Malfunction Description and Correction

Cause:
Malfunction of the oxygen sensor is possibly due to "thermal shock." Thermal shock may occur only under certain environmental conditions when a cold engine is started and moisture/condensation in the exhaust, passes over the heated oxygen sensor causing a rapid cooling of the sensors outer ceramic shell. This rapid cooling may cause the ceramic shell to shatter or crack.

Vehicles produced from 9/92 up to 5/93 have a time-delay before the activation of the oxygen sensor heater relay based upon engine temperature. While this has proven to be quite successful, we are still seeing a small number of cases of oxygen sensor failure due to thermal shock. This is due to the fact that the exhaust system cools down much more rapidly than the engine block, causing an insufficient delay before activating the heater relay in cases of a restart on a warm engine. This latest update includes the previous delay based upon engine temperature, and will also evaluate the engine load when determining the proper time to switch on the heater relay.

Correction:
Vehicles Produced from Model Year '93 (VANOS)

In the event of an oxygen sensor failure, update the engine control module EPROM if it is not of the latest BMW Hardware Number. Please refer to the Parts Information section of this bulletin for the applicable part numbers.

Latest control modules have BMW Hardware Number 1 703 563.

Procedure:
Please refer to the procedure for complete instructions on the replacement of the EPROM in the engine control module.