On 86 - 87 vans, the ECU temperature sensor failing would typically be associated with a code #4. This sensor is very important to the correct running of the engine. Since the sensor is inexpensive and easy to replace I recommend replacing it ASAP any time a code 4 is read. Here is a picture that shows it's location:
The sensor is Napa part #TS5520 for around $15. If you're rolling in the bucks, then it's Toyota part #89422-12010 for around $80. The part number you stated (89422-30020) is the old number..........was superseded by #89422-12010.