Poor connection to 12v Battery negative.
I recently experienced a strange fault. The Check Engine Light and IMA light came on together. Codes flagged were P0420 and P1558. Traction battery state indicator went down to one bar and the car started, but used the 12v starter motor which was sluggish in operation. Under normal circumstances, I reset error codes by removing the 12v battery positive terminal, short it to earth for 60 seconds and then replace it. This mostly clears the codes and extinguishes the fault lights. On this occasion the fix did not work. I applied the fix, started the engine. After 5 seconds both lights and fault codes returned. This situation was repeatable and I could not clear the codes and lights.
In order to correct the fault, I charged the traction battery with my grid charger. Since the 12v battery volts were a little low, I decided to charge that as well. In removing the 12v battery terminal connections to charge it, I noticed that the nut on negative terminal clamp bolt was barely finger tight. Much of the bolt thread had rusted away and the nut was rotating fairly aimlessly and not doing much clamping. I replaced the clamp bolt, charged both batteries applied the fix and the error codes were reset and the fault lights extinguished.
I conclude that the connection between the negative terminal post and the terminal clamp was higher resistance than normal and sufficiently high resistance to upset the vehicle electronic systems even though the car would start on the 12v starter motor.
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