Car Won't Start Hybrid Interlock Circuit Open

Hybrid vehicles ranging from Toyota, Honda, Ford, Audi, VW, BMW, and Mercedes-Benz utilize interlock circuits for the hybrid system.  

Difficulty: Intermediate


If the interlock circuit is tampered with, the vehicle will not start. A fault code such as "The interlock circuit of the high-voltage on-board electrical system has a malfunction." will get triggered, disables the engine start.  

The high voltage system on hybrid vehicles can utilize up to 360 volts. Such voltage can cause injuries or even cause death when an untrained technician works on the hybrid system. The hybrid vehicle interlock circuit is designed to disable engine start for safety reasons. One of the main functions is to prevent injuries to anyone who has not been trained to work with the high voltage system.

The following problems can cause the interlock circuit to be open and prevent engine start: 

  • The interlock safety connector has been removed.
  • High voltage (orange) cables are damaged.
  • The high voltage loop is broken.

Interlock Safety Connector


The high voltage safety connector, also known as the interlock safety connector, is typically mounted on the power electronic module. An interlock safety connector may also be mounted near the 12-volt or high voltage battery on some makes.

This connector will de-energize the high voltage system if removed accidentally or when anyone is tampering with the system. 

Once the high voltage safety connector is removed, the interlock circuit is broken, and the high voltage system will automatically turn off. In most cases, you will not be able to start the vehicle.

A fault code related to a high voltage interlock loop will be stored in the ECU or hybrid system control module. The vehicle may not start even after connecting the safety connector. Fault codes will often need to be cleared to start the engine. 

The high voltage safety connector should only be removed when the ignition is off. 

Damaged Cables


The cables for the high voltage system can get damaged, especially if the vehicle has been in an accident. In addition, the high voltage cables may have an inline fuse that may blow. If the fuse is blown, the high voltage system gets switched off. 

When reconnecting the orange hybrid cables, make sure they are fully connected and secured. 

Damaged high voltage cable

If the high voltage cables are damaged, it is recommended to replace them instead of repairing a damaged cable. 

If the high voltage cables are disconnected, you will also see a hybrid battery warning symbol on the instrument cluster. 

The high voltage loop typically runs the high voltage battery to the electrical A/C compressor than the electric machine. The electric machine is mounted between the engine and transmission. You can see the electric machine power electronic unit (also called electrical machine electronics) mounted to the side or top of the transmission. 

Fault codes such as the interlock circuit of the high-voltage on-board electrical system have a malfunction may stay present even after the interlock safety connector has been connected. To start the vehicle, you will need a professional diagnostic scanner to reset such fault codes. 

 Get Help

If you are having issues with the hybrid, please "contact the YOUCANIC" team with any questions that you may have.

We can help you troubleshoot the problem FREE of charge as long as you send us the codes.  We need the codes from these four modules:

  • Engine ECU,
  • DC/DC converter,
  • BMS,
  • Power Electronics Module

Once we have the codes, we can tell you if you have a bad power electronic module, hybrid battery, or both.

If your vehicle is at the dealership or mechanic, ask them for the list of codes and send them to us.    If you want to ship the car to Baltimore, we can repair it at a fraction of the dealer cost.  We love these cars and believe that it is a sin to pay over $10k to keep these cars running. 

Here is the proof. 

mercedes hybrid problems