I own a very well maintained and almost trouble free 00 Accord AT 4cyl with 250k miles on it. It runs great with minor issues due to it's age like a recent (original) starter problem.

3 1/2 weeks ago I changed the starter motor (AZ Delco Replacement) until I have the current one (original) rebuilded. Since then I experienced a strange issue I couldn't find any info about or able to replicate. When this happens (morning), the car takes a few starter cranks to start. Idle is rough like almost stalling with low RPMs (less 700) with a sudden jump to high RPMS (over 1,500); and coming down to the regular 1,000rpm. Then, when switching to reverse you can feel a semi-hard shift; same when going to D4. Also 1st & 2nd gear is a little rough on my 15min commute to work. When starting the car again it's symptoms free; car starts quickly and engine/transmission shifting is very smooth. No CEL or D4 blinking previously recorded.

Today I experienced the same issue (3rd time overall). Fortunately a CEL was generated (probably was pending). So I got my ODBC scanner and the pending/logged fault was P1486. Apparently this is "Coolant Thermostat Assembly".

Could this part be the cause of car symptoms? I think that part was changed more than 120K ago. Or could it be the start of the end of my tranny/engine? Maybe some kind of computer problem? Solenoids? Any other advice tests / ideas?

I'm gonna try to replace the Thermostat today.

What I have done so far:
-Checked transmission fluid and it's topped and pink in color. I changed it almost 20k ago.
-Replaced PCV valve.

Some forum searches pointed as possible causes:
-Transmission going bad
-Trans Solenoids
-Oxygen Sensor
-MAP
-PCV Valve *Replaced today with an OEM one I had brand new from a few years ago.
-ECU
-Faulty 2nd Clutch Pressure Switch (Some say it could cause sporadic idle issues)
-Faulty ELD (underhood fusebox) or ECU

P.S. My car is due for a Timing Belt replacement. I already have all the parts waiting for my mechanic appointment mid next week. I only use OEM Honda fluids in my car even for the coolant. In fact I think that the only aftermarket parts in my car right now are "Catalytic Converter", "Starter Motor", "Passenger Side Window Regulator" and "Main Relay". All other replaced parts are Honda OEM.

 

https://www.autocodes.com/p1486_honda.html

I think you have air in the system or a bad temp sensor. If you bleed air out and you haven't had any coolant system work done, I would suspect a failing head gasket. Leak down test may be indicated.

 

Thanks! I replaced the thermostat almost 12 days ago. When checking coolant I noticed the radiator cap inside tip was broken so I replaced that the next day with an OEM one. So far, no more symptoms (knock on wood)...

Maybe it was air in the system due to the broken cap or a bad thermo; dunno know. But I never imagined that a bad thermo/air in the system would cause those transmission issues.

Hope this helps someone in the future....

 

I think what happens is air in the coolant system causes a local hot spot and therefore incorrect temp reading on the sensors, thus ECU does not set the fuel mixture correctly (injectors and IACV). As a result engine then runs poorly. This makes it seem like a transmission problem but in reality its just the engine being mismanaged by the ECU (and air pocket). Good you used an OEM cap ! Glad I could help.