Light is on all of the time. On occasion, it blinks. We have it in "Test" mode.

Where are the easiest places to start to find out the problem, and where do I go from there?

If it ends up being a ridiculously expensive repair, can I just drive it in "Test" with no issues?

Thank you for your time.

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2004 E320 Wagon W211
2000 CLK430 Convertible W208
1985 300SD W126
1983 300D W123