The "D" light started flashing today along with message "Check Transmission". No engine malfunction light.
The car drives and shifts fine. Fluid level is fine.
Would there be a diagnostic code due to this even though the EML is not lit? If so, can a cheap ThinkOBD 100 read those codes?
Any hints on how to approach this problem?
Assuming it's a pressure sensor, what voltage should be read when I probe the sensor with a multimeter?


Would this cheap $23 ThinkOBD 100 reader be useful here:

 

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did you ever find out what the issue was ? My tsx started doing it out of nowhere just now.
transmission fluid looks good and still bright colored.
car shifts fine
but blinking D light & D3 light is solid on.

 

Here's my plan:

1. Drive the car until the D blinks, then switch to manual and note which gear numbers blink.
2. Without turning off the ignition, plug in an ODB2 reader and see which codes (if any) are current or pending.
3. Inspect each transmission oil pressure switch (#s 9 and 10 in diagram) to see if any one is leaking transmission fluid (engine off).
4. Test each pressure switch with a multimeter.

For step 3, you can test with the switches in situ. A pressure switch is obviously bad if it's leaking fluid. The fluid can breach through either where the terminal is, or around the housing. Just unplug the connector and view with a borescope to see if any fluid is leaking from where the terminal is, or use a small cotton swab.

For step 4, you can also test with the switches in situ. I believe these pressure switches are of the "normally open" type--meaning no continuity (open circuit) when no oil pressure is present. Unplug the connector. Using a multimeter set to test continuity, touch one lead to the connector side contact (or perhaps even a ground), then touch the other lead to the terminal on the switch. If there is continuity, then the switch would be bad if my assumption about the type is correct. I believe you can also test for DC voltage--if the meter reads either 5V or 12V, then the switch is bad if my assumption is correct. I believe it should be no volts when not in operation.

To uninstall a switch, use 22 mm hex deep socket. 7/8" can also work.

 

I know this might not be any help. But the D was flashing on my 04 and I scanned it and found it was 3-4 pressure switch. I had ordered the switch and before it came in my cooling fan went out and I was having temp issues. I replaced the fan and the trans light hasn't flashed since. It may not be related at all since this car has been a nightmare since I bought it a year ago after replacing my previous 04 that went almost 300k without any major issues. But I thought I'd throw it out there

 

Was the flashing "D" consistently on? In my case, everything is normal again when I restart the car, and the "D" won't flash again until some weeks later. It's unlikely to be due to any of the fans or temp issue, but persistent high heat will cause damage to the transmission. The 2004 TSX has two clutch pressure switches, and they are both the same part #, unlike the 2006 TSX.

 

Which OBD2 scanner did you use to get the code? The ThinkOBD 100 and the Autel AL-319 did not do the job.