Main code 61 indicates the ECU has detected a hot fluid condition in the output retarder.

Possible causes (but not all causes) for hot fluid are:
1. Prolonged retarder use.
2. Low fluid level.
3. High fluid level.
4. A retarder apply system that allows the throttle and retarder to be applied simultaneously.
5. Cooler inadequately sized for retarder.