Starts but won't stay running?

That is great news. I am greatful that it turend out well for you.
If you ever need anything else we will be here.

Dr. C

A plugged catalytic converter is easy to check, your mechanic should have checked this. Also, check that the throttle position sensor is working as it should. A good scanner with live data feed can check all the sensors and actuators are working and pick up a problem quickly in the right hands. You really need to start at the beginning and work through all the systems. Also, fuel pressure and leaking injectors should not be over looked, start here.

Try cleaning both ends of the battery cables, if it has poor connections, there may be voltage drop to the ECM, also unplug the MAF and see if it runs in limp mode, if yes replace MAF.

DTC P0115: ENGINE COOLANT TEMPERATURE (ECT) SENSOR CIRCUIT Circuit Description The ECT sensor is a thermistor sensor that monitors engine coolant temperature. DTC is set when ECM detects an open or short in ECT sensor circuit. Possible causes are: à   Open or short in ECT sensor circuit. à   ECT sensor. à   ECM. Diagnosis & Repair 1. Connect scan tool to DLC No. 3 connector. See Fig. 1 . Turn ignition on. Using scan tool, monitor ECT sensor temperature. If temperature displayed is same as actual coolant temperature, check component and ECM connections. Problem is intermittent. 2. If temperature displayed is 284 °F (140 °C) or more, go to step 5). If temperature displayed is -40 °F (-40 °C), turn ignition off. Disconnect ECT sensor connector. ECT sensor is located on coolant housing at flywheel end of cylinder head. Connect a jumper wire between ECT sensor wiring harness connector terminals. Turn ignition on. Using scan tool, monitor ECT sensor temperature. 3. If temperature displayed is 284 °F (140 °C) or more, replace ECT sensor. If temperature displayed is less than 284 °F (140 °C), turn ignition off. Remove jumper wire. Access ECM behind bottom center of instrument panel, in front of console. Connect a jumper wire between terminals No. 4 (White wire) and No. 9 (Brown wire) at ECM E5 connector. See Fig. 2 . 4. If temperature displayed on scan tool is 284 °F (140 °C) or more, locate and repair open circuit in wiring harness between ECT sensor and ECM. See appropriate wiring diagram in WIRING DIAGRAMS article. If temperature displayed on scan tool is less than 284 °F (140 °C), replace ECM. 5. Turn ignition off. Disconnect ECT sensor connector. Turn ignition on. If temperature displayed on scan tool is -40 °F (-40 °C), replace ECT sensor. If temperature displayed is not -40 °F (-40 °C), turn ignition off. 6. Access ECM behind bottom center of instrument panel, in front of console. Disconnect ECM E5 connector. See Fig. 2 . Turn ignition on. If temperature displayed is -40 °F (-40 °C), locate and repair short circuit in wiring harness between ECT sensor and ECM. See appropriate wiring diagram in WIRING DIAGRAMS article. If temperature displayed by scan tool is not -40 °F (-40 °C), replace ECM. CAUTION: If ECM replacement is instructed in following testing, always ensure ECM connectors and ground circuit are okay. If either are suspect, repair and repeat testing to confirm ECM malfunction. NOTE: If DTCs P0105, P0106, P0110, P0115 and P0120 are output together, sensor ground circuit may be open. Repair as necessary. See appropriate wiring diagram in WIRING DIAGRAMS article. Page 1 of 1 TESTS W/CODES -1998 Toyota RAV4 9/2/2008

You cannot, what happens is the catalyst matrix which is like a large honeycomb breaks and collapses and blocked the exhaust flow. See picture for a view of a section of catalyst matrix.

You said the rav4 runs ok with the mass air flow sensor unplugged, so check the air duct for any splits or cracks, if none found replace the MAF. Also test the ect, and IAT sensors, here's the range chart.

The best time to test both is befroe the first start of the day, they should be within 5 degrees of each other on your scan tool, also do the resistance check.

Yes, they are easy to replace most are just bolt on some are welded, but an exhaust shop can replace a faulty catalytic converter quote easily.

Have the fuel pressure check, mass airflow and throttle position sensors checked/tested

Use a hand held vacuum pump/gauge and apply some vacuum, do this first with the car off, the MAP should hold vac, if not replace it, then test voltage at the sensor with and without vac, does the voltage change? If no on either part, replace it.