What Exactly Does That TPMS Light Mean?

Thanksgiving is a special time of year, during which some friends and extended families gather to share a meal as well as each others’ company. I’m fortunate to have seen some friends who live a state away, as well as family who live in town but I don’t get to see very often. Being known as a “car guy”, friends and family often ask me car-related questions when we happen to see each other. This holiday weekend, I had that experience again.

While I was able to happily answer relatives’ questions about many different automotive topics (the Chevy Volt and its promotional $199/month lease was a favorite for conversation this year), I got two questions from late model hatchback owners about the same topic: the “tire warning light”, or TPMS (Tire Pressure Monitoring System) indicator lamp.

Close friends from out of state who were visiting drive a 2011 Hyundai Accent five-door; a relative drives a 2010 Scion xD. My relative confided to me that she was hesitant to make the twenty-mile drive to dinner in her Scion because there was a warning light on the dashboard- the “tire light” which came on the previous day when she was running errands. She told me she knew the light had to do with the tires, but that her car was serviced about two months ago and they told her that her tires were in good shape, so she walked around her car, looked each tire and poked it with her forefinger (yes, seriously), and decided to chance it since she would be able to ask me about the light when she arrived at dinner.

Our Hyundai-driving friends overheard her asking about the light, and said they had the same light on in their Accent, and that it had been on since “August or September,” but that they had looked at the tires and none of them looked flat, so they kept on driving. Because the tires didn’t look flat, they assumed there was a fault with the sensor… and the car seemed to drive just fine.

With the owners of both cars sitting in my living room, I talked to them about how the TPMS system in both of their cars works, causing the lamp on their dashboard to illuminate when when one or more of their tires falls far enough below the recommended inflation pressure to trigger the sensor which is mounted to the wheel. I explained that it doesn’t mean you should panic, but that it should prompt them to check the pressure in their tires.

Savant’s Corner: How TPMS Works
TPMS systems in the US are required by the TREAD (Transportation Recall Enhancement, Accountability and Documentation) Act.  By law, any vehicle with a gross vehicle weight rating less than 10,000 lbs. sold in the United States on or after September 1, 2007, must be equipped with a tire pressure monitoring system (TPMS). For most mainstream cars, the system consists of an air pressure sensor in each wheel, that transmits a signal to a receiver on the stationary part of the vehicle. The sensor may transmit tire actual tire pressure for display in the vehicle, or may only transmit a logic signal indicating whether the air pressure is above or below a pre-determined value, with the TPMS lamp illuminating if one or more tires is below the required value.

By law, the TPMS light has to come on when the tire is 15 percent below the certified pressure, (the tire pressure number that’s printed on the vehicle certification label located on the driver’s door jamb). In Ford vehicles, the warning light will reset itself, (go off) once tire pressures are set to recommended specifications and the vehicle has been driven above 20 mph for at least two minutes. Know that on some systems with in-wheel sensors, vehicles may need to have the sensors re-programmed if new tires or new wheels are fitted to the car. For example, when I switch between summer and winter tires on our Ford Flex, we have to “pair” the new set of wheels to the vehicle for the season, or else we get a TPMS system error message.

In the case of the Thanksgiving hatchbacks with illuminated TPMS lamps, owners of both cars told me they had tire pressure gauges in their glove boxes, but neither one was able to actually find it when I asked them to (so now I have Christmas ideas for them!). I showed each of them where to find the recommended inflation pressure (on the B-pillar with the driver’s door open in both cases), and then we checked pressures using my pressure gauge from my garage. In the Scion, the first three tires were at the recommended 33 PSI, with the fourth tire (right front) at 22 PSI. I filled it from my air compressor and recommended checking it again in a couple of days, as there is probably a slow leak in the tire.

My friends’ Accent has a recommended pressure of 32 PSI, and all four of their tires were around 26 PSI. Based on their anecdotal evidence of the TPMS light having been illuminated for a few months, they’ve probably just been losing a pound or so each month, which is even more likely to happen as the weather becomes cooler. I inflated each of their tires to 32 PSI, and told them that if they saw the light come on, they needed to check the tire pressures.

For each of these cars, the owner (hopefully) learned something. My relative learned that seeing the TPMS indicator illuminated on her instrument panel is nothing to panic about- it just means she needs to check her air pressure. My friends learned that TPMS light doesn’t necessarily turn on just because the system is broken- it turns on because one or more tires is low.

Notably, neither of these drivers had checked their owners manual to see what the light actually meant, and what they were supposed to do about it. Both cars did have the manual in the glove box. I also told them that if they saw any warning light on the dash, that book in the car would explain what it meant and what they should do about it.

Hopefully, reading this piece will help you better understand how your TPMS system works (and what to do if the light turns on), or will help you explain it to a friend or family member. TPMS systems were developed to help prevent catastrophic tire failures (like blow-outs) due to extended operation at low pressure- but the system only truly works if the driver understands what do do if the TPMS light illuminates.

Author: Kevin Miller

As Autosavant’s resident Swedophile, Kevin has an acute affinity for Saabs, with a mild case of Volvo-itis as well. Aside from covering most Saab-related news for Autosavant, Kevin also reviews cars and covers industry news. His “Great Drive” series, with maps and directions included, is a reader favorite.

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  1. One other thing to mention as the weather turns colder – ambient temperature has a BIG impact on tire pressure. Depending on the tire and wheel, he same number of air molecules may show 35 pounds in the summer and 30 pounds in the winter. As it gets colder, it’s important to check your tires and add air as necessary. In the spring as the weather warms, it’s important to ensure that they’re not overinflated.

  2. Ambient temperature does play a big role in tire pressure. It also depends on where you live and the terrain you are driving on. I am on rough terrains in the country and I am always needing to check my tire pressure and so I got a portable tire pressure monitoring system from which has been a great and handy device.

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