Vice President of Marketing and Sales
Schrader Electronics Ltd.
3255 West Hamlin Road
Rochester Hills, MI 48309
Dear Mr. Wacker:
This responds to your letter requesting an interpretation clarifying specific issues with respect to Federal Motor Vehicle Safety Standard (FMVSS) No. 138, Tire Pressure Monitoring Systems. Specifically, you asked whether a tire pressure monitoring system (TPMS) must warn drivers of low tire pressure in particular circumstances, such as up to the maximum speed of the vehicle, and under all road surfaces and road conditions (including ice, snow, rain, gravel, dirt, and so forth). You also asked whether a TPMS must comply with FMVSS No. 138 if a vehicle dealer changes the tire and wheel combination prior to first sale. Additionally, you asked whether a TPMS must warn a driver if a tire begins a journey underinflated, but within 20 minutes passes the under-inflation threshold by warming up while traveling. Based on the information you have provided and our analysis below, our answers are as follows.
By way of background, the National Highway Traffic Safety Administration (NHTSA) is authorized to issue FMVSSs that set performance requirements for new motor vehicles and items of motor vehicle equipment (see 49 U.S.C. Chapter 301). NHTSA does not provide approvals of motor vehicles or motor vehicle equipment. Instead, manufacturers are required to self-certify that their products conform to all applicable safety standards that are in effect on the date of manufacture. NHTSA selects a sampling of new vehicles and equipment each year to determine their compliance with applicable FMVSSs. If our testing or examination reveals an apparent noncompliance, we may require the manufacturer to remedy the noncompliance, and may initiate an enforcement proceeding if necessary to ensure that the manufacturer takes appropriate action.
Whether FMVSS No. 138s requirements must be met under various operating conditions:
Two questions in your letter addressed the issue of whether FMVSS No. 138s requirements must be met under various vehicle operating conditions. First, you asked whether TPMS must be able to warn a driver of low tire pressure up to the maximum speed
of the vehicle. The test procedures of FMVSS No. 138 specify that a vehicles TPMS may be tested at speeds between 50 km/h (31.1 mph) and 100 km/h (62.2 mph), and that the vehicle must meet the applicable requirements when tested at any point within the range. See S5.3.2 and S5.3.6 of FMVSS No. 138. We note, however, that while vehicles are not required to meet requirements beyond those specified in the standard, it is the agencys expectation that TPMS will function normally over a wide range of operating conditions.
The same would be true regarding your second question, whether TPMS must be able to warn a driver of low tire pressure on all road surfaces and road conditions. (For Example: Ice, Snow, Rain, Gravel, Dirt, etc) S5.2 of FMVSS No. 138 specifies that the road surface is dry during testing. Again, however, we would expect a TPMS to function normally over a wide range of roadway surface conditions beyond the dry conditions specified for compliance testing.
The applicability of FMVSS No. 138 to dealer-altered vehicles:
You also asked in your letter whether a TPMS must comply with FMVSS No. 138 if a new vehicle dealer upgrades a Tire and Wheel Combination prior to the original sale of the vehicle. The answer is yes.
S5.3.7 of the standard states:
The vehicle is tested with the tires installed on the vehicle at the time of initial vehicle sale, excluding the spare tire (if provided). . . .
In the final rule establishing FMVSS No. 138, NHTSA stated that
After considering these comments related to TPMS functionality with replacement tires, we have decided to adopt the approach presented in the NPRM to require the TPMS-equipped vehicle to be certified with the tires originally installed on the vehicle at the time of initial vehicle sale. We emphasize that it would not be permissible for dealers to install tires on a new vehicle that would take it out of compliance with the TPMS standard, and to do so would violate the prohibition on manufacturing, selling, and importing noncomplying motor vehicles and equipment in 49 U.S.C. 30112.
70 Fed. Reg. 18159 (Apr. 8, 2005). NHTSA explained that If the consumer cannot expect to acquire a vehicle that meets all applicable safety standards at the time of first purchase, the purpose of Standard No. 138, and in fact all Federal motor vehicle safety standards, would be severely undermined. Thus, it would be impermissible for a dealer to sell a vehicle at first sale with tires and rims that are incompatible with the vehicles TPMS.
After first sale, the make inoperative provision of 49 U.S.C. 30122(b) would be applicable. We note that the agency discussed this provision in the context of TPMS and replacement tires and rims, including concerns that a small population of replacement tires and rims may be incompatible with a vehicles TPMS, at 70 Fed. Reg. 18160-61 (Apr. 8, 2005) and 70 Fed. Reg. 53086 (Sept. 7, 2005). Essentially, NHTSA explained that in such a situation, as long as the TPMS malfunction indicator light illuminated to warn the vehicle operator that the tires and/or rims were preventing the TPMS from functioning properly, we would consider the TPMS to be functioning properly. However, we noted that this result might be different where it could be shown that the installer of the aftermarket or replacement tires or rims knew of the incompatibility beforehand or took some other action to disable a functioning TPMS unit. NHTSA will consider whether these situations result in violations of the make inoperative provision on a case-by-case basis.
Whether the low tire pressure warning telltale must illuminate if the low-pressure situation is remedied within 20 minutes of starting to drive:
You further asked whether a TPMS must warn the driver (i.e., illuminate the low tire pressure warning telltale) if a tire is 25% below cold placard pressure at the beginning of a journey and within 20 minutes of that journey the air in the tire warms and pressure increases to above the 25% threshold for warning. FMVSS No. 138 requires a TPMS both to calibrate and to be able to detect low tire pressure (and illuminate the low tire pressure warning telltale) within 20 minutes of commencing driving under the specified test conditions. See S6 and S4.2 of FMVSS No. 138. From a safety standpoint, it is desirable to have a low tire pressure warning activate as soon as possible. The 20-minute time period was developed based on the agencys careful balancing of safety and practicability concerns, given the data that we had at the time. There would, however, be no requirement to illuminate the telltale once the air in the tire warms and pressure increases above the low-pressure threshold.
I hope this information is helpful. If you have any further questions, please do not hesitate to contact Rebecca Yoon of my staff at (202) 366-2992.
O. Kevin Vincent
 Id. These safety standards include, among other things, requirements for the vehicles braking system and, if so equipped, electronic stability control system.