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P305F Code – What Does It Mean & How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.


P305F is an OBD-II Code that refers to Dual Battery Control Module Performance


A P305F code is a manufacturer specific diagnostic trouble code (DTC) and refers to a system uses a dual battery configuration which includes a 12 V primary battery and 12 V auxiliary battery. This code is mostly found in Chevrolet vehicles such as the Malibu and Impala. 

The dual battery control module switches between the two in and out at appropriate times to support vehicle loads and battery charging. The auxiliary battery is switched in to provide voltage to the vehicle’s loads at the start of the engine turn (except for the starter motor, which receives electricity from the primary battery). The ECM monitors the auxiliary battery voltage through IGN, which is an input. If the signal is not within parameters, P305F is set.


Common causes for this code include:

  • Battery circuit poor electrical connection
  • Bad Battery
  • When the battery is getting low.
  • There is an open or shorted battery harness


The symptoms of a P305F code are:

  • Service engine soon light is on
  • Battery not charging
  • Engine cranks slowly or not at all (due to low battery power)
  • Auxiliary or primary battery is not providing enough power to start


To diagnose a P305F DTC code, a technician would:

  1. Scan for codes in the ECM and look at the freeze frame data
  2. Check the battery to ensure there are no open circuits
  3. Verify the ECM is sending a voltage signal to IGN
  4. Replace ECM if needed
  5. Replace primary battery if needed
  6. Repair battery harness as needed, and replace secondary battery if necessary.
  7. Rerun DTC after repairs are made 
  8. Since this code is manufacturer specific, it’s important to consult with your local dealer to ensure proper diagnosis and repair.

Common mistakes

The following are some of the most common mistakes that a technician may make when diagnosing a P305F code:

  • Not checking the freeze frame data to determine the conditions under which the DTC was set.
  • Not checking the documents for the vehicle to ensure that the code is correct and not a different code.
  • Using diagnosis equipment that does not properly interpret the DTCs and advise them of their scan results.
  • Not performing all tests or swap tests necessary to determine what components are faulty. It may be easy to identify one or two components that are faulty but to really ensure the diagnosis, every test should be done.
  • Before replacing any components or parts, always check that they are compatible with the vehicle.

How serious is this?

A P305F code is considered to be severe. When a battery gets too low, it cannot produce enough power to start the vehicle and will eventually be unable to maintain sufficient voltage for auxiliary loads. This means that if your car’s battery is getting low, you need to take it in to be charged or replaced as soon as possible so that you don’t get stranded or have a systems failure at high speeds.

What repairs can fix the code?

The following are solutions that may fix this problem:

  • Replace or repair any faulty components
  • Replace or repair Wiring Harness
  • Replace Battery
  • Cleaning the terminals may fix this issue.

Related codes

A P305F is related to and may be accompanied by the following codes:

P0A80 – Charging System Voltage Too Low

P1123 – Low Battery Voltage Detected


In conclusion, the P305F code is a diagnostic trouble code that refers to the auxiliary battery voltage not reaching the ECM. This can be caused by an open or short circuit in the harness of either the primary or secondary battery, a faulty auxiliary battery, a faulty wiring harness, poor electrical connection in any of these areas, a bad primary battery, a bad ECM, or a combination of these factors. It is important to properly diagnose and repair this code in a timely manner so that it does not lead to future issues with starting and charging.