Code 41 - No Distributor Reference Signal

Code 41 indicates there are no distributor reference pulses to the ECM at a specified engine vacuum. This code could set with the ignition on, engine "Not Running" if the MAP or vacuum sensor indicates "Engine Running" voltage with the ignition on. With a constant open or ground in the reference signal circuit, Code 12 would be set along with Code 41. Use Chart 12, if 12 and 41 are set. Code 41 alone indicates problem is intermittent. When distributor reference line signal is lost, engine runs full rich and with retarded (base) spark timing. The result is poor performance, poor fuel economy, and possibly rotten egg odor from the exhaust.

1. This test checks to see if MAP or vacuum sensor voltage changes with loss of vacuum supply. A good sensor will change voltage at terminals "A" to "B" by 1 volt or more.
2. This test checks for cause of an intermittent open or ground in the distributor circuit. Fault could also be a MAP or vacuum sensor that is intermittently stuck, at the same voltage output as an engine "running" condition, with the key only in "ON" position. This condition will produce no reference signal. Terminals must be removed from connector to properly check them. The distributor pick-up coil should also be checked.
3. Since voltage change was less than 1 volt, problem is in MAP/VAC system. The ECM has seen engine "running" vacuum equivalent with no distributor reference signal, with key in "ON" position and engine "not running".