Dear Ask a Mechanic,
Every few weeks the tire pressure warning light in my 2017 Ford F-150 turns on. The dash display shows that it is the left rear tire that has low pressure, but the only tire that needs air when I check is the front left wheel. It has a leak that I plan to get fixed. I am confused why it warns me the wrong tire needs air. – Under Pressure
Get that tire looked at sooner rather than later. An underinflated tire can result in increased fuel consumption, shortened tread life and even a disastrous blowout.
The tire pressure monitoring system you are referring to is a relatively new addition to automobiles. It first appeared in Porsche’s 959 supercar in 1986 and came into widespread use more than a decade later.
Although this system has been mandatory in new vehicles sold in the United States since 2008 – it was one of the actions taken in the wake of the deadly Firestone tire failures and Ford Explorer rollovers in the late 1990s – it is not required in Canada.
These systems use two types of tire pressure monitoring: direct and indirect.
With indirect monitoring, it uses the anti-lock braking system’s wheel speed sensors to look for differences in the average tire rotation speeds. An underinflated tire will rotate faster and change the diameter of the tire.
Direct pressure monitoring, which is utilized in your F-150, uses sensors that measure the actual pressure in each tire. They are usually attached to the valve stem inside the wheels. Each sensor has a transmitter and an ID code so the system knows the tires “belong” to that vehicle and where they are located.
While several tire pressure systems used by several automakers will figure this out on their own (“the same four sensors keep following me around, therefore they must be mine”), others, like in your Ford, need to be trained.
I believe the issues you are describing it that the location of your tires was not properly learned by the system the last time they were installed.
Ford has two methods for training the system. One method is, with the key on, to switch the hazard light on and off three times in 10 seconds. The other is a brake and ignition sequence that is too lengthy to explain here. Luckily you can use the first method with your F-150.
Once the horn chirps to tell you training’s been initiated, in a clockwork order starting with the front driver’s side wheel, add or remove air from each tire until the horn chirps to confirm it recognizes it. After, inflate all four tires to the proper pressures indicated on the placard located in your door jamb.
And get that tire fixed.
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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