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Grid heater or batteries?

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Ok cooler weather here and after truck sits for 2 or more days I get a P0122 DTC 3-5 seconds after start. 

the code is low volt at APPS. I can clear code and it will stay away until the grid heaters cycle after start. I had batteries load tested atl. is good. The batts are 5 years old. All connections are clean and tight. If started and heaters don't cycle then no code.

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Another thing to check is the negative cable on the passenger side. There is a heavy cable/wire going from there to the firewall this is the ECM, PCM and VP44 ground cable. Make sure the connection and the plug is good too.


Another way to check cable is doing the voltage drop test.


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  • 3 weeks later...


Did the checks in above post. Cleaned neg wire. Also pulled heater relays and wiring cleaned and dielectric grease. Now waiting for colder weather. This morning it was 48 the grid heaters cycled 3 or 4 times and for once no p0122 code, also the voltage reading was higher this morning. Before when heaters cycled on cold morning I would get 11.3, today 12.2 was lowest reading.Later this week temps are going to be 38 over night so will see then. Thanks for all the help. 

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