I know this was posted here some time ago, but every time I try to look for it I can't find it and I've seen some recent questions relating to this topic. I found it on another website so I figured I would post it.

Of course, the way to enter the diagnostic mode on the instrument cluster is to depress and hold the instrument cluster (Trip odometer) SELECT/RESET button, turn the ignition switch to the RUN position, and then continue pressing the SELECT/RESET button (5 seconds) until tESt is displayed in the odometer. The SELECT/RESET button must be released within 3 seconds of the odometer tESt display to begin the dealer test mode.

Depress the SELECT/RESET button to advance through the following steps until dtc is displayed. Depressing the SELECT/RESET button will display any stored continuous DTCs before proceeding to the next step.
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Odometer Display Description

GAGE Activates gauge sweep of all gauges, then displays present gauge values. Also carries out the checksum tests on ROM and EE. If the gauge sweep is inoperative, INSTALL a new instrument cluster.

All segments illuminated Illuminates all odometer segments. If any odometer segment is inoperative INSTALL a new instrument cluster.

bulb Illuminates all micro-controlled indicators and LEDs. Install a new indicator or LED as necessary.

r Returns to normal operation of all micro-controlled indicators and LEDs and displays hexadecimal value for ROM level. (used when requesting assistance from the hotline). If alternating flashes for FAIL and ROM level are displayed, INSTALL a new instrument cluster.

EE Displays the hexadecimal value for EE level (used when requesting assistance from hotline). If alternating flashes of FAIL and EE level are displayed replace instrument cluster.

dt Displays hexadecimal coding of final manufacturing test date (used when requesting assistance from hotline).

dtc Displays continuous DTC's in hexadecimal format. Pressing the SELECT/RESET button will display any DTCs stored before proceeding to the next step.

enG Displays the English speed in MPH. Speedometer will indicate present speed within tolerances. Display will show 0 if input in not received. If input is invalid for one second or more, or if speed is 0.

m Displays the metric speed data in kph. Speedometer will indicate present speed within tolerances. Display will show 0 if input in not received. If input is invalid for one second or more, or if speed is 0.

tAc Displays the tachometer data received from the PCM via the SCP network within tolerances. Tachometer will indicate present RPM. Display will show 0 if input is no received, if input received is invalid for one second or more, or if engine RPM is 0.

FUEL Displays the code (0-255) for the fuel sender input to the HEC. The fuel gauge will display a filtered fuel level value. This filter will keep the pointer from moving suddenly or erratically.
255 open send +/- 0
232 full stop +/- 0
215 Full mark +/- 10
178 3/4 mark +/- 8
138 1/2 mark +/- 7
93 1/4 mark +/- 5
41 E mark +/- 4
54 Low Fuel (0-59)
0-18 short (0-20 max)

OIL Displays the code (0-250) for the oil pressure switch input to the HEC. Oil pressure gauge will indicate present oil pressure. Normal oil pressure (greater than 6psi) will display a value between 000 and 176. A low oil pressure or an inoperative engine oil pressure switch (less than 6 psi) will display a value greater than 176.

dEGC Display of engine temperature in Degrees C input from cylinder head temperature sensor.
49 C "C" mark
60 C Normal band start
120 C Normal band end
-40 C No SCP message for 5 seconds

bAtt Displays the code (0-255) for the battery voltage input to the HEC. Battery voltage gauge will indicate present battery voltage. 93-102 6.2-9.1 volts, low voltage
115-124 8.5-10.7 volts, Normal band start
215-225 15.8-18 volts, Norm band end
230-241 16.9-19.1 volts, high voltage

rhEo Displays the present decimal rheostat dimming input, 0-255 (used when requesting assistance from the hotline)

rhi, rhS, rho Not used.

Cr Displays the current RUN/START sense input. Display will show -h for high input with the ignition switch in the START position and -L for low input with the ignition switch in the RUN position.

PA-PE7 not used.

GAGE Repeats the display cycle
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DTC Trouble Code List

9202 Fuel sender open circuit
9204 Fuel sender short to ground
9213 Anti-theft number of programmed keys is below minimum
A103 or 9232 Antenna not connected-defective transceiver
9317 Battery Voltage high
9318 Battery voltage low
9342 ECU is defective
9356 Ignition run circuit open
9364 Ignition Start circuit open
9600 PATS Ignition Key Transponder Signal is Not Received - Damaged Key or non-PATSKey
9601 PATSReceived Incorrect Key-Code from Ignition Key Transponder (unprogrammed Encoded Ignition Key)
9602 PATS Received Invalid Format of Key - Code From Ignition Key Transponder (Partial Key Code)
9681 PATSTranceiver Signal is Not Received (Not Connected, Damaged, or Wiring)
A139 PCM ID does not match between Instrument Cluster and PCM
A141 NVM Configuration Failure (No PCM ID exchange between Instrument Cluster and PCM)
A143 NVM memory failure
5284 Oil Pressure Switch Failure
D027 SCP Invalid or Missing Data for Engine RPM
D041 SCP Invalid or missing data for Vehicle Speed
D043 SCP Invalid or missing data for Traction Control
D073 SCP Invalid or missing data for engine coolant
D123 SCP Invalid or missing data for Odometer
D147 SCP Invalid or missing data for vehicle security
D262 Missing SCP message.

 

I used to go into that diags everytime I drove the truck. I had it on either tac or degC in the warmer days, especially after the Mezieres went on.