Tire Pressure Monitor

[Kmvreter]

All five tires are monitored by TMPS. It sounds like your spare is missing the TMPS which is causing the light to come on.

I agree, kind of.

DesmoST4,
What year is your vehicle? I assume from your description that you have a regular (rubber) stem in the spare. The light isn't blinking ON & OFF so it isn't a fault in the system or a missing/bad/dead sensor. The light is ON solid so the system is working and sensing a low pressure. This makes me think that particular sensor is somewhere in the vehicle, maybe in the glove box or console? The TPMS is no longer checking the spare's pressure but it is still registering a low, ambient air pressure, result.
The other possibility is that you in fact have a low pressure in one of your road tires. This will cause the steady ON that over rides a system fault caused by the missing sensor. Many pressure gauges are not accurate so I would suggest overfilling to test this out and see if you can force the light off (then return your pressures to "normal".)

The forum has some good TPMS threads full of info on how the system works.

 

[DesmoST4]

Well I checked pressure again and have them all set at 35psi, using a digital pressure guage. I checked the spare again and the valve stem is a regular rubber set up so I am guessing this might be the issue. I think the next step to run by Toyota and see how much to get a TPMS stem installed on the spare. I was also thinking about doing the by-pass setup to just get rid of the problem all together. I check my tires regularly anyway on all my vehicles but I don't mind having the system as long as it works. I guess I will see how much money it will cost and then decide.

Thanks for the input fella's

 

[Kmvreter]

Well I checked pressure again and have them all set at 35psi, using a digital pressure guage. I checked the spare again and the valve stem is a regular rubber set up so I am guessing this might be the issue. I think the next step to run by Toyota and see how much to get a TPMS stem installed on the spare. I was also thinking about doing the by-pass setup to just get rid of the problem all together. I check my tires regularly anyway on all my vehicles but I don't mind having the system as long as it works. I guess I will see how much money it will cost and then decide.

Thanks for the input fella's

Look around inside your truck. I'm guessing it's there somewhere because of the solid ON light. Maybe the stealer can diagnose/verify this by getting a temperature and pressure reading from it on their scan tool?
A replacement and scan tool programming aren't cheap; the bypass is.