My Trooper underwent 2 complete injector replacement -- which included RPS, RPCV and TPS relays (these conidered to be maintenance items and may be replaced in the future -- of course, on the account of the owner

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Based on my experience, there were variable symptoms depending on the suggestive extent to replace 1 or 2 or 3 or all 4 injectors (note that injector can be replaced individually and not necessarily altogether). Some I notice were the erratic idling (you see the RPM needle play), power lose (some consider limp mode), hard starting (which should not be overdone or the starter may get damaged and battery be drained/damaged) to name a few.
But there is no best way to confirm what the problem may be without the Tech 2 gadget. That way, the Isuzu-trained technician would be able to pinpoint the culprit.