Engine code 21 on 3406e

Timing sensor supply voltage, sounds silly but clean and check all batt and alternator connections. Then look for a bad or chaffed spot in harness. Sense it is supply voltage related a sensor most likely won't help it.

 

This is a problem in the 5 volt sensor supply. It is not a specific sensor, all 3 wire sensors use it. Most likely a wiring problem, but a shorted sensor could cause it. That code means that the 5 volt sensor supply voltage is too high or too low. Without getting it hooked up to Cat ET, all you can really do it check all the connections and wiring.

 

check the wire harness as it makes the bend over the left front corner of the valve cover and front of valve cover its a favorite wear spot