P0071
PowertrainAmbient Air Temperature Sensor Performance
Usually the first thing you notice is the outside temperature reading on the dash doing something daft, showing 30 degrees on a frosty morning or jumping around as you drive. Sometimes the climate control gets confused with it, and on a lot of newer cars the stop-start packs up. Behind all that, the ECU has worked out that the ambient air temperature sensor isn't agreeing with the intake air and coolant sensors. When those three readings drift too far apart from each other, the ECU flags the ambient sensor as the odd one out and logs P0071.
ⓘ Information only. This page provides general educational information about fault code P0071. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0071 mean?
P0071 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Ambient Air Temperature Sensor Performance.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light on, often with no change at all to how the car drives
- • Outside temperature display reading wildly wrong or flickering up and down
- • Stop-start system refusing to work (common on VAG and Ford models that rely on the ambient reading)
- • Air con or climate control behaving oddly, not cooling or heating to the temperature you've set
- • A small dip in fuel economy on some vehicles
Possible causes
Causes commonly associated with P0071, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. The sensor itself has failed. It's a cheap little component and they do just die, especially after years of sitting out in the weather behind the grille
- 2. Corroded or loose connector at the sensor. It lives in the firing line of road spray and salt, so the pins corrode and the contact goes intermittent
- 3. Damaged wiring in the sensor circuit, chafed, snapped or green with corrosion from water getting into the loom
- 4. Sensor knocked out of position or left dangling after front-end work, so it's reading engine bay heat instead of outside air
- 5. Debris packed in around the sensor, a build-up of leaves, mud or even a road-kill insect blocking airflow over it
- 6. Less often, a fault in the body control module or PCM that handles the sensor reading
- 7. extreme cold or heat pushing the sensor briefly outside its expected window, which can throw a one-off code
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Pull the freeze-frame data and check whether any temperature-related codes (P0111, P0112, coolant sensor faults) are logged alongside it. That tells you whether you're chasing one sensor or a wider electrical problem.
- 2. Find the sensor, usually tucked behind the front grille or bumper, and have a proper look. Check it's clipped in where it should be, not hanging loose, and that nothing's caked over the front of it.
- 3. Inspect the connector and the wiring back from it. You're looking for green corrosion on the pins, chafed insulation, or water sitting in the plug. Clean the terminals up if they look tired.
- 4. With the ignition off and the car cooled right down, put a multimeter across the sensor and check the resistance against the spec for the current outside temperature.
- 5. Watch the live data with a scan tool after the car has sat overnight. Ambient, intake air and coolant should all read close to the same on a cold start. If the ambient sensor is the only one out of step, that's your answer.
- 6. If the sensor and wiring both check out, clear the code and drive it. Comes straight back, replace the sensor. Comes back now and then, you've got an intermittent wiring or connector fault to track down.
Common questions about P0071
If I clear the code will it stay gone, or just pop back up? +
Depends entirely on what caused it. If it was a genuine one-off, say a freak cold snap or a bit of debris that's since blown clear, clearing it and driving a few miles may be the end of it. But if the sensor's actually failed or there's corrosion in the connector, it'll be back within a journey or two. The honest test is to clear it, drive normally, and see if it returns. A code that comes straight back is telling you the fault is still live, so don't keep wiping it and hoping.
What's the worst that happens if I just leave it? +
The car won't strand you. The engine runs the same and you can drive it indefinitely. The real nuisances are the dead stop-start, the wrong temperature display, and climate control that doesn't quite do what you ask. The one thing worth keeping an eye on is the cause: if it's electrical corrosion getting into the loom, that same damp and salt can spread to neighbouring circuits over time, so a sensor fault you ignore today can turn into a more annoying wiring hunt later.
How quickly do I actually need to sort this? +
It's not urgent in the sense of get-it-towed urgent. There's no danger to the engine and nothing's going to leave you stuck on the motorway. But if the warning light is on it'll cause an MOT advisory or fail depending on how the tester reads it, so book it in before your test is due. Realistically you're looking at a low two-figure part and under an hour's labour if it's just the sensor, more like mid three figures if there's wiring repair or trim removal involved. Cheap enough that there's no good reason to live with the light on for months.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →