FAQ: Dealing with ECU CMOS errors
Autronic ECUs contain CMOS random access memory (RAM) for storage of error history, internal Data Logger set-up configuration, internal Data Logger data, and many adaptive variables including Throttle Position Sensor limit, closed loop lambda control, closed loop idle speed control, and cold start history. This CMOS RAM's complete memory content is lost if its electrical supply is not sustained. Electrical supply is maintained during normal engine operation by the ignition supply; during ignition off periods by a +12 volt permanent supply connection to the ECU; and during periods of complete power disruption or while removed from the vehicle by a super capacitor within the ECU. The super capacitor is normally able to maintain supply to the CMOS RAM for more than one week, usually several months, depending upon the age of the ECU and its storage temperature after power supply disconnection.

Once the super cap is discharged, all CMOS RAM data including adaptive variables and ECU internal  data logger set-up are lost, and a CMOS error is logged. Engine operation following this loss will be temporarily disturbed. Throttle response, idle speed control, boost control, closed loop mixture control, cold start-up and running, and other functions may be affected until the ECU fully re-adapts to the engine. Throttle response can be restored immediately by performing a throttle limit learning procedure (see: How do I calibrate the Throttle Position Sensor?).

CMOS memory loss will be present in all ECUs that have been stored disconnected from a 12v power source for an extended period (typically occurs within 3 months of disconnection from power source). It is normal behaviour for new ECUs that have been stored prior to installation.

CMOS memory loss can be prevented by maintaining power supply to the ECU +12 volt RAM supply connection.

This memory loss predominantly occurs in race car installations that have +12 volt RAM supply connection interrupted by a battery isolator switch (Kill Switch).

It should be noted that since the +12 volt RAM supply connection current draw is typically only several mA and its contribution to vehicle battery self-discharge is negligible, permanent connection to the vehicle battery (via suitable protective fuse or circuit breaker) is always recommended.

CMOS memory loss that occurs while correctly connected permanently maintained power supply source (> 6.0 volts DC) is indicative of an ECU fault condition. It should be returned to Autronic for repair.

After a CMOS memory loss, use the PC calibration software to reconfigure the ECU internal data logger and clear the logged CMOS loss error. Under the software Menu ‘M0’ select the ‘ERROR HISTORY’ screen, hit ‘r’ key to clear the CMOS loss error. This procedure does not function if the CMOS error display value is ‘NOW’. Cycling the Ignition power OFF then ON should change the CMOS error history display value to ‘RECENT’. In the ‘RECENT’ state clearing is possible. If the Ignition power OFF then ON cycling does not change the value to ‘RECENT’ the ECU is faulty. Return it to Autronic for repair.

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