240 ECT or O2 sensor?

So this is the same 1990 240 that I work on a lot. I've replaced the IAC, AMM, FPR, and a bunch of other stuff. The car has a problem where it has a bouncing idle and eventually stalls when cold, unless you hold your foot on the gas. Also gets pretty poor mileage.

It's throwing the following codes:
1-1-3
2-2-1
2-3-1

I'm thinking it's the O2 sensor or the ECT sensor under the intake manifold. Is there a way to test and see which one it is? The car seems to run perfectly fine when the engine is warm. Any ideas?

What RPMs is it bouncing to and from?

500-1000. The longer you let it bounce, the lower it bounces, until it stalls.

UPDATE! I have not had time to look at her 240 yet, but it now stalls whether the engine is warm or cold. It died on her 3 times yesterday in traffic, because she couldn't get her foot on the gas fast enough. I'm gonna try and look at it today. I can't figure out if it would be the ECT or the O2, even after reading through the guides on brickboard. Any other suggestions?

Is "ECT" an abbreviation for intake temp sensor? I'd have guessed that to be the most likely, followed by a clogged cat, and then followed by the O2 sensor (these old systems aren't very s.m.r.t are they? ie. the O2 sensor doesn't have a massive amount of input at anything below operating RPM's and above "middling" RPM's and above about 30% throttle?).

No Forg, It stands for electronic coolant temp sensor, It's what your ecu reads from the resistance to know when it's cold or hot, When it fails it usually makes your car idle and run rich for the first few minutes you start it, or when you accelerate really hard blow black smoke.

To test the ECT, you need a multimeter. At ECU connector, with ignition off and car cold, test terminal 13 and ground the multimeter to a suitable bolt on the ECU bracket or some other ground. Measure the resistance, it should be about 3000 ohms if 60 degrees F outside (and about 5000 ohms if 40 F). Next, test when engine is fully warmed up (and turn ignition off). It should be about 100-200 ohms.

To test O2 sensor, remove rubber cover off of wiring harness connector by the firewall under the hood. You should be able to access the wires. With engine at idle, hook up multimeter to wires, the voltage should fluctuate between 0.4 and 1.0 volts. To test oxygen sensor heater: disconnect the two-wire harness and connect ohmmeter across terminals to measure heating resistance. Cold sensor 3 ohms. Hot sensor 13 ohms. Note: the oxygen sensor needs to be heated up in order to function properly.

Also, vacuum leaks could be causing said symptoms. Check all hoses. Intake manifold and exhaust manifold gaskets.