My air conditioning stopped working and I have this code. Are they connected?

Almost certainly, and this is the most valuable thing to know about P0073. Manufacturers block the air conditioning compressor from engaging below a threshold outside temperature — usually somewhere around freezing — to stop the evaporator icing up and to protect the compressor from starting with refrigerant pooled in it. A vehicle that believes the outside temperature is minus 40 will refuse to run the compressor no matter the weather. The system can be perfectly charged and mechanically sound. Fix the temperature circuit first and the air conditioning normally returns on its own.

Why does a 'high input' code report an extremely cold temperature?

Because resistance and temperature run in opposite directions on this sensor. It is a thermistor whose resistance climbs as it gets colder, and the module converts the resulting high signal voltage into a low temperature. An open circuit is effectively infinite resistance, which the module interprets as the coldest value its table contains — commonly minus 40. So the extreme cold reading is not a separate fault; it is what an open circuit looks like once the module has done its arithmetic.

Can I confirm the sensor is bad without buying one?

Yes, in about two minutes. Unplug the sensor and bridge the two terminals on the harness side with a piece of wire while the ignition is on. Zero resistance is what maximum heat looks like to the module, so the displayed temperature should shoot to the top of the range. If it does, every wire and the module input are proven good and the sensor is the open circuit. If nothing changes, the break is upstream and a new sensor would not have fixed it.

The display was reading a bit cold for months before the light came on. Is that related?

It very likely is, and it is the normal progression for this circuit. This sensor lives outside the sealed engine compartment in the road-spray and salt path, so its terminals corrode gradually rather than failing in one event. Added contact resistance biases the reading cold long before the joint opens far enough to trip the code threshold. A reading that was mildly pessimistic for weeks is the early stage of the same fault, which is also why cleaning or replacing the connector is often the real repair rather than the sensor.

OBD-II trouble code

P0073: Ambient Air Temperature Sensor Circuit High Input

Signal voltage on the outside air temperature circuit has risen above its valid ceiling, which the module converts into an impossibly cold reading. Cold is the direction with consequences — on many vehicles it shuts the air conditioning off entirely.

Low severityPowertrainTemperatureDrivable short-term

Quick facts

System: Powertrain
Category: Temperature
Severity: Low severity
Drivable: Usually safe to drive short-term
Repair cost range: $80 – $420
DIY difficulty: Beginner DIY

What does P0073 mean?

P0073 means the signal voltage arriving from the outside air temperature sensor has climbed higher than any genuine temperature could produce. In practice that almost always means the circuit is open somewhere: the module's pull-up has nothing to pull against, so the line floats up to the reference voltage and stays there.

What makes this code worth understanding is not its cause, which is usually mundane, but its consequence. Because the sensor's resistance rises as it gets colder, maximum resistance reads as maximum cold, and the vehicle will report something near the bottom of its range — commonly minus 40 degrees, which happens to be the same number in Celsius and Fahrenheit. Unlike a falsely hot reading, a falsely cold one collides with real vehicle strategies that exist to protect hardware in genuinely freezing weather, and those strategies do not politely announce themselves.

The one that generates the most confusion is air conditioning inhibit. Manufacturers block compressor engagement below a threshold outside temperature, typically somewhere between minus 2 and plus 5 degrees Celsius. The reasons are sound: below that point the evaporator can ice over, and refrigerant migrating to the compressor while it is cold makes for a punishing start-up. A vehicle that believes it is minus 40 outside will therefore refuse to run the compressor at all, in July, with a system that is perfectly charged and mechanically sound. The customer arrives complaining that the air conditioning has failed, nobody thinks to look at a powertrain code, and it is entirely possible to spend money on refrigerant, a pressure switch or a compressor clutch chasing a fault that lives in a two-wire circuit at the front of the car. If air conditioning has stopped working and P0073 is stored, treat the code as the explanation until proven otherwise.

Other cold-side behaviours follow the same logic. Ice warning chimes and dashboard snowflake indicators may fire permanently. Some vehicles extend warm-up enrichment or adjust idle speed on the basis of a cold ambient. Remote start and engine block heater reminders on vehicles that have them can behave as though the car were in an Arctic winter.

As for the cause, the mechanism on this circuit is worth naming precisely. This is one of a very small number of sensors deliberately mounted outside the sealed engine compartment, in the road-spray path, where it is exposed to salt in a way that no engine-bay connector ever is. Terminals in that environment do not usually snap open in a single event. They corrode from the outside inward and go progressively high in resistance first, which means a P0073 is frequently preceded by weeks or months of a reading that was merely a few degrees too cold — believable enough that nobody acted on it — before the joint finally opens far enough to trip the threshold. Owners who recall the display having been slightly pessimistic for a while are describing the early stage of the same fault.

The confirming test is the mirror image of the low-input one. With the sensor unplugged, bridge the two terminals on the harness side with a jumper wire. That is zero resistance, which the module reads as maximum heat, so the reported temperature should slam to the hot extreme. If it does, the wiring and the module are proven good end to end and the sensor is open. If it does not move, the break is in the harness or the module and the sensor is not the problem.

Common causes

Corroded sensor terminals gone progressively high in resistance from salt and road spray exposure
Open circuit in the signal or ground wire between the sensor and the module
Connector unlatched, back-pinned or not fully seated after front-end or mirror work
Failed thermistor that has gone internally open with age
Broken wire inside the insulation where the loom flexes, with no visible damage outside
Green corrosion product forcing the terminal and its mating pin apart inside the connector body
Sensor ground circuit lost, which raises the signal line just as effectively as a broken signal wire
Rodent damage to the exposed front-of-vehicle harness section
Aftermarket accessory wiring spliced into the circuit and later removed, leaving a poor joint

Symptoms

Outside temperature display stuck at an impossible low value such as minus 40
Air conditioning refusing to engage in warm weather with a correctly charged system
Ice warning chime or snowflake indicator permanently active
Check engine light on with P0073 stored
History of the displayed temperature reading a few degrees too cold for weeks before the light came on
Automatic climate control running the heater hard in mild conditions
Reading unchanged by weather, road speed or how long the car has been standing
No change in engine power, idle quality or drivability

Diagnostic steps

1.Read the ambient air temperature parameter and confirm it is pinned at the cold extreme rather than merely reading low, which separates an open circuit from a drift fault.
2.Ask specifically whether the air conditioning has stopped working, because the compressor inhibit strategy makes that the leading customer complaint on this code and it saves a wasted refrigerant diagnosis.
3.With the ignition on and the sensor unplugged, bridge the two harness terminals with a jumper wire. The reading should swing to the hot extreme, proving the wiring and module are intact and condemning the sensor.
4.If bridging the terminals produces no change, the open is in the harness or the module and the sensor is not at fault.
5.Check for supply and ground at the harness connector before condemning any wire, since a lost sensor ground raises the signal line exactly like a broken signal wire.
6.Open the connector and inspect for salt crust, green corrosion product and terminal spread, all of which are typical of a component mounted outside the sealed engine bay.
7.Measure resistance across the sensor terminals at a known temperature and compare against the manufacturer's table to confirm an internally open thermistor.
8.Wiggle-test the harness along the exposed front-of-vehicle run and through any door jamb section while watching the live value, to catch a break inside intact-looking insulation.
9.Confirm the connector latches fully and is correctly oriented, especially on a vehicle that has recently had front-end panels off.
10.After repair, verify the reported temperature against a separate thermometer and confirm the air conditioning compressor now engages normally in warm conditions.

Repair cost

$80 – $420

Diagnosis is $80 to $150 and the jumper test settles sensor versus wiring in a couple of minutes. A sensor replacement is $12 to $110 in parts, giving $80 to $200 fitted where it sits behind the grille and $250 to $380 where a bumper cover or mirror assembly has to be dismantled. Cleaning or replacing a corroded connector is often the whole repair at $90 to $220, and it is the correct one — fitting a new sensor into a corroded connector reproduces the fault. Repairing an open in the harness runs $120 to $300. There is real money saved by finding this code before the air conditioning is investigated, because a compressor or refrigerant diagnosis chased on a false cold reading can cost several hundred dollars and fix nothing.

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Open the Repair Cost Estimator with ambient air temperature sensor replacement preselected. Adjust labor rate and vehicle category to fit your situation.

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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.