Otto Explains — Vehicle Electronics
What Do the Main Sensors in Your Car Actually Do?
Short Answer
Sensors measure what is happening in and around the car and report it to the control units, which use the readings to run the engine, brakes and safety systems. Engine sensors track things such as engine speed and position, the air coming in, the coolant temperature, the oxygen left in the exhaust and knocking. Wheel speed sensors feed the ABS and stability control, tyre pressure monitoring warns of a low tyre, and cameras and radar support driver assistance. A faulty sensor can cause poor running, higher fuel use or a warning light, but a sensor fault code does not always mean the sensor itself has failed.
Which sensors keep the engine running properly?
Several sensors work together so the engine control unit can time the fuel and spark correctly. The crankshaft position sensor tells it the crankshaft's position and the engine speed, and the camshaft position sensor tells it which cylinder is due to fire. The mass air flow sensor, usually in the intake duct between the air filter and the throttle body, measures the air coming into the engine, and with the intake air temperature the control unit works out how much fuel to inject. The coolant temperature sensor reports how warm the engine is, so injection and ignition can be adjusted to it. The oxygen sensor in the exhaust measures the oxygen left after combustion, so the fuel mixture can be fine-tuned for the catalytic converter and low emissions. The knock sensor on the engine block listens for the vibration of knocking combustion, and the control unit then retards the ignition on that cylinder until the knocking stops.
What do the wheel speed and tyre pressure sensors do?
Wheel speed sensors report how fast each wheel is turning to the ABS and stability control (ESP) unit, which uses the readings to stop the wheels locking under hard braking and to reduce loss of grip. Tyre pressure monitoring works in one of two ways. Direct systems have a sensor at each wheel that sends its reading by radio. Indirect systems have no pressure sensor; they use the wheel speed signals to spot a tyre that is turning differently, which makes them less precise and unable to show which tyre is low. When the tyre pressure warning comes on, one manufacturer advises slowing down, avoiding sudden steering and braking, then stopping safely to check the tyres, and warns that ignoring it is dangerous. A light that flashes at start-up and then stays on usually points to a fault in the monitoring system itself.
Which sensors do driver assistance systems use?
Features such as automatic emergency braking and adaptive cruise control rely on a camera, usually at the top of the windscreen, and radar sensors in or near the bumpers. Their view matters. Manufacturers warn that dirt, snow, ice or water on the sensors, heavy rain or fog, tint or coatings on the windscreen in front of the camera, and stickers or accessories near the sensors can reduce detection, so the systems may not work properly. The car usually tells you when a system is unavailable, but it is worth keeping these areas clean and clear.
What happens when a sensor goes wrong?
The effect depends on the sensor. A crankshaft or camshaft sensor fault can cause stalling, hesitation, misfires, or a car that starts only some of the time or won't start at all. A faulty or dirty air flow sensor can cause poor starting, an unstable idle and hesitation. A faulty coolant temperature sensor can raise the idle speed and the fuel use and make starting harder, and a faulty knock sensor can reduce power and increase fuel consumption, which is one reason behind high fuel consumption. Many of these faults also switch on the check engine light.
Does a sensor fault code mean the sensor needs replacing?
Not necessarily. A fault code tells the technician which signal looks wrong, not why. Broken or shorted wiring, corrosion and poor connections cause sensor faults too, and a code about the engine not warming up properly can come from a faulty thermostat rather than the temperature sensor. That is why a diagnostic scan is followed by testing before a part is replaced. AutoTronix Lab carries out diagnostics and electrical repairs, including sensor and wiring faults.
Common Questions
- Why doesn't my tyre pressure warning show which tyre is low? Your car may use an indirect system. It has no pressure sensor in the wheels and works out a low tyre from the wheel speed signals, so it cannot always say which tyre it is. Check all four.
- Can stickers or a tinted windscreen affect the car's sensors? They can affect the driver assistance camera and radar. Manufacturers warn that tint or coatings in front of the camera and stickers or accessories near the radar can reduce detection, so the systems may not work properly.
- Can a bad sensor make my car use more fuel? Yes. A faulty coolant temperature sensor or knock sensor can increase fuel consumption, and air flow and oxygen sensors help the engine control unit get the fuel mixture right.