Renix uses no flash-code system. You need an ICheck box, Renix-era scan tool, or DIY voltmeter test at the diagnostic connectors.
The Renix system (1987-1990) is the only era of XJ without a usable flash-code or OBD scan capability for most home wrenchers. The system has two diagnostic connectors near the airbox/cowl labeled D1 and D2 (Diagnostic 1 and 2). Renix-era dealer used the OTC/Kent-Moore MILLER tool; modern equivalents are extremely rare. The DIY method involves connecting a digital voltmeter (high impedance, 10 MOhm) to the various sensors and checking specific voltage values against the FSM with key on engine off and engine running. The lunghd.com Renix diagnostic guide is the gold-standard reference and walks through each sensor test. Common diagnostic targets on Renix: TPS reference voltage 4.5-5.0 V, MAP signal 4.5-5.0 V key-on engine-off dropping to ~1.5 V at idle, CTS resistance 8500 ohm cold to 350 ohm hot. Plan to spend an afternoon with the multimeter and FSM open.
Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.