This does sound like the ecm is at fault here.The ecm controls the check engine light.It should come on every time the key is switched on before you start the car.If it turns off and does not come on intermittently with the key on and the engine is not starting I suspect the ecm is at fault needing replaced.Below is the diagnostic procedure for this fault code you have set here.
The power steering ECU assembly receives signals from the ECM, skid control ECU, and steering control ECU via CAN communication.
- When two or more DTCs starting with "U" are output simultaneously, inspect the connectors and wire harness of each ECU.
- If the power steering ECU assembly detects DTC U0121 only, it adjusts the amount of power assistance for a speed of 100 km/h (62 mph) and continues system control.
1. CHECK WIRE HARNESS (+B1, +B, E)
(a)Turn the engine switch off.
(b)Disconnect the ECM connectors (E4) and (A5).
(c)Measure the resistance according to the value(s) in the table below.
(d)Measure the voltage according to the value(s) in the table below.
NG -- REPAIR OR REPLACE HARNESS OR CONNECTOR
OK -- REPLACE ECM
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