ECU failure is very highly unlikely. Follow the OBD-II self-diagnostic trouble codes.
Suspect the TPS. Test the TPS.
A Jeeper had OBD-II Check Engine Light trouble codes P0123, 0122, and 0121, Installing a new genuine Jeep TPS sensor did not solve the codes or cure the symptoms. On the highway the driver noticed the cruise control didn’t work either, and the Jeep was sputtering/stalling, The turn signal was acting weird, sometimes it would turn on, sometimes not...sometimes it would blink 10 times a second like it had a bulb gone out. Others Cherokee owners report the odometer randomly goes blank. The cure to all the symptoms was a new clock spring.
COMPLAINT: TPS TROUBLE CODE WILL NOT CLEAR
A late model Jeep with a 2.5 or 4.0 liter engine and the AW4 transmission comes in with the “MIL” Check Engine Light illuminated and a complaint of late shifts. A scan reveals a code P0123, “TPS Voltage High”, is stored. The scan tools data list indicates that the TPS signal wire voltage is at 5.0 volts at closed throttle, and then ranges up to 11.5 volts. A back probe of the TPS signal wire (usually is an Orange/Dk Blue wire), shows 5.7 volts at closed throttle, with a similar rise in voltage as the throttle is opened. A check of the TPS 5 volt supply and ground circuit prove that both of these circuits are good.
A check of the TPS signal wire with the TPS disconnected shows the same 5.7 volts. Inspection of the wiring between the TPS and the PCM and TCM shows no faults. A replacement TPS does not cure the problem, neither does replacement of the PCM or TCM.
TESTING: Unplugging a faulty Clock Spring should return the TPS voltage to its’ normal readings. Or, you can remove the Horn fuse in your fuse block - this will remove the 12 VDC supply to the clock spring. Once the short circuit is temporarily disconnected, you should be able to reset the trouble code, and confirm you diagnosis of a faulty clock spring.
CAUSE: The Switch Sense circuit in the Clock spring in the steering wheel hub has shorted to battery voltage from the horn button circuit. When the Switch Sense Circuit shorts to the 12 volt HORN circuit the TPS signal voltage is driven high and remains there as shown. This is because the 5 volt reference voltage used by the Cruise Control “Switch Sense” circuit and the TPS are shared inside the PCM.
CORRECTION: Replace the Clock spring assembly. Once the Clock spring has been replaced be sure to check the sensor 5volt reference voltage at the PCM to insure that circuit has not been damaged by the short to power. Also check the TPS signal range to insure the TPS has not been damaged.
