Under the Hood: A tire-pressure puzzle

McClatchy-Tribune Information Services

October 30, 2017

Q: Hereís an odd problem to consider: My car just began showing a low tire pressure warning for the left front tire. It said it was down to 31 pounds. The other three were 35. I added air at the machine at my gas station. The gauge on the hose said it was at 35, so I figured it was wrong and added about 5 pounds. Then when Iím driving home, the message said my front tire was still low but my rear tire now had 39 pounds. Iím planning to buy a tire gauge to try to see what is really going on. Why would the car do this?

óBecky C.

A: Good move buying a pressure gauge! I have a hunch that when you last had your tires rotated, the tire pressure monitoring system sensor matching procedure (also called re-registering) wasnít performed, or the procedure wasnít successful. And Iím guessing your car isnít one that automatically identifies the proper wheels after theyíve been switched around. Direct tire pressure monitoring systems, or TPMSs, employ a battery-powered sensor/transmitter in each wheel, right behind the valve stem.

On vehicles that require a relearn procedure, it can vary widely, even within a car brand (I found 26 differing procedures for just GM vehicles!) Some require a reset request to be made using the driver info panel, then a lengthy road test or sequential (temporary) dropping or increasing of each tireís pressure. Others require the use of a scan tool to access the system, and/or a re-learn tool, or magnet that prompts each TPMS sensor to identify itself to the system. There are a dizzying number of possible procedures! A good way a consumer can check for the possibility of a DIY-no tool procedure is to go to www.tpms.com. Select "resources," then TPMS "reset and relearn." Choose the manufacturer, and the needed info is provided.

Performing a monthly physical check of all four tires can keep one safely inbounds with either an unmatched system or an indirect TPMS system that might not indicate which tire is low. Indirect systems look at the anti-lock brake systemís wheel speed sensors to infer a low tire (rolls faster than siblings due to smaller diameter). One advantage of an indirect system is avoiding potentially costly sensor renewal each seven years. Remember to go with the cold tire pressure listed on the driverís door pillar sticker, not the maximum pressure listed on the tire.

Q: My check engine light just came on with a P0401 code. What do I do to fix this?


A: This may be tough to fix yourself. The code P0401 indicates insufficient exhaust gas recirculation (EGR) flow. The cause could be a faulty EGR valve, a fault in its control method, or carbon clogged within either a slender exhaust feed tube or intake manifold passage. During typical driving, a small quantity of exhaust is routed back through the engine a second time to reduce combustion temperature (exhaust is inert, takes up space that would have been air and fuel).

On older vacuum-controlled EGR systems, applying vacuum to the EGR valve while at engine idle should result in a dramatic slow-down or the engine dying. If the vacuum holds but limited slow-down occurs, the fault is either a stuck valve or clogged passage. Removal and inspection should confirm which. Newer vehicles employing an electric EGR are best tested by commanding the valve open with a scan tool while observing idle quality and/or the valve position sensor, MAP sensor, or exhaust pressure sensor. Unfortunately, there are so many variations in EGR systems from car to car, and test methods, the best I can do is provide this limited overview. Donít throw parts at it! A pro with an emissions test license should be able to diagnose this blindfolded!