Why is this code rated more serious than the other battery temperature codes?

Because of which way the charging system errs. A high signal is read as a very cold battery, and cold batteries get charged at a higher voltage. Applied to a battery that is actually warm, that sustained overvoltage boils off electrolyte, sheds plate material and can swell the case — real, cumulative damage on a timescale of weeks in summer heat. Its sibling P0516 pushes voltage the other way, which shortens battery life but does not actively cook it. Same family, opposite consequence.

There is a bad smell around my battery. Is that connected?

Very likely, and it is the symptom worth acting on quickly. That sulphurous, rotten-egg odour is hydrogen sulphide venting from a battery being gassed by excessive charging voltage — exactly what a false extreme-cold reading causes. Measure voltage at the battery posts with the engine warm and running. If it is sitting above about 15 volts, get the vehicle looked at soon and avoid long drives in the meantime, since the gassing gets worse the longer the alternator runs.

I unplugged the sensor to test it and got this exact code. Does that mean it is fine?

It means the test worked as expected, not that the sensor is fine. An open circuit and a genuinely frozen battery produce identical signals, so unplugging the sensor will always set P0517. To get a real answer, run the test the other way: bridge the two harness terminals with a jumper wire. Zero resistance should drive the reading to the hot extreme and change the code to P0516. If it does, the harness and module are healthy and the sensor is your fault.

Could my alternator be causing this instead?

It is worth distinguishing, because both produce high charging voltage. A failed voltage regulator overcharges regardless of what any sensor reports, and will do so with no battery temperature code stored at all. With P0517, the module is being told to raise voltage and is doing exactly as instructed. The way to tell them apart is the live data: if battery temperature reads a plausible value and voltage is still high, look at the alternator. If battery temperature is pinned at the cold extreme, fix that circuit first and re-measure before spending anything on charging components.

OBD-II trouble code

P0517: Battery Temperature Sensor Circuit High

Signal voltage on the battery temperature circuit has risen to the top of its range, which the module reads as a deeply frozen battery — so it raises charging voltage, and in warm weather that can cook the battery.

Medium severityPowertrainCharging / ElectricalDrivable short-term

Quick facts

System: Powertrain
Category: Charging / Electrical
Severity: Medium severity
Drivable: Usually safe to drive short-term
Repair cost range: $70 – $500
DIY difficulty: Beginner DIY

What does P0517 mean?

This is the member of the battery temperature family with teeth. The sensor is a negative-coefficient thermistor, so high resistance means cold and high resistance produces high signal voltage. When the module sees the signal sitting near the top of its range — commonly above about four and a half volts for longer than the calibration allows — the temperature it computes is extremely cold. An open circuit produces the same reading, because an open is simply infinite resistance, which is why a disconnected sensor and a genuinely frozen battery look identical to the module.

The consequence runs the opposite direction to P0516 and is considerably more destructive. A cold battery needs a higher charging voltage to accept charge, so the strategy responds to the apparent cold by raising its target. On a July afternoon, with a battery already at 50 degrees under the hood, that is precisely the wrong move. Sustained overcharging drives electrolysis inside the cells: water splits into hydrogen and oxygen, electrolyte level drops, plates begin to shed material, and the case can swell. The tell-tale is a sulphurous, rotten-egg smell around the battery after a long drive, and it is worth taking seriously rather than dismissing.

That is also why this code earns a slightly higher severity than the rest of its family. Nothing about it makes the vehicle unsafe to drive in the short term, but a battery being overcharged through a hot summer can fail in weeks rather than years, and a swollen or gassing battery in a sealed engine bay is not something to leave indefinitely. The single most useful thing an owner can do before the car reaches a shop is measure voltage at the battery posts with the engine running and warm. Anything sustained above roughly 15 volts confirms that the false cold reading is genuinely reaching the charging system rather than being ignored.

Diagnostically, the good news is that an open circuit is the easiest fault in automotive electrics to isolate. Because a disconnected sensor mimics the fault exactly, the test is to do the reverse: bridge the two harness terminals together with a jumper wire and watch the reading. Zero resistance should drive the value to the hot extreme and change the stored code to P0516. If it does, everything from the connector back to the module is intact and the fault lies in the sensor or its pigtail. If the value refuses to move, the break is in the harness or at the module end.

Common causes

Open circuit in the signal or ground wire between the sensor and the module
Battery temperature sensor failed open — the most common single part failure in this family
Connector unplugged, backed out, or terminals spread so they no longer make contact
Broken pigtail where the wire flexes as the battery is removed and refitted
Signal wire shorted to a voltage source, which also drives the reading to the top of its range
Corroded terminal producing high resistance that mimics an open without being one
Sensor omitted entirely after a battery tray replacement or an engine bay repair
Module input circuit failure, uncommon and only after wiring and sensor have been cleared

Symptoms

Check engine light on with P0517 stored
Battery temperature in live data pinned at the coldest end of the scale regardless of weather
Charging voltage measured at the battery higher than normal, often 15 volts or more when warm
Rotten-egg or sulphur smell around the battery after highway driving
Battery case bulging, or electrolyte level low on batteries with removable caps
Headlights and dash lights noticeably brighter than usual, sometimes flickering brighter at idle
Short battery life — a battery failing within a year or two of installation
Corrosion building rapidly on the battery terminals from vented gas

Diagnostic steps

1.Measure voltage directly at the battery posts with the engine running and fully warmed. This comes first because it tells you whether the false reading is actually affecting charging. Sustained readings above about 15 volts confirm overcharging and make the repair urgent rather than routine.
2.Read battery temperature in live data. A value stuck at the extreme cold end on a warm day confirms the direction and matches an open circuit.
3.Bridge the two sensor terminals on the harness side with a jumper wire, key on. The reading should snap to the hot extreme and the code should change to P0516 on the next check. Success proves the harness and module are fine and puts the fault in the sensor or pigtail; no change puts it in the harness.
4.Measure the thermistor's own resistance across its terminals. A failed-open sensor reads infinite at every temperature and will not respond to being warmed by hand.
5.Inspect the connector for backed-out or spread terminals rather than assuming a broken wire. A terminal that has retracted in its housing looks perfectly connected from outside and is a frequent finding on a connector that gets disturbed every time the battery is changed.
6.Once repaired, inspect the battery itself before calling the job done. Check for a swollen case, low electrolyte on serviceable batteries, and heavy terminal corrosion. Load test it — a battery that has been overcharged for a season may not survive even after the circuit is fixed.

Repair cost

$70 – $500

A failed-open sensor is inexpensive, typically $15-$60 for the part, and the job runs $70-$180 including the time to access it. Repairing a broken pigtail or a spread connector terminal sits at $90-$220. Tracing an open in the harness can reach $150-$350. The figure that pushes this code's total higher than its siblings is the battery: if it has been overcharged through a warm season, replacement adds $150-$320, and that is the more likely outcome the longer the fault has been present. Diagnosis alone is $80-$150.

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DIY vs shop

This is a beginner-friendly repair. Common hand tools, a free afternoon, and a willingness to follow a procedure are usually enough. The risk of causing a bigger problem is low if you read up on your specific vehicle first.