Control Module Information Modes

The control module has three modes for transmitting information. The following information will describe the system operation in each of the three modes.

ROAD TEST (OPEN) MODE
On engines that can be monitored in the "Open" mode, certain parameters can be observed without changing the engine operating characteristics. The parameters capable of being read vary with engine families. Most scan tools are programmed so that the system will go directly into the DLC mode if the "Open" mode is not available.

DIAGNOSTIC MODE (ALSO CALLED 10K)
In this mode, all obtainable data is readable. However, in this mode, the system operating characteristics are modified as follows:

- "Closed Loop" timers are bypassed.
- Ignition is advanced.
- Idle Air Control (IAC) valve controls engine idle to 1000 RPM + 50 RPM (if applicable).

BACK-UP FUEL (ALSO CALLED BACK-UP OR 3.9K)
In this mode, the control module is operating on the fuel back-up logic used to control the fuel delivery if the control module fails. This mode verifies that the back-up feature is OK. The parameters that can be read on a scan tool in this mode are not of much use for service.