My 2008 ultima tires will leak air to the extent they are almost completely flat. It started with the passenger side rear almost completely flat. I would fill with air and it will last for 2-5 days. Now the drivers side rear tire is going flat nightly and the passenger side rear is staying inflated. I have also had to add air to both front tires. ALL THIS IS BEING DONE WITH COLD TIRES. THANK YOU

put soapy water around the bead (where the tire meets the rim) aluminum wheels have corrosion there often and they will leak ,, check inside and outside pump it to about 50 lbs and lay it on its side , if that is the problem you are best to have it unmounted cleaned and reseated

IF YOU HAVE REAR HEAT AND A/C THERE ARE 2 LINES THAT RUN UNDER CAR ALL THE WAY TO THE BACK PASSENGER SIDE WHEEL WELL!! THEY DO ROT OUT!!. YOU COULD REPLACE OR JUST CLAMP OFF LINE, BUT YOU WOULD NOT HAVE REAR HEAT. THE LINE HAS ANTIFREEZE IN IT. CONNECT THE 2 LINES TOGETHER TO COMPLETE THE LOOP

If its reading low and the tire pressure is correct there is a problem with that sensor. Typically they need to be replaced in order to correct the problem. A TPMS tool capable of displaying TPMS sensor data would confirm the bad reading.

They display the tire pressure as a convenience. They don't have to display the pressures and many makes/models do not.

The level of sophistication with these systems is slowly improving. Many newer models will automatically know where the tire is and display it correctly. Unfortunately yours need to be relearned manually after every tire rotation which is very common with earlier systems. It is assumed that this is done after every tire rotation which would then make the displayed tire locations correct.

If you are confident that the correct tire location is being displayed than I wouldn't see any issue with replacing that sensor. If you are not confident the correct tire location is being displayed than it wouldn't be a bad idea to do a relearn procedure.

The TPMS matching process is outlined below:
1. Set the parking brake.
2. Turn the ignition switch to ON/RUN with the
engine off.
3. Press and hold the Remote Keyless Entry (RKE)
transmitter's LOCK and UNLOCK buttons, at the
same time, for about five seconds to start the TPMS
learn mode. The horn sounds twice indicating the
TPMS receiver is ready and in learn mode.
4. Start with the driver side front tire. The driver side
front turn signal also comes on to indicate that
corner's sensor is ready to be learned.
5. Remove the valve cap from the tire's valve
stem. Activate the TPMS sensor by increasing
or decreasing the tire's air pressure for about
eight seconds. The horn chirp, can take up to
30 seconds to sound. It chirps one time and then
all the turn signals flash one time to confirm the
sensor identification code has been matched to
the tire/wheel position.
6. The passenger side front turn signal comes on to
indicate that corner sensor is ready to be learned.
Proceed to the passenger side front tire and
repeat the procedure in Step 5.
7. The passenger side rear turn signal comes on to
indicate that corner sensor is ready to be learned.
Proceed to the passenger side rear tire and
repeat the procedure in Step 5.
8. The driver side rear turn signal comes on to
indicate that corner sensor is ready to be learned.
Proceed to the driver side rear tire, and repeat
the procedure in Step 5.
9. After hearing the single horn chirp for the driver
side rear tire, two additional horn chirps sound
to indicate the tire learning process is done.
Turn the ignition switch to LOCK/OFF.
If no tires are learned after entering the TPMS learn
mode, or if communication with the receiver stops, or
if the time limit has expired, turn the ignition switch to
LOCK/OFF and start over beginning with Step 2.
10. Set all four tires to the recommended air pressure
level as indicated on the Tire and Loading
Information label.
11. Put the valve caps back on the valve stems.

This info can also be found on page 5-66 of your owners manual.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

I have run into this before on vehicles and find it is leaking around the bead of tire/rim the cooler air lowrs pressure slightly and will leak around bead. this is hard to find wheel will have to be off vehicle sitting flat pour soapy water around rim/tire seal and watch for tiny bubbles might take a while lower air pressure and check again on both sides.sometimes with this concern will only leak when parked in certain position around the wheel weight clamp is a common leak at bead.

if your truck has rear air/heat you will need to check the rear heater lines-they may be leaking

TPMS Sensor Matching Procedure

Each TPMS sensor has a unique identification code. Any time you rotate your vehicle's tires or replace one or more of the TPMS sensors, the identification codes will need to be matched to the new tire/wheel position. The sensors are matched to the tire/wheel positions in the following order: driver side front tire, passenger side front tire, passenger side rear tire, and driver side rear tire using a TPMS diagnostic tool. See your dealer/retailer for service.

The TPMS sensors can also be matched to each tire/wheel position by increasing or decreasing the tire's air pressure. If increasing the tire's air pressure, do not exceed the maximum inflation pressure indicated on the tire's sidewall.

To decrease air-pressure out of a tire you can use the pointed end of the valve cap, a pencil-style air pressure gage, or a key.

You have two minutes to match the first tire/wheel position, and five minutes overall to match all four tire/wheel positions. If it takes longer than two minutes, to match the first tire and wheel, or more than five minutes to match all four tire and wheel positions the matching process stops and you need to start over.

The TPMS sensor matching process is outlined below:

1. Set the parking brake.
2. Turn the ignition switch to ON/RUN with the engine off.
3. Press the Remote Keyless Entry (RKE) transmitter's LOCK and UNLOCK buttons at the same time for approximately five seconds. The horn sounds twice to signal the receiver is in relearn mode and TIRE LEARNING ACTIVE message displays on the DIC screen.
4. Start with the driver side front tire.
5. Remove the valve cap from the valve cap stem. Activate the TPMS sensor by increasing or decreasing the tire's air pressure for five seconds, or until a horn chirp sounds. The horn chirp, which may take up to 30 seconds to sound, confirms that the sensor identification code has been matched to this tire and wheel position.
6. Proceed to the passenger side front tire, and repeat the procedure in Step 5.
7. Proceed to the passenger side rear tire, and repeat the procedure in Step 5.
8. Proceed to the driver side rear tire, and repeat the procedure in Step 5. The horn sounds two times to indicate the sensor identification code has been matched to the driver side rear tire, and the TPMS sensor matching process is no longer active. The TIRE LEARNING ACTIVE message on the DIC display screen goes off.
9. Turn the ignition switch to LOCK/OFF.
10. Set all four tires to the recommended air pressure level as indicated on the Tire and Loading Information label.
11. Put the valve caps back on the valve stems.

AIR SUSPENSION BAG IS LEAKING AIR.

Don't go to the dealer, unless you just have too much money. Call around to tire shops in your area and see if anyone has a "Bar-Tech" tool or a similar device that will reset your sensors for you and HUGE savings.

yes that is the expansion valve! there is a hissing sound from the restriction in it for a pressure drop thats what makes the a/c cold the hissing is normal