MRCOOL / DIY 5th Generation
PH 91 error code
Low temperature protection of the evaporator.
- Severity
- protection
- Verified
- Jul 18, 2026
What triggers it
Low temperature protection of the evaporator.
What to do
MRCOOL's service manual lists this code without a documented procedure.
Which systems show it
- MRCOOL DIY 5th Generation single-zone 9k, 12k, 18k, 24k, and 36k D25 indoor/outdoor models listed in manual section 1.1.
Related codes
Technician scope
No MRCOOL service-manual procedure is published for this code; do not infer repairs until a better source exists.
Source
- Document
- data/sources/mrcool-diy5-service.pdf
- Section
- 4.3 Error Display (Outdoor Unit with Auxiliary Board)
- Page
- 37