TPMS Service
Tell direct from indirect TPMS, service sensors and valve stems correctly, and perform the right relearn for the vehicle.
Table of contents
A tire pressure monitoring system (TPMS) warns the driver when a tire is badly under-inflated. Low tires overheat, wear quickly and can fail, so this is a safety system, not a convenience. Almost every tire job touches it. Handle the sensor badly or skip a relearn, and the customer comes back with a warning light on the dash.
What You Will Be Able to Do
- Explain what the federal TPMS rule requires and what the two warning light patterns mean.
- Tell direct TPMS from indirect TPMS.
- Service a sensor and its valve stem with the right parts and the right torque.
- Choose and complete the correct relearn procedure.
- Explain why a shop may not disable the system.
What the Law Requires
Federal Motor Vehicle Safety Standard 138 has required TPMS on new passenger vehicles and light trucks since the 2008 model year. It covers vehicles of 10,000 pounds gross vehicle weight rating or less, with some exceptions such as dual-wheel vehicles. The system must:
- Light the low pressure warning within 20 minutes of a tire falling 25 percent below the pressure on the door placard, or to a minimum set in the rule, whichever is higher.
- Warn the driver when the system itself has a fault.
The light has two meanings:
| Telltale | Meaning | Where to look |
|---|---|---|
| Stays on solid | One or more tires are low | Check all tire pressures |
| Flashes for 60 to 90 seconds, then stays on | The system has a fault | Scan for codes, test sensors |
Worked Example: The Warning Point
A vehicle placard says 36 psi. A 25 percent drop is 36 x 0.25 = 9 psi, so the light can stay off until the tire is near 27 psi. A tire at 28 psi is badly low, yet the light may not be on. TPMS never replaces a pressure gauge.
Tires also lose about 1 psi for every 10°F drop in temperature. If a tire set to 34 psi on a warm 70°F day meets a 30°F morning, it will read about 30 psi. That is why the light often comes on during the first cold snap and goes off after a few miles.
Why a Shop Cannot Disable It
Federal law, 49 USC 30122, bars manufacturers, dealers, rental companies and repair businesses from knowingly making a required safety system inoperative. National Highway Traffic Safety Administration (NHTSA) letters apply this to TPMS: disabling the warning light is a violation. So is installing wheels and tires that leave a working system with no way to function. Taping over the light, removing the bulb, or coding out the system is not an option, even if a customer asks. Some states add their own rules for TPMS in safety inspections.
Direct and Indirect Systems
| Feature | Direct TPMS | Indirect TPMS |
|---|---|---|
| Hardware | Battery sensor in each wheel | No sensor in the wheel |
| How it detects low tires | Measures pressure and sends it by radio | Uses ABS wheel speed sensors |
| What the driver sees | Often pressure per tire | One warning light |
| After service | Often needs a relearn | Reset through a button or menu |
Direct TPMS puts a sensor inside each wheel, usually on the back of the valve stem. Each sensor has a unique ID and a sealed battery, and sensors often last 5 to 10 years. You replace the sensor when the battery dies, since most batteries cannot be replaced.
Indirect TPMS watches wheel speed. A low tire has a slightly smaller rolling circumference, so it turns faster than the others. The ABS sensors see the difference and the system warns the driver. It needs a reset after tires are inflated, rotated or replaced, so that it can learn the new baseline. It may not catch four tires that all lose air evenly.
Sensor Parts and Torque Values
Direct sensors come in two main styles.
- Clamp-in: A metal stem passes through the wheel and is held by a nut, with a grommet sealing it.
- Snap-in: A rubber stem pulls through the valve hole like a standard stem. The sensor is fastened to the stem by a screw.
Different parts have different torque figures, and these are easy to confuse. Always use the vehicle or sensor maker's number and an inch-pound torque tool.
| Part | Typical torque | Notes |
|---|---|---|
| Valve core | 3 to 5 in-lb, often 4 in-lb | Very low, use a preset tool |
| Clamp-in stem nut | About 35 to 80 in-lb | Varies widely by maker |
| Snap-in sensor screw | Preset by the tool | Usually a one-time screw |
Note the unit. An inch-pound is one-twelfth of a foot-pound, so 35 in-lb is less than 3 ft-lb. Over-torque a clamp-in nut and you can crack the stem or separate it from the sensor, which often shows up as a slow leak weeks later. Under-torque it and the grommet will not seal. Do not tighten by feel.
Service Kits
When a tire comes off a wheel with a direct sensor, service the valve so that it stays sealed. A TPMS service kit includes a new core, cap, grommet, nut and washer, as the sensor requires. Replace these parts every time a tire is dismounted, or whenever a sensor is replaced.
- Use the nickel-plated valve core made for the stem. A plain brass core in an aluminum stem can corrode and seize.
- Use a plastic cap, or the cap the sensor maker supplies. A metal cap on an aluminum stem can seize onto the stem.
- Do not put lubricant or sealant on a clamp-in stem's threads or seal, because it changes the torque.
- A corroded stem can snap off when you remove the cap. Tell the customer if you see heavy corrosion, and quote a new sensor before you start.
Replacing a Sensor
- Test all sensors with a TPMS tool before the job starts. Record which ones respond. This proves what was working before you touched the car.
- Deflate the tire and break the beads, keeping the tool away from the sensor.
- Remove the old sensor. Clean the valve hole and check the wheel for corrosion.
- Fit the correct sensor for that vehicle. Aftermarket sensors may need to be programmed, or cloned from the old sensor's ID, with a TPMS tool.
- Fit the sensor according to the instructions, with new seals, and torque it to spec.
- Mount the tire, set the pressure, and confirm the sensor sends a signal.
- Perform the relearn if the vehicle needs one.
Cloning copies the old sensor's ID to the new one, so the vehicle treats it as the same sensor. That can avoid a relearn, but you must be able to read the old sensor.
Relearn Procedures
A relearn tells the vehicle which sensor IDs belong to it and, on many cars, where each sensor sits. You may need one after you replace a sensor, and on many vehicles after a rotation. Look up the exact procedure in your TPMS tool or the service information, because it can change by model and year.
| Type | How it works | Tool needed |
|---|---|---|
| Stationary | Put the car in learn mode and trigger each sensor in order | TPMS tool, or a magnet on some older cars |
| Self-learn (drive) | Drive the vehicle and it finds the sensors itself | None for the relearn |
| OBD | Write sensor IDs to the module through the diagnostic port | Scan or TPMS tool |
Procedures commonly differ by brand. Use these only as a guide to what to expect:
| Brand group | Common method |
|---|---|
| GM | Learn mode, then trigger each sensor in order with a tool or by changing pressure |
| Ford | Learn mode, then trigger each sensor in order with a tool |
| Toyota and Lexus | A SET button for pressure on many models, with IDs written by a tool through the port |
| Chrysler group | Often self-learn by driving |
| European makes | Often scan tool or menu based |
Self-learn usually needs steady driving above a set speed for 10 to 20 minutes. The owner must know this if the light does not go out in the bay.
Diagnosing a TPMS Light
| Symptom | Likely cause | What to do |
|---|---|---|
| Solid light, tires low | Real low pressure or a leak | Set pressure, find the leak |
| Solid light, tires at spec | Placard pressure not set, or needs a reset | Set pressure, reset or relearn |
| Flashing, then solid | Dead sensor, or module fault | Scan the system, test each sensor |
| Sensor does not respond | Dead battery or damaged sensor | Replace the sensor |
| Light after a rotation | Positions not relearned | Do the relearn |
| Light after new wheels | No sensors, or wrong sensors fitted | Fit and program the correct sensors |
| Light comes on in cold weather | Pressure fell with temperature | Set cold pressure to the placard |
A spare with a sensor can also set the light on some vehicles, so set its pressure when you check the others.
Common Mistakes
| Mistake | What it costs |
|---|---|
| Not testing sensors before starting | Being blamed for a sensor that was already dead |
| Tightening a nut by feel | Cracked stem and a slow leak weeks later |
| Reusing old seals | Slow leak and a comeback |
| Skipping the relearn | Warning light, or pressures shown at the wrong wheel |
| Fitting a sensor for the wrong frequency | No signal at all |
| Disabling the light for a customer | A federal violation and a safety risk |
Check Your Understanding
- The warning light flashes for a minute after start-up and then stays on, and all four tires are at the correct pressure. What does that point to? A system fault, such as a dead sensor. Use a TPMS tool and scan the system.
- You tighten a clamp-in nut with a ratchet until it feels tight. What can happen? It can crack the stem or damage the seal. The leak may not show for weeks. Use a torque tool and the maker's value.
- A customer with indirect TPMS has just had two tires replaced. What do you do? Set the pressures, then perform the reset through the button or menu, so that the system learns the new tire size and speed.
- A customer asks you to turn off the light rather than pay for a new sensor. What do you say? You cannot do that. It is a required safety system and a repair business may not make it inoperative. Offer to replace the sensor.
- A placard says 40 psi. About where should a low warning come on? At about 30 psi, which is 25 percent below the placard.
Key Takeaways
- A solid light means low pressure, and a flashing then solid light means a system fault.
- Direct systems use sensors with batteries in each wheel. Indirect systems use ABS wheel speed sensors.
- Know which part you are torquing. Valve cores take about 4 in-lb, and clamp-in nuts take much more.
- Replace seals and service parts every time, and test sensors before you begin.
- Relearn procedures vary by vehicle, so look them up.
- A shop may not disable TPMS.