Just replace head gasket on sons 2001 honda civic ex. Test drove it was fine. Parked last night and this morning when to drive and check transmission light stays lit, the d on dash column flashes green and engine revs in drive. What could this be. Replaced head gasket because it would occasionally run hot. Son is active military and getting read to deploy. Money is tight. Just sent $450 for this job.

Attach a code reader to the Data Link Connector, under the dash, and check for codes. Many auto parts stores will do this for free.

Here are some recommended maintenance tips:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563.html#post219991

Be sure to use a 50/50 mixture of the recommended coolant and distilled water. Also, change the transmission fluid, ideally full-synthetic, every 30,000 miles.