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It sounds like you're attempting the OBD resets correctly. It doesn't matter when you do the reset just as long as the probe is in the right socket (#2 in this case for the fuel system ECU), the ignition is on and you are not in the middle of an advanced diagnostic test. If the LED is on steady after holding the button for 5+ seconds then the unit should be ready for resetting. It doesn't matter if you've read any stored codes. Each unit (fuel ECU and ignition ICU) can store up to three codes. The 1-1-1 end code only comes up if there are less than three stored codes, otherwise the codes simply repeat after the third is read.
I'm guessing your ECU (OBD pin 2 mode) is constantly detecting an ECT problem and won't reset because of that. The engine may need to be firing for the ICU (OBD pin 6) to detect the fault it saw or the problem may be intermittent (suggesting a wiring/ground problem). Whenever you have OBD trouble you should try the ultimate reset by disconnecting the battery for a few minutes. That will also reset all the trim patterns it's stored coincidentally often giving a temporary improvement in performance. Note that the OBD reset button does not reset trim patterns.
The ECT on the LH 2.4 system (used on your '89) is a dual sensor type, one half for the ECU (the grey/white wire from the LH 2.4) and the other half for the ICU (the red/black wire from the EZ-116K). The ECT grounds directly to the block.
Because you're getting an ECT failure in both modes that leads me to believe there's only two possibilties: a) The ECT is indeed faulty. b) There is a faulty electrical connection. In the latter case, because both ECU and ICU wiring would have to be involved, the most likely possibility is a bad wiring connection either at the ECT or at the intermediate block connectors under the right front strut tower brace (a known trouble spot in the pre-'90 700's). There's also a possibility that the case of the ECT isn't properly grounding to the block -not unheard of. Connection problems at the ECU and ICU themselves are rare unless there's a water leak or someone's been poking around. Note that '89 wiring harnesses are the newer ones not known for insulation degradation, but wiring damage is always possible.
Start by inspecting the wiring connections -remove, clean and reconnect if there is any doubt. The next step would be to test the ECT signal. The test for the ECU sensor side of the ECT can be done at the ECT connector, but is best done at the ECU connector and is fully described in the FAQ. The FAQ does not make it clear that the test is only for the B230F/B234F LH 2.4 ECU -checking between pin #13 (grey/white wire to ECT) and pin #5 (ECU internal ground). As an aside for others reading this, tests for the B230FT LH 2.2 ECU (which may well get different temp readings as it uses the older style single sided ECT) would be between pin #2 (grey wire to ECT) and pin #5 (black wire grounding ECT through ECU internal ground). Always probe the ECU and ICU connectors by removing the shell and probing through the side slots to avoid damaging the connector pins. Failing all that, remove and clean the ECT in case the block grounding is bad. If that doesn't fix it then it's ECT replacement time. The ECT can be a bear to get out depending on how frozen it is in the block -it's easy to break off the ceramic connector so it's best to only remove it as the last step. With penetrating oil, a large, snug wrench, a little patience and keeping the wrench from slipping, I've usually had success once I got enough stuff out of the way, like the idle control valve, manifold strut and flame trap. If needed, take the intake manifold off for proper access.
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Dave -own 940's, prev 740/240/140/120's & quasi-expert only on a good day
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