AS button in a car: decoding, purpose and how to use it correctly

Mysterious button with the inscription AS: why does it appear in modern cars?

You get into a new or used car, start the engine - and suddenly notice an unfamiliar button on the panel with the inscription AS. What is it? Why isn't it in older cars? And most importantly, is it possible to press it without fear of breaking something in the electrical system?

Button AS (from English Auto Start or Auto Stop) is an element of the system Start-Stop, which automatically stops and starts the engine to save fuel. This technology first appeared in Toyota Prius back in the 90s, but spread massively only after 2010, when European environmental standards became stricter. Today AS installed on Volkswagen, BMW, Ford, Hyundai/Kia and even for some models LADA (for example, Vesta and XRAY with engines 1.8).

But here lies the catch: different manufacturers AS works differently. Somewhere it's just a system switch Start-Stop, and somewhere - part of a more complex energy management algorithm. Next, we’ll look at all the nuances so that you know exactly when to press this button and when it’s better to leave it alone.

Explanation of AS: Auto Start or Auto Stop?

The biggest confusion arises from the double meaning of the abbreviation. In 80% of cases AS stands for Auto Stop - that is, a system that automatically turns off the engine when stopping (for example, at a traffic light). But in some models (especially BMW and Mercedes-Benz) the same button is responsible for Auto Start - a function that starts the engine in advanceto warm up the interior or battery.

How to understand what it means AS in your car? Here are the key signs:

• 🔋 Auto Stop: The button is usually located next to the gear selector or on the center console. When pressed, the indicator lights up A OFF on the dashboard - this means the system is disabled.
• 🔥 Auto Start: the button can be labeled as AS/RS (Remote Start) and is located on the key fob or in the multimedia menu. Includes remote start.
• ⚙️ Hybrid systems: in Toyota and Lexus AS often means Accessory Mode - a mode in which the electronics work, but the engine is turned off.

If there is no clear explanation in the car manual, try this test: turn off the car, press AS and look at the dashboard. If the battery icon or the message READY - this is Auto Start. If nothing happens, it is most likely Auto Stop.

How the Auto Stop (AS) system works in detail

System Auto Stop (aka Start-Stop, ISS at Mazda or MSA at Mitsubishi) is built on the interaction of several sensors:

1. Speed sensor — determines that the car has stopped (speed = 0 km/h).
2. Brake pedal position sensor — checks that the driver has pressed the brake (in automatic transmissions) or depressed the clutch (in manual transmissions).
3. Battery charge sensor — if the charge is below 70-75%, the system will not work.
4. Engine temperature sensor - with a cold engine (+20...+30°C) Auto Stop may not turn on.

The operating algorithm is simple:

1. You stop at a traffic light, put it in neutral (manual) or keep your foot on the brake (automatic).
2. After 1-3 seconds the engine turns off.
3. As soon as you release the brake pedal (on an automatic) or press the clutch (on a manual), the engine starts instantly.

But there are nuances! For example, in Volkswagen Golf and Audi A3 The system only works if:

• 🔌 The battery is more than 75% charged.
• 🌡️ Engine temperature is higher +30°C.
• 🚗 Doors and hood are closed.
• 💺 The driver's seat belt is fastened.

Why doesn't the engine turn off when idling?

If the Auto Stop system does not operate, check:

- Battery charge (may be below threshold).

- Engine temperature (the system turns off when cold).

- Is the air conditioning or heated windows turned on (some models do not turn off the engine when the load on the generator is high).

- Condition of the clutch/brake pedal position sensor (a common cause of malfunction).

Table: Auto Stop trigger conditions in popular models

Make/Model	Minimum battery charge	Engine temperature	Additional terms
Volkswagen Polo, Skoda Rapid	75%	≥ +25°C	The air conditioner is turned off or in eco mode
Toyota Corolla (hybrid)	80%	≥ +30°C	Foot on the brake pedal (even on manual)
Ford Focus 3rd generation	70%	≥ +20°C	The steering wheel is turned no more than 45° (steering angle sensor)
Hyundai Solaris/Kia Rio	72%	≥ +28°C	Doors closed, seat belt fastened

Important: On vehicles equipped with an automatic transmission, the Auto Stop system may not function if the selector lever is in the N (neutral) instead D (drive). This is done for safety - to prevent the car from rolling on a slope.

When should you turn off Auto Stop (AS)? 5 real cases

Many drivers immediately turn off AS, because the system seems intrusive. But there are situations when necessarily you need to deactivate it - otherwise you risk damaging the car.

⚠️ Attention: If your car has a non-original battery (especially a cheap acid battery), the system Auto Stop can kill him quickly. Modern cars require AGM or EFB-batteries that can withstand frequent starting cycles.

Disable AS in the following cases:

• 🔋 The battery is weak or old (more than 3-4 years). Each automatic start eats up 0.2-0.5% charge.
• 🚗 You are driving in a traffic jam with frequent stops (more than 10 times in 5 minutes). The system does not have time to restore oil pressure.
• 🌡️ It's colder outside -10°C. The oil thickens, and frequent starts wear out the starter.
• 🔧 You are towing a trailer or transporting a heavy load. The engine needs stable operation to cool.
• 🛠️Caught on fire Check Engine or battery failure indicator. This means the system is operating in emergency mode.

How to disable AS:

1. Click the button AS once. The icon should appear on the dashboard A OFF or Start-Stop OFF.
2. In some models (for example, Renault) you need to hold the button for 3-5 seconds.
3. On BMW and Mercedes the shutdown can be reset after each engine start.

Myths about Auto Stop: what is true and what is not?

There are a lot of rumors circulating on the Internet about AS. Let's look at the most popular:

• ❌ "Auto Stop ruins the starter" — Not really. Modern starters are designed to 300 000–500 000 startup cycles (this is 10-15 years of operation). But if the starter is already worn out, frequent starts will finish it off faster.
• ❌ "The system saves 20% fuel" — Exaggeration. Real savings in the city - 3-7%, on the highway - almost zero.
• ✅ "Auto Stop wears out the battery faster" — True, but only if the wrong battery is installed. AGM-batteries cope with the load.
• ❌ "You can disable AS permanently via the diagnostic connector" — Dangerous. This will cause errors in the ECU and may disable other systems (for example, regenerative braking in hybrids).

Auto Start (AS): How does remote start work?

If in your car AS means Auto Start, then that's a completely different story. This system allows start the engine remotely - from a key fob, mobile application or even by timer. Most often found in cars with:

• 🔑 Keyless Access (keyless entry),
• 📱 Support Car-Net (Volkswagen), ConnectedDrive (BMW) or Toyota Remote.

How it works:

1. You press the button AS on the keychain or in the app.
2. The system checks:
   • Are the doors and hood closed?
   • Is the car in neutral (or in P for automatic transmission),
   • Is the fuel level sufficient?

If everything is OK, the engine starts and runs. 5-15 minutes (depending on settings).

Benefits Auto Start:

• ❄️ Warming up the interior in winter (especially important for Diesel-engines).
• 🔋 Recharging the battery during long-term parking.
• 🚗 Possibility to run on a timer (for example, every day at 7:00).

⚠️ Attention: Remote start prohibited by law in some countries (eg Germany and Austria) due to environmental regulations. There are no restrictions in Russia, but if the car is parked in an enclosed space (garage, parking lot), this can lead to carbon monoxide poisoning.

How to set up Auto Start (for example Volkswagen Tiguan):

1. Go to menu Settings → Comfort → Remote start.
2. Select engine running time (5, 10 or 15 minutes).
3. Activate the option Trigger by temperature (for example, when -10°C).
4. Save the settings and check the operation from the key fob.

- Is the key fob within range (usually up to 50 meters).

- Is the mode turned on? Valet (service mode that disables some functions).

- Are there any errors in the immobilizer system (may block startup).-->

Common malfunctions of the AS button and how to fix them

If the button AS does not respond to presses or the system does not work correctly, the reasons can be either simple (dead battery) or serious (ECU malfunction). Let's look at typical problems and ways to solve them.

1. The AS button is not pressed or the indicator does not light up

Possible reasons:

• 🔋 The battery in the key fob is dead (if the button is on the key).
• 🔌 The fuse has blown (look in the fuse box F30 or Start-Stop).
• 🛠️ The button contacts have oxidized (you need to disassemble the panel and clean it).

2. Auto Stop does not turn off the engine

What to check:

• 🌡️ Engine temperature (should be higher +25...+30°C).
• 🔋 Battery charge (if below 70%, the system turns off).
• 🚗 Position of the pedals (on the manual the clutch pedal should be depressed, on the automatic – the brakes).
• 🔧 Oil pressure sensor (if it shows low pressure, Auto Stop will not work).

3. Auto Start does not start the engine remotely

Common reasons:

• 📶 Key fob out of range (maximum 30-50 meters).
• 🔑 Mode enabled Valet (disables remote functions).
• 🚪 One of the doors or the hood is not closed (the system blocks the start).
• 🔌 Low battery (remote start requires more power than regular start).

If all else fails, check for errors using a diagnostic scanner (for example, ELM327). The most common codes are:

• P050A — low pressure in the brake system (blocks Auto Stop),
• P0615 - starter circuit malfunction,
• B1188 — Clutch pedal position sensor error.

AS in hybrid cars: features and differences

In hybrids (for example, Toyota Prius, Lexus NX, Ford Fusion Hybrid) button AS often serves a dual function:

1. Auto Stop — turns off the internal combustion engine (ICE) when stopping.
2. Accessory Mode — puts the car into a mode where only the electronics work (without starting the internal combustion engine).

Differences from conventional cars:

• 🔋 In hybrids AS can activate the mode EV Mode (propulsion only on electricity).
• 🛢️ The engine turns off more often - even when slowing down to 7-10 km/h (in ordinary cars the threshold is 0 km/h).
• ⚡ Hybrid battery (Ni-MH or Li-ion) is not afraid of frequent starts, unlike conventional lead ones.

How to use AS in a hybrid:

1. To turn off Auto Stop click the button AS once. An icon will appear on the dashboard A OFF.
2. To activate Accessory Mode press and hold the button 3-5 seconds (the machine will go into mode READY without starting the engine).
3. In some models (for example, Lexus ES Hybrid) AS can be configured via menu Drive Mode Select.

⚠️ Attention: In hybrids Toyota and Lexus when disconnected AS The regenerative braking system is also deactivated. This may increase fuel consumption by 10-15%.

FAQ: Answers to frequently asked questions about the AS button

Is it possible to drive with the AS system permanently switched off?

Yes, you can. This will not cause any harm to the car. However, please note that:

• 🛢️ Fuel consumption in the city will increase by 3-7%.
• 🔋 The battery will last longer (fewer cranking cycles = less wear and tear).
• 🌿 In some EU countries (for example, France) driving with the Auto Stop may be considered a violation of environmental standards (fine up to €135).

Why does the engine not turn off after pressing AS, although all conditions are met?

The reasons may be the following:

• 🌡️ The engine is not warmed up (temperature is below +25...+30°C).
• 🔋 The battery is discharged (even if the light is not on, check the voltage with a multimeter - it should be at least 12.4 V).
• 🚗 The air conditioning or heated windows are turned on (some models do not turn off the engine when the load on the generator is high).
• 🛠️ The oil pressure sensor or clutch/brake pedal position sensor is faulty.

Try turning off the car, turning off the ignition 30 seconds, then run again - sometimes this resets temporary errors.

How do I know which battery is needed for a car with AS system?

Cars with Auto Start-Stop require special batteries:

• AGM (Absorbent Glass Mat) - the best option, withstands 300 000+ startup cycles.
• EFB (Enhanced Flooded Battery) - a budget alternative, suitable for cars without regenerative braking.

How to check compatibility:

1. Look at the sticker on the old battery - if there are inscriptions AGM, Start-Stop or EFB, buy the same one.
2. Look in the car manual - the recommended type is indicated there.
3. Use online directories (eg. Bosch, Varta or Mutlu), where you can select a battery based on your car model.

Never install a regular acid battery! It will fail in 6-12 months due to deep discharges during frequent starts.

Is it possible to disable AS myself permanently without contacting the service?

Technically yes, but not recommended. Methods:

• 🔧 Physical shutdown: remove fuse Start-Stop (but this may also disable other systems, such as recuperation).
• 📱 Software shutdown: via diagnostic adapter (ELM327 + program VCDS for VW or ISTA for BMW).
• 🔌 Sensor decoy: Connect a resistor to the clutch pedal sensor so that the system “thinks” the pedal is depressed.

Risks:

• ⚠️ May catch fire Check Engine.
• ⚠️ In hybrids, shutdown AS sometimes blocks transition to EV Mode.
• ⚠️ When updating the ECU firmware, the settings will be reset.

Does the AS system affect engine life?

Depends on engine type and operating conditions:

• ✅ Modern engines (after 2015) with system Start-Stop They have a reinforced starter and flywheel, so wear is minimal.
• ❌ Old engines (until 2010) are not designed for frequent starts - the life of the starter and bearings may be reduced by 15-20%.
• ⚠️ Diesel engines more sensitive to AS, since the glow plugs turn on every time you start.

To reduce wear:

• Use approved oil C2/C3 (low ash content, resistant to frequent starts).
• Change the oil every 10,000 km (instead of 15,000 km).
• Disable AS in traffic jams and when towing.