Testing the O2 sensor would be a good start.

A constant 7.2 volts would correspond roughly to a 50% duty cycle, which in the 124 world is either a bad sensor or wiring.

Try unhooking the sensor and monitoring its voltage with the engine warmed up and running.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe