Defect LED detection for car lights

Question

The schematic beneath is a new project of me.

The theory behind this is: My car has a very fool proof bulb failure detection and warning system. CANbus proof LED bulbs can't fool the car and just a parallel resistor will make a faulty LED bulb undetectable. So when the parallel resistor (wich imitates as a incandescent bulb) can be "switched" off when a short or open circuit LED bulb is "detected", the car will see a faulty bulb.

But... another common fault in LED bulbs is that the LEDs are burned, but without a full open or short circuit. They just take less current/voltage when this happens. So there should be a third "detection": the voltage/current drop. How can I managa that, without heavily increasing the number of parts and most important, the costs.

At this moment, it is only for the rear brake and indicator bulbs and every bulb needs a circuit as the example below, with 2 circuits combined in 1 box and 1 box for each side.

Thanks in advance.

^{simulate this circuit – Schematic created using CircuitLab}

^{simulate this circuit}

Figure 2 by @Transistor. A more conventional drawing with voltage decreasing from to to bottom, chassis symbol used for the vehicle groun and ground symbols all pointing down (towards the ground).

Answer 1

A product I designed has a diagnostic test for the LED that simply measures the voltage across it and compares it to high and low limits (it would be possible to temperature-compensate those limits).

That's done as part of the supervisory functions of an MCU that is there anyway, so it occupies one GPIO pin and requires an extra resistor or two (one or both of which may have been otherwise unused in networks). The software overhead is negligible, in program size and speed, even with a lower-end MCU.

Answer 2

The voltage of diodes and transistors might vary significantly with temperature, not to mention the variation between different pieces and types, and therefore I do not think that you can do anything better than measuring both the current and voltage, save them and compare to their previous values. You have to do these all in a relatively noisy environment. Not an easy task!