P0071 on 2017-2018 Ford Fusion: Causes and Fixes for the Ambient Air Temp Sensor

What's Unique About the 2017-2018 Ford FUSION

For the 2017-2018 Ford Fusion, this code has distinct causes depending on the engine. On models with the 2.5L engine, Ford issued Technical Service Bulletin TSB 17-2215 for vehicles built on or before August 18, 2017, indicating a simple software glitch is often the culprit, requiring a PCM reprogram instead of a part replacement. For Hybrid and Energi models, TSB 17-2072 notes that the code can be triggered under specific temperature conditions, such as moving from a warm garage into the cold, which also requires a PCM software update to fix the sensor comparison logic. This highlights that for many Fusions of this era, the first step should be checking for software updates, not immediate part replacement.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• Check Engine Light is on.
• Inaccurate outside temperature display on the dashboard or console (may be stuck at -40°F, 50°F, or another incorrect value).
• Automatic climate control (A/C and heat) may not work correctly or may blow air at the wrong temperature.
• Reduced fuel efficiency in some cases.
• Occasional rough idle or stall just after a cold start.
🎬 See a breakdown of common symptoms and causes for P0071.

Most Likely Causes

1. PCM Software Requires Update (2.5L & Hybrid/Energi Engines) 🔴 High Probability Ford issued TSB 17-2215 specifically for 2017-2018 Fusion models with the 2.5L engine (built on or before 18-Aug-2017) identifying a software calibration issue as a primary cause for P0071. Similarly, TSB 17-2072 was issued for 2017 Hybrid/Energi models to correct faulty sensor comparison logic.
   How to confirm: A Ford dealership or a qualified shop with Ford's diagnostic software (IDS/FJDS) can check the current PCM calibration level against the one specified in the relevant TSB. This is the definitive first step for these specific powertrains.
   Typical fix: Reprogramming the Powertrain Control Module (PCM) to the latest software version. The TSB for the 2.5L notes this also resolves code P068A.
   Est. part cost: $0
2. Faulty Ambient Air Temperature (AAT) Sensor 🟡 Medium Probability The sensor is located at the front of the vehicle, making it susceptible to damage from road debris, moisture, and corrosion. It is a common failure point across many Ford models.
   How to confirm: Use a scan tool to compare the AAT sensor reading to the actual outside temperature. If the reading is stuck (e.g., at -40°F or 50°F) or clearly incorrect, the sensor has likely failed. You can also test the sensor's resistance with a multimeter; it's a thermistor, so its resistance should change predictably with temperature. An open circuit (infinite resistance) or a short (zero resistance) indicates a bad sensor.
   Typical fix: Replace the AAT sensor. This is a simple, push-in or clip-in part. 🎬 Watch this step-by-step guide to replacing the sensor yourself.
   Est. part cost: $15-$40
3. Damaged Wiring or Connector ⚪ Low Probability Wiring to the AAT sensor is in an exposed location behind the grille and can be damaged by road debris, impacts, or corrosion over time. The connector pins can also become corroded.
   How to confirm: Visually inspect the wiring harness and connector for any signs of fraying, breaks, or green/white corrosion. Perform a continuity test on the wires between the sensor and the PCM. With the ignition on and sensor unplugged, one pin on the connector should have a 5-volt reference signal.
   Typical fix: Repair the damaged section of wire or replace the connector pigtail.
   Est. part cost: $10-$30

Rare But Worth Checking

• Faulty Powertrain Control Module (PCM): This is very rare. All other possibilities, especially software updates, sensor failure, and wiring issues, should be thoroughly ruled out before considering PCM replacement.

Diagnosis Steps

1. Scan for Codes: Use an OBD-II scanner to confirm P0071 is the primary code. Note any other codes present, like P068A, as this can point directly to a TSB.
2. Check for TSBs: For a 2.5L engine (built before 18-Aug-2017), immediately suspect the need for a PCM update per TSB 17-2215. For 2017 Hybrid/Energi models, be aware of TSB 17-2072 for temperature logic issues and SSM 46410 for a stuck 50°F reading.
3. Check Live Data: Use a scan tool to view the live data from the AAT, IAT, and ECT sensors after the car has sat overnight. They should all read within a few degrees of each other. A reading from the AAT that is fixed, such as -40°F, points to a sensor or wiring issue.
4. Visual Inspection: Locate the AAT sensor. On the 2017-2018 Fusion, it is typically found clipped into the active grille shutter assembly, visible through the lower grille opening. 🎬 Watch: How a faulty sensor affects your dashboard and A/C. One Reddit user noted it was on the top left corner of the shutter grille when facing the car. Inspect the sensor and its connector for physical damage, corrosion, or debris.
5. Inspect Wiring: Carefully check the wires leading to the sensor for any breaks, chafing, or loose connections. Check for a 5V reference signal at the connector with the key on.
6. Test the Sensor (Optional): If you have a multimeter, disconnect the sensor and measure its resistance. The resistance should change when you warm it (e.g., with a hairdryer). An open circuit (infinite resistance) or a short (zero resistance) indicates a bad sensor.
7. Perform Fix: Based on your findings, either replace the AAT sensor, repair the wiring, or have the PCM reprogrammed at a dealership or qualified shop.

Parts You'll Likely Need

• Ambient Air Temperature Sensor (OEM #DY1160 (Motorcraft Service Part Number), AU5Z-12A647-B (Ford Part Number), AU5T-12A647-AC (Engineering Number)) — This sensor is the most common hardware failure for this code. It is relatively inexpensive and easy to replace. The part is a simple 2-pin thermistor.
  Trusted brands: Motorcraft, Standard Motor Products, Dorman
  OEM price range: $25-$48
  Aftermarket price range: $15-$30

Related Codes That Often Appear With This One

• P068A — TSB 17-2215 for the 2.5L engine explicitly states that P0071 may appear alongside P068A (ECM/PCM Power Relay De-Energized Performance - Too Early), and both are resolved by the same PCM reprogram.

Technical Service Bulletins (TSBs) & Recalls

• tsb17-2215: States that 2017-2018 2.5L engines (built on or before 18-Aug-2017) may exhibit P0071 and/or P068A, fixed by a PCM reprogram.
• TSB 17-2072: Notes that 2017 Hybrid/Energi models can set P0071 after being stored in a warm location and driven on a cold day, fixed by a PCM reprogram.
• SSM 46410: Mentions P0071 on 2017 HEV/PHEV models in conjunction with degraded HVAC performance and a 50°F ambient temperature reading.

Platform-Specific Known Issues

• A PCM software update is required to fix P0071 on some 2017-2018 Fusions with the 2.5L engine (built on or before 18-Aug-2017), as documented in TSB 17-2215.
• 2017 Hybrid and Energi models may set this code after being stored in a warm location and then driven in the cold, which is a PCM logic issue fixed by a software update per TSB 17-2072.
• Hybrid and Energi models may also exhibit P0071 along with poor HVAC performance when the outside temperature display is stuck at 50°F, per SSM 46410.
• A YouTube user with a 2016 Fusion demonstrated a successful DIY replacement of the sensor to fix a reading that was stuck at -40°F, confirming the location and ease of repair.

Mechanic-Grade Diagnostic Values

• AAT Sensor Resistance vs. Temperature — expected: Resistance should correlate with temperature. Representative values are: 32°F (0°C) ≈ 32,000 Ohms; 50°F (10°C) ≈ 19,900 Ohms; 68°F (20°C) ≈ 12,500 Ohms; 86°F (30°C) ≈ 8,000 Ohms.. Failure: Infinite resistance (open circuit), near-zero resistance (short circuit), or a resistance value that does not change when the sensor is heated or cooled.
• AAT Sensor Connector Voltage — expected: With the sensor unplugged and ignition on, one pin should show a 5-volt reference signal from the PCM. The other pin is the signal return/ground.. Failure: No voltage indicates a wiring problem or PCM issue. Voltage significantly different from 5V could also indicate a circuit problem.

Scan Tool Commands That Help

• HVAC Control Panel: AAT Sensor Data Reset — After replacing the AAT sensor, this procedure may force the system to update to the current raw temperature value instead of waiting for the filtered data to update over a drive cycle. On the HVAC panel, press and hold the A/C and Recirculation buttons simultaneously for several seconds until the display flashes.

Wiring & Ground Locations

• C146 — A key harness connector located behind the driver's side (LH) headlamp assembly.. The AAT sensor wiring passes through or near this major connector. Damage or corrosion at this point could interrupt the sensor signal to the PCM.
• G102 — A chassis ground point located on the passenger's side (RH) of the engine compartment.. A poor ground connection at this point for the sensor circuit could cause erratic or incorrect readings, leading to a P0071 code.

Real Owner Repair Stories

• YouTube video by 'Repairs and Stuff' (2016 Ford Fusion SE) — Outside temperature display was stuck at -40°F on startup, then would change to 50°F after a few minutes. This caused the automatic climate control to function incorrectly.
  ❌ Tried (didn't work) The user went straight to the most likely hardware cause.
  ✅ What actually fixed it Replacing the ambient air temperature sensor (Part No. DY1160) located in the front lower grille. The repair was simple, requiring no tools or just a pair of pliers, and immediately corrected the temperature reading and A/C function.

When the Usual Fixes Don't Work

• In some cases, the fix is not straightforward. A Ford Fusion owner on Reddit reported that after replacing the AAT sensor and repairing a break in the wire, the P0071 code immediately returned after clearing. This indicates that while the sensor and wiring are the most common culprits, the issue can sometimes be more complex, potentially involving the PCM's internal logic or another interacting system, even after the TSB-related software updates have been performed.

OEM Part Supersession History

• FC4Z-12A647-A, DY-1345 → AU5Z-12A647-B — Standard part revision and consolidation by Ford.
  Heads up: The newer part number AU5Z-12A647-B is the correct replacement for the listed predecessors.