BMW Technical Information System
https://www.bmwtechinfo.com/

ACDelco Technical Delivery System is General Motors' service information
https://www.acdelcotds.com/subscriptions

Honda Independent Repair Website - available subscription terms: 1 https://techinfo.honda.com/

Nissan Publications
https://www.nissan-techinfo.com/

Mazda Service Info
https://www.mazdaserviceinfo.com/

Subaru Technical Information System
http://techinfo.subaru.com/index.html

Toyota Technical information system
https://techinfo.toyota.com/

Volvo Technical and Owner bookstore
https://www.volvotechinfo.com/

HelmInc.com Ford, Honda / Acura Service manuals online, CD/DVD service manuals for many brands.
http://www.helminc.com/

These two websites offer monthly and yearly subscriptions to service manuals adopted for DIY use:

ALLDATAdiy Paid online access to TSBs and DIY service manuals for many brands
https://www.alldatadiy.com/

Mitchell1DIY Paid online access. TSBs and DIY service manuals.
http://www.eautorepair.net

AutoZone - some free repair guides and technical information is available for registered users. Follow the menu 'Repair Help' then select 'Vehicle Repair Guide'. If you want to find the information on testing of various sensors, follow the menu 'Engine Performance & Emission Controls'.
http://www.autozone.com/