TPMS Service: Tire Pressure Light Diagnosis and Sensor Repair

Tire pressure monitoring has been required on new passenger vehicles in the US since the 2008 model year, and the same federal rule sets the trigger: the light has to come on once a tire runs 25 percent or more below the cold pressure printed on your door placard. On a placard calling for 35 psi, that is roughly 26 psi. A flashing light at startup that then stays solid points to a problem with the system itself, not the air.

A light is not a diagnosis. Our warning lights guide covers the other symbols on your dash.

NHTSA's tire safety page (opens in a new tab) explains why correct tire pressure matters.

Direct TPMS uses a battery-powered sensor inside each wheel that reports actual pressure. Indirect TPMS has no sensors in the wheels. It uses the ABS wheel speed sensors to notice when one tire spins faster, which happens when it loses air. The fixes are different, so we identify your type first.

Direct systems

• A sensor in each wheel, usually attached to the valve stem
• Many cars can show each tire's pressure on the dash
• Sensors have sealed batteries, commonly good for about 5 to 10 years, and are replaced whole
• Tire changes need sensor service parts and sometimes a relearn

Indirect systems

• No sensor in the wheel
• Needs a reset after inflating or rotating, usually through a button or menu
• A reset done with tires at the wrong pressure teaches the system the wrong baseline

If you are not sure which you have, the owner manual says, or we can tell from the valve stems and a scan.

We connect a TPMS tool to each wheel and read what the sensor reports: pressure, battery status and sensor ID. Then we compare that to a pressure gauge. If the sensor reports correctly and a tire is low, it is an air problem. If the sensor does not respond, reports a weak battery, or reads wrong, the sensor is the problem.

What the test shows for each wheel:

• Sensor ID: confirms the car knows which sensor is where
• Reported pressure: compared against a gauge reading
• Battery status: many tools show a low battery warning
• Temperature: helps spot odd readings
• No response: a dead or damaged sensor

You see these readings before you approve anything. If a tire is low because of a puncture, we move to flat tire repair instead.

Direct TPMS sensors run on sealed batteries that cannot be swapped. Most are built to last somewhere between 5 and 10 years, depending on the sensor and how much the car is driven. When the battery fades, the sensor stops reporting and the light comes on. Replacing the sensor is the fix.

Planning ahead

If the car is past the typical life and the tires are coming off anyway, it can make sense to talk about sensors at the same visit. That saves removing the tire twice. We show you each sensor's reported status so you can decide. This is a want, not a need, unless a sensor has already failed.

Heat and age

Inland Empire summers are hard on everything rubber and electronic under a car. Sensor grommets can harden and leak, which is why service kits are part of every tire change.

Mixed ages

If one sensor dies from old age, the others from the same year may not be far behind. We tell you what we see and leave the call to you.

The tire comes off the wheel so we can reach the sensor. We install a new sensor, either a factory part or a programmable replacement set up for your vehicle, with a new seal kit. Then the tire is remounted and balanced, and the car is taught the new sensor ID through the relearn procedure your vehicle maker specifies.

1. Read and record the old sensor ID and status.
2. Dismount the tire.
3. Remove the old sensor and install the new one with fresh seals.
4. Program the sensor for your vehicle, if it is a programmable type.
5. Remount, inflate and balance.
6. Run the relearn.
7. Confirm all four sensors report and the light is off.

Sensor nuts and valve cores tighten in inch-pounds, not foot-pounds, and the number is small. Overtightening cracks the sensor body, which is one reason we do not rush this job. The same care applies during any tire installation.

A relearn teaches the car which sensor is on which wheel. Some vehicles relearn on their own after a short drive. Others need a scan tool, a sequence of key and button steps, or a connection through the diagnostic port. If the relearn is skipped, the light can stay on or the dash can show pressures at the wrong corners.

Relearn types:

• Automatic: the car learns sensor positions while driving.
• Stationary: a tool triggers each sensor in order while the car is in learn mode.
• Scan tool through the diagnostic port: sensor IDs are written directly to the car.

When a relearn is needed:

• After a sensor is replaced
• After a tire rotation on cars that display pressure by position
• After wheels are swapped, such as a second set for the season

We do the relearn before you leave and check that the light stays off.

Air pressure drops as the temperature falls. A common rule of thumb is about 1 psi for every 10 degrees Fahrenheit. A cool morning after a warm week can push a tire that was already a little low past the warning point. As you drive and the tires warm up, pressure rises and the light may turn off.

That does not mean the light is wrong. It means the tire was near the edge.

What to do:

1. Check pressure before driving, or at least 3 hours after parking, with the tires cold.
2. Set every tire to the door placard, not the maximum stamped on the sidewall, which is the tire's own ceiling.
3. Check again in a week to see if one tire keeps dropping.

If one tire keeps losing air, it likely has a slow leak. Our tire pressure and care guide covers how and when to check.

A light that returns days after you filled the tires usually has a cause that air alone does not fix. Either a tire is still leaking, the system is reading a wheel it does not expect, or a sensor is failing. Sorting out which one takes a gauge, a TPMS tool and a look at recent tire work.

Tell us what happened before the light came back: a flat, a rotation, a cold snap or new wheels. That history often points to the cause before the tool is even connected. A slow leak at the valve is covered on our valve stem page.

The basics are the same, but some vehicles add a twist. Electric vehicles are heavy and sensitive to pressure, so a low tire costs range as well as tread. Trucks may use different front and rear pressures, and some full-size spares carry their own sensor that can set off the light by itself.

• Electric vehicles: many show live pressures on the screen and need a relearn through the vehicle menu after tire work. See electric vehicle service.
• Pickups that tow: some trucks let you choose a loaded or towing pressure setting so the warning point matches the load.
• Dual rear wheels: inner duals may need extended valve stems to check and fill.
• Full-size spares with sensors: a low spare can turn the light on even when the road tires are fine.
• Seasonal or custom wheel sets: each set needs its own sensors programmed to the car, or the light stays on.

When you book, mention the vehicle type and any second wheel set. That way the right sensors and relearn steps are ready when you arrive.