The first step in any diagnostic strategy is to figure out whether or not your customer’s vehicle actually has a TPMS problem. Any number of things can cause the TPMS warning light to come on or flash. The light should illuminate when a tire is low, and should eventually go out after the low tire has been inflated to its recommended pressure. If the light remains on after checking/inflating the tires, or if it flashes and remains illuminated, it may signal a TPMS problem that will require further diagnosis.
TPMS problems can include any of the following:
• A tire pressure sensor that has stopped functioning because the battery has died.
• A tire pressure sensor that is working intermittently due to a weak or failing battery.
• The TPMS module is not receiving a signal from one or more sensors because of an antenna or wiring fault.
• The TPMS module itself is not functioning properly or has failed because of a voltage supply, wiring or internal electronics fault.
• The tires were serviced or rotated recently and the relearn procedure or was not done correctly.
• The vehicle owner does not understand how their TPMS system works.
One of the diagnostic mantras that is preached by service experts today is "Test Before Touch." Basically, you should always use a TPMS tool to activate and check the response signal from each tire pressure sensor in each wheel before you do anything else. This will tell you
(1) whether or not each sensor is capable of generating a signal, and
(2) if the sensor is generating a signal whether or not the pressure reading is accurate.
The pressure reading from a sensor can be easily verified by checking the actual pressure in the tire with a gauge. If the pressure value displayed on your TPMS tool from a sensor reads 32 PSI (or whatever), you should find 32 PSI when you check the pressure with a gauge.
A key point here is that your tire pressure gauge MUST be accurately calibrated. Those cheap spring-loaded stick-style tire pressure gauges often vary up to 5 PSI or more! The most accurate gauges are the electronic digital ones because many have a self-calibrating feature that compensates for changes in ambient air temperature.
WATCH OUT FOR BAD VALVE STEMS AND AGING SENSORS
Something else to watch out for are corroded or damaged TPMS valve stems. The valve stem on each wheel should be visually inspected for corrosion or other damage that might affect the integrity of the valve stem.
Consider the age and mileage of the vehicle when doing your diagnosis. The average life of the battery inside a brand new factory TPMS sensor is around 7 to 10 years depending on use. The more the vehicle is driven, the more often the TPMS sensors generate their signals and the faster they use up their remaining battery life.
HOW TO PROCEED
If you find a tire pressure sensor that is not functioning or reading accurately, the natural assumption is that the sensor is the problem and that replacing the sensor will fix it. Usually it will. But until you check the rest of the TPMS system, there’s no guarantee a bad sensor is the only problem that may be affecting the operation of the system.
If all of the sensors appear to be working normally, and all of the tires are inflated to the recommended pressure, but the TPMS warning light is remaining on or flashing, you’ll have to dig deeper to uncover the fault.
For the next step, you’ll need a TPMS tool or scan tool that can communicate with the TPMS system via the OBD II diagnostic connector under the instrument panel. After plugging in your tool, read out any fault codes that are found and write down the code(s) so the information isn’t lost when you clear the module’s memory.
You might find a code indicating one or more bad tire pressure sensors because there is no signal coming from the sensor. But if you already checked each sensor with your TPMS tool and didn’t find any problems, you know the problem isn’t the sensor. Consequently, the only explanation is that the sensor signal is not getting through to the TPMS module. The problem could be a damaged or shorted antenna near the wheel, or a wiring fault between the antenna and the TPMS module.
If you suspect the TPMS module is not receiving a good signal from one or more sensors, check the antenna wiring for continuity and problems such as shorts, opens or high resistance. A voltage drop test across any wiring connections should read 0.10 volts or less. If you find a higher voltage drop reading, it indicates excessive resistance that is affecting the quality of the signal.
If the antenna wiring checks out but the TPMS module is still generating a "no sensor signal" code, the fault is likely within the module itself. But before you condemn the module and tell your customer the TPMS module needs to be replaced, make sure you check the TPMS module voltage supply and ground connections because low voltage can make any electronic module misbehave and act quirky.
On some vehicles, the signals to the TPMS module are shared or go through the keyless entry system so a wiring problem that affects the keyless entry system could cause a problem with the TPMS system. The TPMS module may be working fine but is not getting the right information from the keyless entry system. That’s why you should always look up the OEM service information for the vehicle you are working on to see how the TPMS system operates — especially if you are having difficulty figuring out a problem. You should also check for any Technical Service Bulletins (TSBs).
Something else that can cause a TPMS system to malfunction or set false codes is EMI (electromagnetic interference). EMI from another source might be messing with the TPMS signals and confusing the system. Electrical crosstalk between adjacent wiring circuits can occur when the magnetic field around a wire that runs parallel to another wire induces a current in the second wire.