code U1421 what does this mean?

The code found is U1421 implausible ignition key off time received. Your truck has a key cycle counter that runs even when the truck is off. It will tell the TIPM or ECM the time when it goes to start. That code indicates typically an invalid time response. This should not be a critical code. Here is the actual test for the code. Note: it requires the factory scanner to check it and the other control modules for any other possible codes:

For a complete wiring diagram Refer to Section 8W.

• When Monitored:
• Ignition on.
• Set Condition:
• The fuel volume message the ECM is receiving is implausible. The circuit is continuously monitored. One Trip fault.

Possible Causes

CAN B OPEN OR SHORTED

CLUSTER MODULE

FCM (TOTALLY INTEGRATED POWER MODULE)

ECM

Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding.

 Theory of Operation

The Totally Integrated Power Module (TIPM) isn't receiving a fuel volume signal over CAN B from the Cluster Module. The TIPM has to send the PCM a fuel volume signal over CAN C. The signal the TIPM sends over CAN C is implausible.

 Diagnostic Test

1. ACTIVE DTC

Ignition on, engine not running.

With a scan tool read DTCs.

Is the DTC active at this time?

Yes

• Go To 2

No

• Refer to the INTERMITTENT CONDITION Diagnostic Procedure.
• Perform the POWERTRAIN VERIFICATION TEST.
  

2. CAN B BUS HARDWARE DTCS ALSO ACTIVE

With a scan tool check for DTCs in the TIPM.

Are any CAN B Hardware related DTCs active at this time?

Yes

• (Refer to 8 - ELECTRICAL/ELECTRONIC CONTROL MODULES - DIAGNOSIS AND TESTING) for the appropriate diagnostic procedures.
• Perform BODY VERIFICATION TEST - VER 1.

No

• Replace and program the Powertrain Control Module per Service Information.
• Perform the POWERTRAIN VERIFICATION TEST.

As you can see you will need the right scanner etc. so this may actually be better serviced at a repair center that has the right tools.