What EV drivers need to know

By Paul Gipe:  Member, Electric Auto Association

New cars in the US  include a Tire Pressure Monitoring System (TPMS) with a sensor in each wheel that sends a wireless signal to the car's computer. A pressure measurement and/or a warning of low pressure is then displayed on the dashboard.

All EVs have some form of TPMS. Our Chevy Volt had a TPMS that displayed actual tire pressure on the dashboard Driver’s Information Center, and our Chevy Bolt has the same. The driver then must toggle between various screens to find an image showing a diagram of the car detailing the pressure for each tire.

Our base model 2015 Nissan LEAF did not report tire pressure on the dash directly. It simply provided an alert when tire pressure was low. I set up Leaf Spy to read the diagnostics from the car’s computer and to  report actual tire pressure for each tire. This was helpful in maintaining  proper inflation for hypermiling in the limited-range LEAF. (That is, the car was capable of reporting tire pressure in each tire but it didn't do so in the base model. One had to purchase the more expensive models to obtain this feature.)

This is all well and good when the TPMS works as it should. Unfortunately, that's not always the case.

Why resetting the TPMS matters

When tires are rotated, the TPMS should be reset to enable accurate reporting of pressure. I've learned the hard way that not all mechanics do their job the way it is described in the books. They either forget to reset the TPMS, don't have  the time to do a reset, or don't know how to do so for the particular vehicle they are servicing.

This has happened twice in the three years we've leased our Bolt. The first instance involved the GM-trained mechanic at our Chevrolet dealer. After a tire rotation, I found the tire pressure readings for the front tires were actually for the back tires. OK, I could live with that.

However, toward the end of the lease, we replaced the tires. Afterwards, I noticed that the tire pressure page was displayed on the dash. Good, I thought. They reset the TPMS as they should.

No, they hadn't, but I didn't learn this right away.

A few weeks later the dash displayed a low pressure reading for a front tire. I pumped it up by hand. That didn't seem to do the trick. Hmm. I inflated the rear tire on the same side. That didn't seem to work either. I took out my trusty pencil tire gauge and measured both tires. Pressure was where I wanted it on both, so I assumed one of the sensors was bad.

Wrong. And, no, I didn't do the common sense thing to do by measuring all four tires. Perhaps I’m just getting a bit lazy with all this technology.

The next day, the dash flashed a low tire pressure warning. I filled the left rear tire again. The dash still displayed a warning but the tire pressure gauge indicated the pressure was fine.

Off we went on one of our COVID-19 escapes from Bakersfield. We were only five miles from home when the handling just didn't feel right and the TPMS display was now on all the time. That should have been a hint.

I pulled over at the next exit and finally did a walk around. Yep, there it was: a nearly flat right rear tire.

I nursed the car back home, and inflated the tire with the hand pump. (This time, I had Nancy help me as that's a lot of air to put in a tire with a hand pump.)

Naturally, I had to drive 20 minutes across the city to a mechanic who knows how to fix the special Michelin. (See Chevy Bolt Self-Sealing Tires Work But More Difficult to Repair.) We were back on the road by mid-morning on our adventure to Oak Flat.

By now, I was convinced that I couldn't rely on others to reset the TPMS. I had to do it myself.

Resetting the TPMS

Resetting the TPMS isn't difficult, but it does require a special tool and procedure. The Bolt manual describes the TPMS on page 279 and the "TPMS sensor matching process" on 280. I can also recommend a hokey video by ACDelco on TPM systems that provides background information, as well as several threads about TPMS on the ChevyBolt.org forum. 

Initially, I'd been put off by the idea of buying a special tool just to reset the pressure sensors,  especially when I learned how much professional mechanics pay for them. But then I discovered a number of inexpensive  Chinese special purpose tools designed just for this one function.  My online order was $20 with express delivery.

For our Bolt, and Chevrolets in general, the reset process is triggered by opening the tire pressure screen on the Driver’s Information Center. Then, the driver can press the check mark on the right side of the steering wheel until the DIC screen changes. When prompted to enter the ‘relearn’ process, and ‘Yes’ is pressed,  the horn will beep twice to signal that two minutes have been provided to complete the reset. The car's marker lights indicate which tire needs resetting. The process can then be repeated for each tire. At the end of the process, the horn will beep twice again. Each tire pressure sensor should now match the display on the DIC tire pressure screen.

TPMS and altitude

Many drivers here in California note that they receive a low-pressure warning when they head into the Sierra Nevada Mountains. It's happened to us several times and I've come to expect it. Previously, I attributed this phenomenon to altitude and the change in air pressure, but now I know that's not the case. Instead, it is due to decreased temperature with altitude.

I use a rule of thumb that temperature decreases from 2.5 F° to 3.5 F° with every increase in elevation of 1,000 feet. For us, the nearly 10,000 foot climb from Bakersfield to Horseshoe Meadows results in a 25 F° to 35 F° temperature decrease, dropping tire pressure by as much as 3 psi. The pressure then rises back to normal when we return to sea level.