Dear Ask a Mechanic,
My 2009 Toyota Corolla has a warning light that stays on. I’ve brought it into the dealership, and they say it has to do with the tire pressure. At the time, they did something to clear the warning light and checked the tires. They said it would cost a lot of money to replace the sensor that is causing the warning light. They said that if I didn’t want to pay for it to be replaced, I could just live with the warning light being on and just be sure to check the tires now and again. But it’s annoying seeing the light on every time I’m driving. What do you think? – Tire tired
Automotive tire pressure monitoring systems, generally referred to as TPMS, have been around for more than 30 years. The earliest design, co-developed by Bosch and Porsche, used specialized wheels with calibrated pressure switches built into them. The system was costly and not widely used.
Improvements in technology eventually allowed sensors powered by internal batteries to do the job. Each sensor transmits its information to the car’s computer using radio signals. While these sensors have become smaller and capable of transmitting more information (such as tire temperature and sensor battery strength), the basic premise of what is known as direct TPMS remains the same today.
At the same time, anti-lock brakes (ABS) have also become more common and its controllers are now much more sophisticated. This is also a way that tire pressure can be detected. Known as indirect TPMS, this method uses the ABS wheel speed sensors to monitor average tire speeds relative to one another.
Low pressure will result in a tire’s overall diameter becoming smaller. The tires sidewalls get smooshed out and the wheel is closer to the road, meaning the tire’s overall height is less and it will rotate slightly faster than a properly inflated tire. Eventually the average tire speeds reveal the imbalance, and the ABS module will turn on the TPMS warning light.
Automakers frequently use either type. Your Corolla uses the direct kind with four sensors in the valve stems. Since they have batteries built in them, they have a finite lifespan. The battery is attached to the sensor, so replacing the sensor is required to replace the battery, and the tires must come off the wheels to do it.
Like you, I hate having warning lights on, and TPMS is a useful system when it works, so I’d want to fix it. Just know that the sensors are probably all the same age, so even if only one is dead, the others may not be far behind.
While factory sensors can be expensive, aftermarket sensors are widely available, functionally interchangeable and potentially far less costly. I would expect that it will cost well under $100 for each sensor to be replaced and installed. You’ll most likely have to go an independent shop for this option, though.
Ask a Mechanic is written by Brian Early, a Red Seal-certified automotive technician. You can send your questions to firstname.lastname@example.org. These answers are for informational purposes only. Please consult a certified mechanic before having any work done to your vehicle.
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