P0073 means the voltage from your car’s ambient air temperature sensor, the small sensor that measures the outside air, is higher than the sensor can produce on its own. In practice that almost always means an open circuit: an unplugged or corroded connector, a broken wire or a sensor that has failed open. The computer then reads the coldest temperature it can, often −40, and acts on it. That is the part worth knowing, because a car that thinks it is −40 outside will often refuse to run the air conditioning. If you have P0073 and the A/C has stopped blowing cold, the fix is usually a sensor costing a few tens of dollars or a connector, not an A/C repair. This article covers what sets P0073, the six causes in order of likelihood, two free tests that separate a bad sensor from bad wiring, and what each repair typically costs.
Start with the table, then read the note under it.
| Cause | Telltale Sign | Typical Cost |
|---|---|---|
| Unplugged or damaged connector | Recent front-end, bumper, grille or mirror work; dash reads a fixed −40 | $0 to $100 |
| Corroded connector terminals | Fault comes and goes with rain or road salt | $0 to $120 |
| Broken or chafed wiring | Reading jumps when the harness or mirror is moved | $50 to $250 |
| Failed sensor | Reading recovers when the harness is jumpered at the sensor plug | $25 to $150 |
| Poor ground or signal return | Other sensors or lights on the same ground also misbehave | $20 to $200 |
| Module or software fault | Wiring and sensor test good; a bulletin covers your vehicle | $0 to $600+ |
Stop and get it fixed before driving in rain, cold or humid weather if the windshield fogs up and will not clear. Many cars run the A/C compressor to dry the air for demisting, and a false freezing reading can stop that. Otherwise P0073 does not affect how the engine runs, and the car is safe to drive while you arrange the tests below.
What the P0073 Code Actually Means
The ambient air temperature sensor is a thermistor, a resistor whose resistance changes with temperature. On almost every car it is the negative temperature coefficient type: its resistance falls as it gets warmer. The module that reads it sends a regulated reference voltage, usually 5 volts, through a fixed resistor to the sensor, and measures the voltage left on the signal wire. A warm sensor has low resistance and pulls the voltage down. A cold sensor has high resistance and lets it rise.
That means the signal voltage is highest when the sensor is coldest. If the circuit opens anywhere, at a connector, a broken wire, a bad ground or inside the sensor, there is nothing to pull the voltage down, so it rises to the full reference voltage. That is the reading P0073 describes: Ambient Air Temperature Sensor Circuit High. The module interprets it as the coldest temperature on its scale, which is commonly −40 degrees. That is the one temperature that reads the same in Fahrenheit and Celsius, which is a useful clue in itself: a dash display stuck on exactly −40 is a circuit reading, not the weather.
Most cars mount the sensor at the front, behind the grille or bumper cover and ahead of the radiator, where it sits in the airflow. Some put it in an exterior mirror: Ford’s bulletin 25-2545 for the 2023 to 2025 F-Super Duty, which lists P0071, P0072 and P0073, places it in the right exterior mirror. Which computer reads it also varies. It may be the engine computer, the body control module, the climate control module or the instrument cluster, and the reading is shared with the others over the vehicle network. The code can be stored in the engine computer even when the sensor is wired to another module, so a full-system scan is worth having.
Symptoms You May Notice Alongside the Code
- The outside temperature on the dash reads −40, shows dashes, or is blank
- The A/C compressor will not engage, so the vents blow outside-temperature air
- Automatic climate control behaves as though it is freezing, with the heater running hard
- A frost or ice warning on a warm day
- The check engine light, on cars where the engine computer monitors the sensor
The engine itself usually runs normally. Some engine computers use the outside temperature when deciding whether to run certain emissions self-tests, so those tests may not complete while P0073 is stored, but drivability is rarely affected.
Why the A/C Stops Working
Air conditioning compressors are not meant to run in freezing weather, so climate systems lock the compressor out below a set outside temperature. When the module believes it is −40, the lockout is active even on a hot day. Ford’s TSB 16-0054 for the 2014 to 2016 Transit Connect with the 2.5L engine describes exactly this. The A/C is inoperative because the ambient reading is “below -1.1 °C (30 °F)” when the actual temperature is above it, with P0073, P0074 and/or U0422 stored. Its fix is a new sensor and a new connector pigtail spliced into the harness. Nothing in the A/C system itself is touched.
So if the A/C stopped working at the same time as P0073 appeared, fix the sensor circuit first. Recharging the system or replacing an A/C part will not help while the module still thinks it is freezing. The site’s guide to why A/C blows warm air covers the other causes, for the case where it still blows warm once the temperature reading is right.
1. An Unplugged or Damaged Connector
This is the most common cause, and it usually has a history. Front-mounted sensors sit right where a bumper cover, grille or headlight comes off for repairs, and the connector is easy to leave unplugged, catch on a clip or crack while refitting. A minor parking knock can also break the sensor’s plastic mount and leave it hanging by its wire. If P0073 appeared after any front-end work, or after a mirror was replaced on a car with a mirror-mounted sensor, check this first.
Find the sensor, usually visible through the grille or from under the front bumper, and make sure the plug is fully seated with the lock engaged. Check the locking tab, the plug body and the short tail of wire behind it.
2. Corroded Connector Terminals
A front-mounted sensor lives in the spray from the road, so water, salt and grit reach its connector constantly. Corrosion adds resistance, and enough of it reads like an open circuit. The giveaway is a fault that comes and goes with the weather, setting in rain or after salted roads and clearing in dry spells. Open the plug and look for green or white deposits and swollen or pushed-back terminals.
Light corrosion can be cleaned with electrical contact cleaner and a small brush, but terminals that are pitted or green to the core need a new connector pigtail. Ford’s TSB 16-0054 specifies just that, a replacement connector pigtail with the wiring taped and folded to relieve strain, which shows how often the plug is the weak point.
3. Broken or Chafed Wiring
The sensor’s two wires run behind the bumper or, for a mirror-mounted sensor, through the door and the door hinge, where every opening flexes them. Look for wires rubbing on the bumper reinforcement or radiator support, damage from earlier collision repairs, and rodent bites. For mirror-mounted sensors, the run through the door is the suspect. One FCA case file for P0073 directs technicians to inspect the circuits from the sensor in the mirror to the driver door module, and from there to the body control module.
Watch the live reading on a scan tool while you flex the harness or open and close the door. A reading that jumps between a normal value and −40 has found the break.
4. A Failed Sensor
The thermistor itself can fail open, from age, a stone strike that cracks its housing, or water getting inside. It is a genuine cause, and the sensor is cheap. But it ranks behind the connector and wiring, because a new sensor plugged into a corroded or broken circuit will read −40 just like the old one. The jumper test below tells you which you have before you buy anything.
5. A Poor Ground or Signal Return
On many cars the sensor’s second wire is a signal return to the module, but on some it goes to a shared ground point on the body. A corroded or loose ground opens the circuit just as a broken wire would. Chrysler’s bulletin 08-175-22 for the 2022 Pacifica lists P0073 alongside other faults caused by water intrusion and corrosion, and its repair is resealing a chassis ground point, G917A, with zip ties and vinyl tape. When P0073 arrives with other unrelated electrical faults, look for the ground they share.
6. A Module or Software Fault
This is the rarest cause. The module that supplies the reference voltage and reads the signal can fail, and some vehicles have software that mishandles the reading. Ford’s TSB 16-0054 starts by resetting the module’s keep-alive memory and checking whether the reading and the A/C recover, before replacing anything. Consider the module only when the sensor, connector, wiring and ground all test good, and ask a dealer whether a bulletin or update applies to your vehicle.
How to Diagnose P0073: Two Free Tests
A scan tool that shows the ambient temperature reading, which most can, and the vehicle’s wiring diagram are all you need. Because “high” means open, the tests work by deliberately opening and closing the circuit and watching what the module sees.
- Read the live value. If it shows a plausible outside temperature with the engine cold, the fault is intermittent; go straight to flexing the harness. If it shows −40 or the coldest possible value, carry on.
- Inspect the sensor and connector for the damage, corrosion and unplugged connections described above.
- The jumper test. Unplug the sensor and connect the two terminals in the harness side of the connector together with a short jumper wire, ideally fused. With the key on, the reading should swing to the hottest value on its scale. If it does, the wiring, ground and module are all good, and the sensor is the fault. If it stays at −40, the open is in the wiring, the ground or the module, and a new sensor will not fix it. Only ever join the sensor’s own two wires; never connect either one to battery voltage.
- Measure the sensor. Unplugged, its resistance should change smoothly as you warm it in your hand. An open reading or one that does not move confirms a failed sensor. Your service information gives the resistance it should have at a given temperature.
- Check the reference voltage at the harness connector with the key on. A reading near 5 volts on the signal wire, with the sensor unplugged, is normal for most designs.
- Clear the code and drive. Many cars do not update the outside temperature reading until the car has been moving for a while, to stop engine heat from skewing it. The FCA case file notes the vehicle must be driven for at least 5 minutes above 15 mph after a repair before the reading updates and the code can be erased. Do not assume the repair failed because the display has not changed in the driveway.
What not to replace on suspicion: A/C components, the climate control module and the engine computer. None of them is likely to be the problem when the circuit reads open, and all of them cost far more than the sensor.
P0073 Repair Cost
These are typical ranges at an independent US shop, including parts and labour.
- Reconnecting or reseating the plug: nothing, or a short diagnostic charge.
- New ambient air temperature sensor: roughly $25 to $150. The part is often $10 to $60, and a front-mounted sensor usually takes minutes to swap, though some need the bumper cover or grille partly removed.
- Mirror-mounted sensor: more labour, because the mirror cover or mirror has to come off, and some vehicles need a camera recalibration afterwards if the mirror carries a surround-view camera. Ford’s F-Super Duty bulletin allows extra time for exactly that.
- Connector pigtail: about $20 to $120 fitted.
- Wiring or ground repair: about $50 to $250, depending on where the damage is.
- Module replacement: several hundred dollars or more with programming, and rarely the answer.
Can You Keep Driving With a P0073 Code?
Yes, in most cases. The ambient sensor does not control fuelling or ignition, so the engine runs as normal. What you lose is comfort and convenience: the outside temperature display, often the A/C, and on some cars automatic climate control that behaves properly.
The exception is visibility. If the A/C is locked out, the system may not be able to dry the air, and the windshield can fog in rain or humid cold weather and be slow to clear. If that happens, fix it before driving in those conditions. Also bear in mind that while the check engine light is on for P0073, it cannot warn you about a second fault, and an engine computer with incomplete self-tests may fail an emissions inspection.
Related Codes
- P0072: ambient air temperature sensor circuit low, the opposite fault. The signal voltage is too low, which the module reads as an impossibly hot day, usually from a short to ground.
- P0070, P0071 and P0074: a general circuit fault, a range or performance fault where the reading is plausible but disagrees with other temperature sensors, and an intermittent or erratic signal. P0074 often appears with P0073 when the open comes and goes.
- P0113: intake air temperature sensor circuit high. It is the same open-circuit logic on a different sensor, the one in the engine’s air intake. See the P0113 guide.
- P0118: engine coolant temperature sensor circuit high. Again an open circuit read as a frozen sensor, but on the coolant sensor, and far more important to how the engine runs. See the P0118 guide.
- P0117: the coolant sensor’s low-input fault, the mirror image of P0118. See the P0117 guide.
If the check engine light comes and goes rather than staying on, this guide to an intermittent check engine light explains why that happens and what it tells you.
Frequently Asked Questions
What does the P0073 code mean?
P0073 means the signal from the ambient air temperature sensor, which measures the outside air, is reading higher in voltage than it should. That almost always means an open circuit, so the computer reads the coldest possible temperature, often -40 degrees, and the outside temperature on the dash may show -40 or nothing.
Can P0073 stop my air conditioning working?
Yes. Climate systems lock out the A/C compressor below a set outside temperature, and P0073 makes the system believe it is freezing. Ford documented this on the Transit Connect, where the A/C was inoperative because the ambient reading was falsely below 30 degrees Fahrenheit. Fix the sensor circuit before working on the A/C.
Is it safe to drive with a P0073 code?
Yes, in most cases. The engine runs normally. The exception is a windshield that fogs and will not clear because the A/C is locked out, which is a visibility problem in rain or humid cold weather.
How much does it cost to fix P0073?
Usually $25 to $150 for a new sensor fitted, nothing to about $120 for a connector, or $50 to $250 for a wiring or ground repair. Mirror-mounted sensors cost more in labour. Module replacement is rare.
Where is the ambient air temperature sensor?
On most cars it sits behind the front grille or bumper cover, ahead of the radiator. Some vehicles put it in an exterior mirror, such as the right mirror on the 2023 to 2025 Ford F-Super Duty. Your service information shows the exact location.
Why does the outside temperature not update after the repair?
Many cars only update the outside temperature reading after the vehicle has been moving for a while, so engine heat does not skew it. One FCA procedure requires at least 5 minutes of driving above 15 mph before the reading updates and the code can be erased.
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