Assisting my girlfriend with her car

[SilentSpanner]

My girlfriend doesn't know anything about cars, but I do, so we've been attempting to diagnose the problem with her 2010 Corolla 1.8 with 155,500 miles. For the past five years that we've owned the vehicle, shifting into gear (auto) causes the engine to gradually decrease in RPM until it trembles and nearly stops. After cleaning the battery posts and clearing the code, the problem that caused it to throw a p0607 yesterday has not returned, so it appears to have been resolved. We have not encountered any other codes since we acquired it. Additional things that I have done include replacing the air filter and plugs, cleaning the MAF, looking for hoover leaks and adding techron. I am competent in the fundamentals, but I am terrible at figuring out what's wrong. Due to the short distances she travels on her way to and from class (just one mile), I changed the oil every three months regardless of the mileage. The trans has behaved better after I drained and filled it a couple of times when it was acting up. It has reached a stage where, when cold, it starts up fine but immediately drops to about 2-300 rpm and occasionally dies; however, after a few seconds of gas, it smooths out and runs normally. The chillier weather appears to be exacerbating the problem. If we can just prolong this ordeal for another six months, it would be ideal, since she is nearly finished with nursing school. Before we bring it to the shop to get it properly diagnosed, is there anything else I should try? I haven't done that yet because I've been sick for four months, my girlfriend is in school, and I'm single-handedly paying all the bills, so money is tight. I genuinely hope she doesn't become stuck or miss class because of it. I was going to attempt cleaning the throttle body, but I'm not sure it would be effective and it might be a bit beyond my skill level. Thanks for any suggestions. 

[JamesStv]

You already have everything you need to give this a go, and it could work. Starting the engine and spraying WD-40 around the intake will cause the gaskets in the intake manifold to contract in colder temperatures; if the engine's revolutions per minute (rpms) change, then there's a vacuum leak; another, less messy method is to pass propane over the area and listen.

After you cleaned the MAF, the next step is to inspect the throttle body. Idle airflow can be severely reduced by even a thin coating of carbon around the circular throttle plate.

Cleaning instructions: Inspect the intake snorkel for debris. After that, use throttle body cleaner on a rag and carefully remove the black carbon ring from the bore and butterfly plate edges. Stay away from spraying the electronics housing.

The most important stage: It is necessary to conduct an idle re-learn after cleaning. To reset the ECU's learned carbon compensation values, disconnect the negative battery terminal for 10 minutes. Put it back in the car's socket, turn it on, and let it run quietly for ten to fifteen minutes until it gets hot enough to run.

[SilentSpanner]

I will definitely try the manifold trick.

I thought you had to remove the throttle body to clean it; I could probably do it, but I'd rather not. At our garage, I had a conversation with a man who agreed with you—just remove the air hose and gently clean the butterfly and body using a rag and some cleaner.
The issue was immediately resolved after I did that. In any case, the difference is quite noticeable.

I will go take care of that right now because I was unaware of the relearn requirement and, thankfully, we did not drive the car anywhere after the repair.

the helpful advice is much appreciated.

[Hypabl]

Follow the advice and re-learn how to use the throttle. To learn how to do this, just search YouTube. At first, the P0607 indicates that power issues may have occurred, and you noticed the battery's dubious condition; nevertheless, you should get it tested under load. A short amount of time spent travelling drains the battery faster than it can be recharged. You did a great job cleaning and tightening it, but it may still be weak even after starting the car. Alternators have it tough with weak batteries. Quite a few recommendations on this website that defines codes. For more information, visit https://www.obd-codes.com/p0607. 

[Hypabl]

https://www.obd-codes.com/p0607?utm_source=google&utm_medium=organic&__cf_chl_rt_tk=WsnAocZQ1PptG1SJ2A9d.SR40Ye8XrVMZ2NwKnevgOQ-1794487870-1.0.1.1-eZVNcOhlv7COx1ooUcm3s59ZFVa0yP7q05m2VQ5IiKs

[SilentSpanner]

With the clamps removed, I was able to test the battery, and it held up fairly well considering its age—four years. It was probably just corrosion, but I'll be sure to get the electrical system checked out soon. Additionally, I verified that all the available grounds were in excellent condition.