You may have an issue with your OVP relay (the ABS light has come on in my 560SEL when my OVP went out).

Here is a fault code list that I got from somewhere about a year ago, and I'm pretty sure this would apply to the e420 you have, but #19 and #20 would make me question that (I know it's for a w124 e320 anyway...)

Display Diagnostic Code
1. turn-engine off;
2. turn ignition switch to #2 position (do not start car);
3. depress switch (at diagnostic port) for 1-second, then release;
4. count number of LED blinks before going to solid/always on;
5. depress switch again to see if there are any other faults in memory (faults are displayed in descending order, and if there are no other faults in memory, the first fault number should re-appear);
6. record the number of blinks you counted for history;

Erasing/Resetting Check Engine Light
within 20-seconds of LED going to solid/always on,

1. depress switch for 6-10 seconds, release;
2. LED should blink ONCE then go to solid/always on;
3. turn ignition switch off;
4. restart car and verify that Check Engine Light is OFF (assuming you have CORRECTED the problem).

Here are the error codes.
Count the blinks after pressing the diagnostic button for 1 sec.

1. No Malfunctions;
2. Full load contact, throttle valve switch implausible;
3. Coolant temp. read by CIS-E implausible;
4. Potentiometer voltage implausible;
5. O2-sensor signal implausible;
6. Not Used;
7. TNA-signal (RPM-signal) read by CIS-E implausible;
8. Altitude pressure signal from EZL control unit implausible;
9. Current to EHA is implausible;
10. Idle contact, throttle valve switch implausible;
11. Air Injection System;
12. Absolute pressure values from EZL is implausible;
13. Intake air temp. reading is implausible;
14. Speed signal read by CIS-E is implausible;
15. Not Used;
16. EGR
17. O2-sensor line shorted positive to ground;
18. Current to idle speed air valve is implausible;
19. Not Used;
20. Not Used;
21. Not Used;
22. O2-sensor heating current implausible;
23. Short circuit to positive in purge switchover valve circuit;
24. Not Used;
25. Short circuit to positive in start valve circuit;
26. Short circuit to positive in shift point delay circuit;
27. Data exchange between CIS-E unit & EZL unit interrupted;
28. Intermittant contact in coolant temp. sensor circuit;
29. Difference in coolant temp. read between CIS-E unit & EZL unit;
30. Not Used;
31. Intermittant contact in intake air temp. sensor;
32. Not Used;
33. Not Used;
34. Coolant temp. from EZL unit is implausible.