Tire Pressure Monitoring Systems (TPMS)

The New Rule and Current Technologies

TPMS Safety Standard:  The National Highway Traffic Safety Administration (NHTSA) established Federal Motor Vehicle Safety Standard (FMVSS) No. 138 to cover tire pressure monitoring systems.  The rule is authorized under the “TREAD Act,” a law passed in 2000 to expand tire safety standards as a result of the Ford/Firestone crisis.

Covered Vehicles & Compliance Deadlines:  The rule applies to all new passenger cars, multipurpose vehicles, trucks and buses weighing 10,000 pounds or less.  The rule was phased-in: 20% of MY 2006 cars were required to comply to have a TPMS, 70% of MY 2007 cars, 100% of all MY 2008 cars (by Sept. 1, 2007).   Vehicles produced by final-stage manufacturers and alterers must be equipped with a compliant TPMS by September 1, 2008.

Detecting Under-Inflation:  Vehicle manufacturers are required to install a system that can detect when one or more of the vehicle’s tires are 25% or more below the recommended cold inflation pressure, or a minimum level of pressure specified in the standard, whichever pressure is higher.  NHTSA has published the minimum activation pressures at Table 1 of FMVSS No. 138 (ex: 20 lbs for P-metric standard tires).

25% Threshold: NHTSA believes a tire is "significantly" under-inflated when the tire pressure has fallen to 25% below the manufacturer's recommended inflation level.

Technology Neutral:  The rule is a performance standard that is technology neutral.  NHTSA estimates that the new systems installed on the auto production line will cost between $48 and $70 per vehicle.

Warning Light & Malfunction Indicator:  A dashboard warning light must alert drivers if there is a loss of 25% of the recommended pressure in any wheel, or if the TPMS system is not operating properly.  The warning systems need not measure tire pressure until a motorist has driven between 30 and 60 miles per hour continuously for 20 minutes.  The yellow warning light must stay on as long as any of the vehicle's tires remain significantly under-inflated and the vehicle is operating.  The TPMS must also include a yellow TPMS malfunction indicator (using the acronym "TPMS") to alert the driver when the system is non-operational.  A malfunction must be detected within 20 minutes of occurrence. At the manufacturer’s discretion, the warning and malfunction lights can be separate lights or they can be combined.  If combined, it shall have the “TPMS” acronym, flash for 60-90 seconds when a malfunction is detected, and thereafter remain continuously illuminated until the situation causing the malfunction has been corrected.

Spare Tire:  The TPMS is not required to monitor the spare tire, even when it is installed on the vehicle.

Owner’s Manual:  The owner's manual must include a statement about the TPMS, including reference to the fact that both aftermarket tires and rims may affect the TPMS's continued functionality, stressing the driver's ongoing responsibility for regular tire maintenance, and alerting consumers that some replacement tires may call for an inflation pressure different than what is reflected on the vehicle placard.

Major Issues Impacting SEMA Members>

Tampering/Make Inoperative Prohibition: when the rule was being drafted, SEMA and the National Automobile Dealers Association (NADA) expressed the need to hold manufacturers, dealers, installers, repair facilities harmless from the “make inoperative” provision (49 U.S.C. 30112) of the Motor Vehicle Safety Act.  This is the provision that forbids a person from knowingly manufacturing, selling or installing a product that will take a vehicle or item of motor vehicle equipment out of compliance with a motor vehicle safety standard.   NHTSA ruled:

  • TPMS Must Be Fully Operational Prior to First Sale:  Vehicle manufacturers must certify vehicle compliance under the standard with the tires installed on the vehicle at the time of initial vehicle sale.  NHTSA recognizes that dealers may replace the vehicle's factory-installed tires with other tires (and wheels), including ones with a different size and/or recommended cold tire inflation pressure. The TPMS must perform properly with any such tires (and wheels), because the vehicle could be equipped with those tires at the time of initial sale. The dealer has violated 49 U.S.C. 30112 if the installed tires or tire/wheel combination prevents the TPMS from functioning properly.
  • TPMS Does Not Have To Be Fully Operational After First Sale:  NHTSA does not consider installation of an aftermarket or replacement tire or rim that is not compatible with the TPMS to be a “make inoperative” situation under 49 U.S.C. 30122, provided that the business entity does not disable the TPMS malfunction indicator light (MIL). 

MIL Disablement Not Permitted:  The MIL may not be disabled when aftermarket tires and rims are installed on the vehicle that are incompatible with the continued proper functioning of the TPMS.  In such cases, NHTSA believes the TPMS MIL is performing its intended function -- alerting the driver that there is a potential problem.  The MIL encourages the motorist to make the TPMS compatible while reminding them that the TPMS is currently unavailable to provide low tire pressure warnings.

Most Aftermarket Wheels And Tires Should Be Compliant, But Compliance Is Not Mandated:  There are millions of TPMS-equipped vehicles in the U.S.  Neither the agency nor the vehicle manufacturers have received reports indicating any significant performance problems with those TPMSs when replacement tires are installed on the vehicle.  NHTSA also notes that aftermarket direct TPMSs are available and that such systems may be capable of functioning regardless of the construction of the tires.  NHTSA believes that only a very small number of replacement tires (estimated at less than 0.5 percent of production) may have construction characteristics and material content that cause the vehicle's TPMS to exhibit functional problems.  For these tires, there is no clear design solution at the moment.  For all of these reasons, NHTSA has decided not to apply FMVSS No. 138 to replacement tires.

Reprogrammability:  SEMA requested that the TPMS be reprogrammable in order to accommodate alternate and replacement tires with different pressure thresholds.  SEMA expressed concern that vast numbers of vehicles will soon be TPMS-equipped annually with a variety of different and evolving monitoring systems.  SEMA said the easiest way to address the issue would be to require reprogramming as a performance requirement under the rule.  In denying the request, NHTSA contends that reprogramming will be a voluntary feature of the TPMS in the marketplace and is unaware of any problems that would prevent reprogramming.  However, it will not mandate reprogramming.  [Example:  The original car comes with tires inflated at 32 pounds.  The warning light will come if a tire drops to 24 pounds – which is 25% of 32.  However, the car owner then puts low profile wheels and tires on the car with a recommended tire pressure of 40.  25% of 40 means the warning light should come on at 30.  But unless the TPMS system is reprogrammed, it won’t trigger until 24.  That means the tire is now 40% under-inflated.]

Service Information:  SEMA requested that service information be made available to the vehicle owner to the extent that such access is available to other parties.  This would ensure that dealers, retailers, and installers would have access to all service information necessary to make replacement and alternate tires and wheels operate correctly in conjunction with the MIL.  NHTSA does not believe a regulatory mandate is necessary.  It believes market forces will operate sufficiently to share information – in the same fashion as cars has been repaired and upgraded for many decades.