Last month a customer brought in her 2014 Subaru Forester 2.5 L Touring because the tire-pressure monitoring system (TPMS) light was on. She had checked the tire pressure on all four wheels and it was correct, yet the light warning of low pressure had been on for weeks.
Before we get into the problem, here’s a quick overview of the TPMS. There are two general systems used across all car lines: direct and indirect. Each system varies among manufacturers.
The direct system uses a tire-pressure sensor in each wheel that reports the pressure back to the TPMS control module. The indirect system works in conjunction with the antilock brake system/traction control system. The ABS/TCS system can see a slower rotation from a wheel if the tire pressure gets too low. It then checks the rest of the car to see if it is in a skid or just has lower tire pressure.
It may seem that the direct system with a tire-pressure sensor makes more sense, but cars using the indirect system don’t have to rely on tire-pressure sensors and can use existing systems.
Diagnosing the problem
Once the car got into the shop, we connected the Subaru scanner and checked for codes. There were no codes in the TPMS. We then checked if we could see all four sensors in the computer; we could. We then used a different tool and went wheel to wheel to check the TPMS battery life. We were able to ping each pressure sensor, which told us the TPMS sensors were transmitting.
While we were testing the pressure sensors, we noticed the tires were old and cracking. No matter what we found in our diagnostics, the tires should be replaced.
We then noticed that the tire pressure sensors were not registered in the TPMS module, so we used the scanner to try and register the tire sensors to the car. After several attempts, they still would not register.
We scanned the rest of the car, finding some communication and low-voltage codes. During the test plan, we discovered that the battery voltage was at 8.2 volts at 250 amps. We suggested to the customer that the battery be replaced before registering the TPMS sensors.
After replacing the battery, the sensors still wouldn’t register. That led us to perform a capacitor discharge on the entire car. This enables us to start from scratch – just like when you turn off your computer, unplug it and then plug it back in. However, we still couldn’t get the sensors to reset.
At that point, we surmised that the TPMS module was probably bad. The customer felt that because the TPMS light did not cause an immediate threat to the car, she would rather replace the tires first and revisit the TPMS warning light in the future. We replaced the tires, performed an alignment and the customer picked up the car.
The next morning, she returned to tell us that her key remotes did not work. At that moment, we had a good idea of what went wrong. We first tried to reinitialize the key remotes, but they would not initialize. On this car, the TPMS module and the KEY module are one unit. This was done because the TPMS sensors, like key remotes, put out a signal. When we performed the capacitor discharge, it caused the TPMS/KEY module to forget the keys, but because the module was damaged, it could not reinitialize the keys.
We replaced the TPMS/KEY module and programmed it. Once the TPMS/KEY module was programmed, we were able to program the keys and tire-pressure sensors.
If one finds a damaged part or a broken wire, it is easy to condemn a part. When it comes to control modules, it is more difficult. We have to trace and test everything back to the control module before we can condemn it. In the process of our testing, we uncovered a second problem – the keys not working after the capacitor discharge. This helped us make the decision to replace the TPMS/KEY module.
Matt Pataky owns Sunnyvale Foreign Car Service, 15 Pioneer Way, Mountain View. For more information, call (650) 960-6988, email [email protected] or visit sunnyvaleforeigncar.com.
