Thank you for giving me some help.
I have some experience with electronics, but I'm not electronic engineer. I used to fix my stuff, but now I am stuck.
My post is for RF but maybe also for general electronics.
I have a Mazda 2008 with remote start. The engine starter not work anymore (because the sensor-decoder with the wire around the key soket need to be changed), but the rest is working. I used it almost always for keyless entry and alarm.
I have 2 remotes. On back is written:
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MAZDA ( I found also images on internet as "Hyundai")
P/N 0000-8F-F06;
FCC ID GOH-PCGEN2 (this one I found to Hynday and Ford too);
IC3954A.TN400;
P3465.
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Inside, the electronic plate circuit is
P/N 1022912 - one can find it with google -for ex. here https://www.ebay.com/itm/266249362363
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One remote is used, other is almost new, but (strange!!) both of them have same problems. Started to refuse from time to time operating from first button push, needed 2 or more pushes (lock or unlock). Then, start needed to push some other buttons too till the unlock works. Then I keep both remotes with me, but both did same thing. I can open door with key but can't start car when alarm is off - and disarm can be done only by remote. So I use only keys, but is not secure.
Then I replace the batteries. Old one has around 2.9-3V, new ones 3.3-3.4V (don't remember exact, but better than old ones). With remote open, test and work normally. There is a led inside and each contact is well seen on LED, and on car. However, after you let car stand a while (30 min or more), it doesn't work well again.
All tests I did in my house.
I discover that on first try (contact with my metallic pense for ex) the led is not lighting, (as not good contact), but after few tries, the led become stronger and stronger, and the car start to respond. With rubber buttons is almost the same, but need more work (more pushes). Then it works normaly indefinitely. Till I pause a while (20-30 min). Then again, need to be "heated". LED is only inside, can't be seen outside whith remote covers closed, so I can't know what is happening near the car. All tests I did from inside my house, and car respond as I described. Near the car, with remote assembled, I can't tell, but it works randomely, sometimes after few pushes, other times after many, many minutes.
I can only think of an degraded element that work well only when heated. Tried with my finger, can't find one. Try to find description of internal electronic CHIPs but there is no match on internet for
101070-2
06522VG
and a smaller one is
100
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0724M02
What I found for RF remotes is that usually is a coder (bigger) and a RF circuit.
What can change the current in LED I can't understand. All circuits are alimented from battery through a resistor of 0.1 ohm.
Attached photos are from circuit found on internet (better quality).
Need help please for what to try.

 

Tried to add a summary, but the 10 minutes expired, so, here it is on short:
Summary:
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Remote transmit well when internal LED is lighting well. First button push not make it lit normal, it needs more or sometimes many many more. 2 remotes, one used daily and one almost not used, both work bad (both made in 2007). Can be the car receiver or damaged part in transmitter (because is old not because is used - the not used remote works bad too - think of degraded capacitor).
Read details in first post.
Thanks.

 

Does the button on the remote switch power to the circuit?
If so then the switch may have developed a high resistance and multiple presses cleans the contacts.

 

Does the button on the remote switch power to the circuit?

No, circuit is allways powered though a diode (??) D6. Each buttons is connected between negative power (-) and one entrance of a diode D. Not well understand how it works 2 buttons on one input in I.C. U1, at least regarding B2 and B3. B1 and B8 can work with 2 differents amps (from pin 5 to B1 goes by comb to negative, but press B8 connect pin 5 to negative by a diode).
Please see new picture of circuit.

 

@ MisterBill2
-Yes, I cleaned all contacts, batteries and sensors with alcohol and wearing gloves. Also,
-I over-pressed batteries contacts with a piece of sponge-rubber put inside cover,
-I put autoadhesive aluminium foil on "Unlock" button, in case of original carbon rubber is not good anymore
all without any luck.
Also, I re-measured batteries, my previous info I was wrong. See actual info:
V=3.02..3.04V (open circuit, average, for all batteries)
Isc=0.018A ... 0.012A depending of wich batery I test (measured on 10A instrument scale).
Batteries were ordered last year, installed this summer, but without using remotes (because they did not work).
Do you think I need to order new batteries?
Thank you

 

Batteries were ordered last year, installed this summer, but without using remotes (because they did not work).
Do you think I need to order new batteries?

This has happened to me. Purchasing inexpensive (cheap) batteries on line I quickly discovered that they do not have a long shelf life. Bought a pack of 10 CR2032's because the price was too good to pass up. Or so I thought. A few years later I needed one. ONE! NINE of them were useless. Huge bargain there? Nope. So I've taken to ordering one or two from REPUTABLE sources instead of someone who's selling brand name batteries. There's no telling how long they've sat around.
Seller: Joe-Schmoe. (fictitious name - not an actual person)
Customer rating? 0
Reviews 3.6 stars out of 5
Top review: "Works great"
Worst review: "Dead on arrival"

There's very little to prevent people from buying up batteries from these reputable sources but these batteries may have failed in inspection and were offered at gross discounts. Those failed batteries are then put on line for sale by people like Joe.

Buy your batteries from a real brick & mortar store and only buy what you will need within a few years. Beyond that - it's a gamble.

I'd try a new battery and see what happens. If nothing happens then you either have a bad remote or the receiver in the car has gone bad.

 

This has happened to me. Purchasing inexpensive (cheap) batteries on line I quickly discovered that they do not have a long shelf life. Bought a pack of 10 CR2032's because the price was too good to pass up. Or so I thought. A few years later I needed one. ONE! NINE of them were useless. Huge bargain there? Nope. So I've taken to ordering one or two from REPUTABLE sources instead of someone who's selling brand name batteries. There's no telling how long they've sat around.
Seller: Joe-Schmoe. (fictitious name - not an actual person)
Customer rating? 0
Reviews 3.6 stars out of 5
Top review: "Works great"
Worst review: "Dead on arrival"

There's very little to prevent people from buying up batteries from these reputable sources but these batteries may have failed in inspection and were offered at gross discounts. Those failed batteries are then put on line for sale by people like Joe.

Buy your batteries from a real brick & mortar store and only buy what you will need within a few years. Beyond that - it's a gamble.

I'd try a new battery and see what happens. If nothing happens then you either have a bad remote or the receiver in the car has gone bad.

Can I use a power source, just for test? Don't want to buy batteries (expensive at brick store) and if no help, drop to garbage.

 

3.3V @ 30mA would work. Just consider that the battery is not capable of producing more than 30mA. Therefore 3.3V ÷ 0.03A = 110Ω. So I'd suggest you use a 100Ω resistor in series with your 3.3V power supply. I wouldn't suggest running the risk that a hard 3.3V might harm some electronics.

Ohms Law says a circuit will only draw as much amperage as it needs, provided that amperage is available. I wouldn't chance that the electronics might draw more than 30mA. That's the reason why I suggest a series 100Ω resistor.

 

I have had a similar problem with the remote unlock for my 2017 Grand Caravan. One remote started taking a bit of button wiggle to operate. The other works perfectly every time. Everybody told me it was a weak battery. A new battery checking full voltage has the same problem. Took it to the "master" lock shop, which has the machines for programming. the keys. Can they replace the worn out PCB with the failed contacts??? NO! they will give me a "god deal" and use the same plastic housing, for $160 USD. The second key cost us $175 when we bought the vehicle. (It was a year old lease turn in, had everything we wanted), half the price of new. Of course it is the older key that wore out.

 

I have had a similar problem with the remote unlock for my 2017 Grand Caravan. One remote started taking a bit of button wiggle to operate. The other works perfectly every time. Everybody told me it was a weak battery. A new battery checking full voltage has the same problem. Took it to the "master" lock shop, which has the machines for programming. the keys. Can they replace the worn out PCB with the failed contacts??? NO! they will give me a "god deal" and use the same plastic housing, for $160 USD. The second key cost us $175 when we bought the vehicle. (It was a year old lease turn in, had everything we wanted), half the price of new. Of course it is the older key that wore out.

Yea, mine is 17 years old and worked well till last summer (of course, I changed the battery once at 4 or 5 years. But we used only one remote, the 2nd one was only "in case of"... Strange is that both get "stressed" in almost same time, starting with the not used one. I should say that is the receiver in bad shape, but... when the LED of remote not light, it not work, so it should be on the transmitter. I am not sure WHEN the LED come on: when it transmit or when it receive from the car the answer "OK" (however, it is not bidirectional system, of my knowledge). The LED is not visible from outside, so have no idea how (or when) it works. I came here to ask because maybe one of the electonists here, somebody, can make the analise of circuit and found where could be the problem. 2 remotes like this plus programming of the alarm, goes over the price of my whole car.
Strange is that I have to make short-circuits on the contacts few times (did it with my pense) and the LED start (when button is pressed) with very low, each time stronger (almost crescendo but not always, possible depending also on the quality of "the short circuit" that I made on the comb sensor on printed circuit), till at one moment when the car answer (and the LED is high). After that, the led is always high, and car works always. Also, it works well with the original rubber button. Is like something works when it get hot, but I have no idea what. After a while (like "when get cold again"??) car not respond and LED is low. It is no button fault, as when one works, all works and viceversa, when it not work, all button not work.

 

That is similar to when, as teenager, I played with some cheap electronics, a relay in colector of a transistor. The relay buzz but not get close when the transistor was cold (the resistance E-C was too high, cheap transistor, from disassemblings), but once it get hot, was no problem, the relay works well. (normal, resistance E-C get lower with rising temperature, the amperage rise and so the temperature getting the resistance even lower - this is a cycle with a positive reaction... and growing logaritmic; if keep long enough, you may burn the transistor). My remote will not burn, but it seems similar. But... why both remotes after 20 years in almost same time? The active parts are degrading with use, (more on improper use), but one remote was almost not used, and do the same. So , I thought to be a resistor or a capacitor. I found a lot of motherboards that stop working because of capacitors, even just becomes old, even not used. So I thought can be similar here. Any thought??