What pressure does the TPMS sensors alert on a low tire? I have 29psi and still no alert. I am getting my new Falkens today so not really concerned about filling them.

 

My alert come on the other day when it was -38°C at 26 psi.

 

Looks like my new Falkens are installed. Seems a little high though.

 

Done!

 

pics

 

These.....

 

For pressure monitor to come on it depends on the accuracy of the individual Sensor and being only able to respond with in +/- one Psi.
I have one that reads 1 Psi off from the other 3 on a consistent basis and what corner it goes to , when rotated to.
The visual display comes on at 28 Psi.
Average pressure increases are about 4 to 5 Psi depending on road surface Temps variations and how aggressive the driving is and the load weight.
There are a number of dynamics all affecting, at the same time.
Different make tires can raise or lower their pressures at different rates.
it takes a lot of long term observations to see these things happening when and where during drive cycles over the same route repeatedly over time and different conditions.
To set base line pressure, I use a digital gauge two 2 places to the left of the decimal point to set pressure after setting over night when all tires 'should' be 'close' to the same temperature. Absolute pressure accuracy is not an important value but the baseline to begin from for observation is a must or you can't see the fine points of pressure changes no matter from what pressure level is set as baseline.
If there is an interest in this, its very telling how the pressures move around in each tire as you drive and able to detect one that is consistently high or low by 1 psi..
That indicates a Sensor that is possibly not quite as accurate as the others.
Good luck.

 

The only time I’ve gotten an alert was when a tire went below 28 psi. It may have been 26, I can’t remember exactly because it was a couple years ago.

 

I don't know an exact number.... I had a similar question back in 2018 (see link below). What I discovered is that the requirement for TPMS is not an exact number. As I recall, the government requirement stated something like: TPMS should report when pressure is significantly low. One can look up the exact TPMS government requirements if you want exact wording. I typically run my tires at about 40 PSI or more, so I changed the TPMS specified reporting number (can do via FORScan) to be 42 PSI for front and 45 PSI in the back tires. My thinking was that if a tire ever got down to about 32 PSI or lower, I'd get an alert (instead of waiting until the tire pressure might be much lower than that). Basically, I don't want to be driving on a tire with less than 30ish PSI. I know this is not an exact answer, and I think each TPMS sensor may report slightly differently. The link below is when I asked the same basic question. You'll have to scroll down in that thread from there as there were multiple conversations active so responses may be several posts later Hopefully this helps with a little more information.

FORScan - software to enable/disable features in your truck - Page 1435 - Ford F150 Forum - Community of Ford Truck Fans

You'll see in post 14357 of that thread, I had set 51 psi as the specified TPMS pressure for the back tires. Then lowered tire pressure on each back tire until a TPMS alert went off. As you can see, with a specified PSI of 51, the tire at 35 PSI set an alert, the tire at 36 PSI did not. Does that mean that greater than 15 PSI is the magic number? (I don't think so) Or as other suggested, maybe it is more of a percentage (approximately 30% in this case). Regardless, I don't think anyone could get a specific number or percentage to be reliable. Again, I refer back to the paraphrased words in the governments TPMS requirements of "significantly low" and you have to interpret that the way you want (lol). BTW, I've had other vehicles alert me at 28 PSI when the tire is specified at 32 PSI. I see nothing consistent in when a TPMS will actually see an alert for a low tire pressure.
By the way, I'm not an expert in this subject, just what I found when I had the similar question.

at least one link to Federal TPMS regulations: (there are likely updated documents)
DEPARTMENT OF TRANSPORTATION (nhtsa.gov)

 

Yes, I believe the NHTSA specifies alarm at 75% of rated pressure. 26 PSI is 75% of 35 PSI.