BMW Coupes and Sedans 1970-1988 Repair Guide

Thermal Reactor System


The thermal reactor system is used on all models except Federal (49 States) versions of 4-cylinder engines. The reactor allows additional burning of exhaust gases, thus reducing HC and CO emissions. The reactor depends on a supply of fresh air supplied by the air injection system to operate.


See Figure 1

Click image to see an enlarged view

Fig. Fig. 1: EGR and thermal reactor triggering box, mounted in the engine compartment. The thermal reactor reset button (1) and EGR reset button (2) can be pressed with the eraser end of a pencil

After 25,000 miles (40,000 km), a warning light will indicate that the thermal reactor should be inspected for cracks or other damage. Other than this inspection, the thermal reactor is maintenance-free. After inspection, the mileage interval switch must be reset. The switch is connected to the speedometer cable on the inside of the left side engine carrier. Two buttons are located on the switch; push the button marked REAC to reset the switch for the thermal reactor.


  1. Remove the air cleaner with air flow sensor.
  3. Remove the heat guards.
  5. Check both reactors for cracks and heat damage, replacing if necessary.
  7. Reinstall heat shields and air cleaner assembly.