The D4 light is on all the time. I've been reading about TCUs and I've got mine out of the car to inspect it. Found 2 caps and 1 resistor burnt out. I replaced those along with all the other electrolytic capacitors and put it back on the car. It didn't make a difference. Then I checked a bunch of resistors and diodes from around the burnt caps. Reheated some solder joints, checked the connectors, the grounding. All looks fine. But the light is still on and the car is sluggish. Then I checked the solenoid connectors, the ground wires under the hood, the fuses (Is there a fuse for the TCU somewhere? I couldn't find it.) I found that some wires (from the ECU/TCU connectors) had the insulation chewed off by rodents (but they weren't shorting) and I masked those with electrical tape. I want to test the wires with a multimeter but I can't find a diagram anywhere. All I could check was the black (ground) wire.

The TCU is a 28100-PXO-A21-M1 (PXO or PX0?) and I couldn't find one like it for sale. On the other hand I'd rather DIY this because I'm short on cash after fixing a bunch of other things on this car.

Any tips, any ideas? What should I do next?

 

Anyone?

 

If the light stays on then there might be a code with it. Did you jump the connector and check for codes?

 

Be sure the TCU is dry and insulated. While I had the carpet out on my 1990 Accord LX, the TCU would malfunction on cool mornings. A tiny bit of condensation I assume. A new dry carpet solved the problem.

 

@Bad_dude: I shorted the blue connector, the D4 light stays solid on. No code (?)

@totalaccord: It's actually dry, no signs of moisture on the carpet or TCU.