How to Troubleshoot a Tire-Pressure Monitoring System ?

When this warning lamp illuminates on your instrument panel, at least one of your tires is 25 percent below its correct pressure. This TPMS sensor uses a lithium battery with a five- to 10-year life span. But you still need to keep a decent tire gauge in your glovebox.

It's hot. Real hot. Fortunately, the new car you bought right before that Arizona spring-break road trip has air conditioning that works great, in spite of the heavy pop-up trailer and loaded roof rack weighing it down. You pull in for gas and lunch, and carefully check the pressure in the trailer tires with the gauge that lives in your glovebox. The tires on your crossover

are fine though, because all new cars have a tire-pressure monitoring system that will tell you if your tires are low from the comfort and safety of your driver's seat. The desert beckons, and 40 miles of heat-shimmered asphalt later, a tire blows. What happened?



After swapping in the spare, you continue your trip at a more sedate pace. The TPMS light is on, and you stay well under the speed limit until you can check the tire pressures with a gauge. Surprise--they're all low. Surprise No. 2: The TPMS light remains on after you top off the tires to the correct pressure, at least until you can get a replacement tire mounted later in your trip.

Be Prepared

A Department of Transportation study dating back to 2001 says that 60 to 80 percent of cars on the road are running tires underinflated by as much as 10 percent. Worse yet, they say that 20 to 50 percent are being driven with tires down in pressure by as much as 20 percent. Yet, here's the scariest part: If your tires are low, even falling into that minus 20 percent category, your TPMS won't tell you--ever. The TPMS warning light is only required to illuminate when the pressure gets 25 percent below the correct value, which is enough to reduce fuel economy, lower the available grip (especially in wet conditions) and make tires run substantially hotter.

Bottom line: Don't trust the TPMS. Once you understand how your TPMS system works, you'll understand why it doesn't obviate the need for regular tire-pressure monitoring.

The Easy Way

There are two types of TPMS on the market, direct-reading and indirect. Indirect systems use only software and readouts from the individual wheel-speed sensors used by the antilock brake system. If all four tires are properly inflated, they will all rotate the same number of revolutions in a stretch of road. If one tire rotates more than the other three, it has a shorter rolling radius because the pressure in it is low.

The indirect type of system is inexpensive, because the only real part that must be added to a car is the display on the dash and some extra code in the vehicle's operating system. Properly inferring tire pressures this way can be a problem when the tires are unevenly worn, or if you replace only two worn tires instead of all four. You can reset the system to allow for wear when all the tire pressures are correct. The procedure varies from vehicle to vehicle, and this one won't necessarily work on yours. Persistent TPMS warnings can often be cured by setting the tire pressures properly and then resetting the TPMS somewhere in the menu on the driver's display, or perhaps with a scan tool. If the system isn't reset this way, TPMS warning lights will also crop up after a normal tire rotation, or if you're replacing tires with ones of a different size from the old ones.

1. TPMS senders are usually attached to the valve stem, although some are banded to the wheel's drop center. These units can cost over $100, but replacements for most domestics can be found on the aftermarket for $50 or less. They're prone to damage from tire-mounting machines, so get your tires mounted by a knowledgeable technician.	2. In an attempt to reduce the sensor's mass and keep the wheel in balance, the metal stem of the sender is commonly made of aluminum. Soft aluminum. Use a torque wrench on the inch-pound scale to tighten these, because it's really easy to strip the threads by overtightening. Ferrous-metal valve caps may interfere with the electronics, so stick with OEM-style plastic valve-stem caps.

The Better Way

There is a much better (read: more expensive) technology to monitor tire pressures. Higher-end vehicles use a direct-reading system of battery-powered senders mounted inside the tire, communicating with the TPMS by means of small antennas in each wheel well. Every few minutes, the TPMS will interrogate the senders in the wheels, acquire a "true" pressure reading and transfer the message to the in-car display.

But what's to keep the system from reporting the tire pressure of the car next to you at a traffic light? In order to keep things straight, the individual senders have a unique serial number to transmit to the vehicle. And that keeps individual wheels on each corner of the car properly sorted in the TPMS's tiny little brain. On many vehicles, there's even a sender on the spare tire. The TPMS is initialized with the position of the four (or five) tires when the car is new. Eventually, those tires could change position, through normal tire rotation or as the result of a flat. That's why there is a procedure to resynchronize the system. For most vehicles, it involves the use of a special tool that communicates with the vehicle and the sender to make everybody play nice. This device knows which wheel is which because it's held next to each valve stem in turn as the system is programmed. Your car dealer will have one, and some independent repair shops might as well. Unfortunately, they're different for every brand of car, and they cost plenty, with basic models going for $600 to $800, and those that work on a variety of vehicles running up to $2500.

Fortunately, it's rarely necessary to use the factory-style tool. Each manufacturer has its own tool and procedure. Look in (surprise!) the owner's manual.

And all of this has to happen within 2 minutes, or the learn mode times out and you'll have to start over. Also, if anyone nearby is adjusting tire pressures on a car with TPMS sensors, the system may be confused. If your vehicle's TPMS includes the spare, burrow into the trunk and get access to the spare first. Don't forget to top off the tires to the correct pressure.

1. The definitive correct tire pressure is on this sticker on the door frame. This info is also in the owner's manual. Ignore what's printed on the tire sidewall. Your car manufacturer has determined what pressures will make your car handle properly. The pressure on the sidewall is the maximum pressure for any vehicle.	2. Tire pressure should be checked in the morning on cold tires, not after you've driven to Starbucks. Tire pressure changes 1 psi for every 10 degrees of outside temperature. A change from 70 F to 40 F will lower the pressure 3 psi, enough to affect wet braking and fuel economy; check your pressures monthly.

On some GM cars you can activate the wheel sensors with a simple, powerful horseshoe magnet. GM has a special tool for this, but I'll bet a nice strong magnet from RadioShack would work. On Toyotas, apparently the only way to activate the sensors is with the Toyota TPMS tool or scan tool. Your mileage may vary. You'll need to find out exactly what procedure your vehicle needs any time you rotate tires or install winter tires. Consult your owner's manual, or the factory service manual. Be wary of advice on TPMS from enthusiast websites: A lot of the procedures I saw there were wrong or just plain out-of-date.

Special Considerations

If you ever need to replace a valve-stem core, use stainless steel rather than brass to avoid corrosion. It's one thing to replace a $2 rubber valve stem and something else entirely to replace a $100 TPMS sensor. And always use that cap. Water, road salt or mud could affect the sensor.

One last thing: That can of aerosol flat-fixer in your trunk may damage the sensor. Yes, it says "Sensor Safe" on the label, but experience in the field says it's a bad idea. The hole-filling compound may well plug up the hole in the sensor that checks pressure. If you must use one of these products, take the vehicle to a properly trained tire technician as soon as possible to have the hole plugged properly from the inside.

Here's a typical procedure for teaching the TPMS system which wheel is which:
1. Place the ignition switch in the "ACC" position.
2. Simultaneously press the keyless entry transmitter's lock and unlock buttons until a horn chirp sounds. This will put the system in "learn mode."
3. Starting with the left front tire, increase/decrease the tire pressure for 5 to 8 seconds, then wait for a horn chirp. The horn chirp may occur before the 5-to-8-second pressure increase/decrease time period has been reached, or up to 30 seconds after the 5-to-8-second pressure increase/decrease time period has been reached.
4. After a horn chirp has sounded, proceed as in step 3 for the next three sensors in the following order: right front, right rear, left rear.
5. After the LR sensor has been learned, a double horn chirp will sound, indicating that all sensors have been learned.