagnostic steps
- 1.Disconnect the sensor at the tank and check the signal again. If it stays low with nothing connected, the element cannot be the cause and the fault is in the harness or the connector. If it rises as expected, the element and its connection are the suspects. Do this first.
- 2.Note whether the reported fluid temperature is merely high or pegged at the top of the scale. Pegged points at a hard short; a plausible but too-warm figure points at a resistive path.
- 3.Open the tank connector and look for moisture, salt crust, green deposits and terminal position. A wet connector creates a resistive bridge that mimics this fault exactly and then dries out.
- 4.Ask when the fault appears. A complaint tied to rain, washing or puddles is almost always water in a joint rather than a damaged conductor, and chasing it as a short wastes hours.
- 5.Measure the element's resistance at a fluid temperature taken directly with a thermometer in the fill neck. An internally low thermistor is obvious once there is a real temperature to compare against.
- 6.Check each sensor pin for a short to the tank assembly body and shell, not just pin to pin. An element shorted to its housing is invisible to a pin-to-pin reading.
- 7.Test the signal conductor to ground along its length with the harness flexed at each clip and pass-through, and inspect any previous splice in the run.
- 8.Check whether the tank heater has been commanded at all during cold operation. A heater that has never run while the module believed the fluid was warm is consistent with this fault and tells you how long it has been present.
- 9.Scan for DEF pressure, pump and dosing faults stored after cold-weather driving. Those suggest the pump has already been asked to work against partly frozen fluid and should be assessed rather than assumed healthy.
- 10.After repair, confirm the reading settles near ambient after a cold soak, confirm the heater is commanded when it should be, and verify dosing is withheld and then released properly on a cold start.
Repair cost
$150 – $1,600
Diagnosis is $120 to $250. A connector clean, reseal and terminal repair is $80 to $300 and is the single most common real fix on the low side, because water in a joint causes more of these than broken wires do. A harness repair is $150 to $500. Where the element has failed, most platforms supply it only within the tank sender assembly at $250 to $900 in parts plus 1 to 4 hours of labour, often including draining or lowering the tank. Budget separately for cold-weather consequences: if the vehicle has been driven through freezing conditions while the module believed the fluid was warm, a DEF supply pump is $400 to $1,200 fitted and a pickup or pressure line repair $150 to $500. Those are avoidable by getting the fault fixed before winter, or by keeping the vehicle above freezing until it is.
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.