MLI/Engine Codes 96' Honda Accord LX

I have a couple mystery codes. Here’s verbatim what the code reader told me.

“P420 – catalyst system deficiency in Bank 1”

“P720 Torque Converter clutch open solenoid/circuit”

Somebody told me the first one meant my catalytic converter is out or something with environmental reqs.(okay I’ll replace that right away…) and the other is even more confusing. I do have a problem with my shifter allowing one to push between gears without depressing the shifter button. Not sure if that’s what it means by the clutch. It’s really annoying these being on, I don’t think they are serious least I hope not…Help please.

They’re not mysteries but they require diagnosis.

Here’s an easy place to start from the P0420:
- it is a code that will set if your catalytic converter is bad. However, it can be set for other reasons as well.

The other code you will have to double-check. My own sources tell me that P0720 is an output speed sensor circuit problem. There is a P0740 for torque converter clutch (TCC) circuit issues.

How many miles are on this car? Has the transmission ever been serviced? You do have a torque converter clutch. Its part of the torque converter which is the thing between the engine & transmission that’s an integral part of auto transmission function. It doesn’t have anything to do with your shift button.

For P0420, some of the other things can be the oxygen sensors, especially the rear sensor or an exhaust leak upstream of the cat.

Now for the transmission, the button only has to be pressed to get the shifter out of park, you also have to have your foot on the brake.

If you are having a problem with the clutch solenoid, one way to tell is when it is fully warmed up, at 60mph on level ground, the tach should be at 2450 rpm. You should also notice that small variations in the gas pedal will not vary the tach much. You can push it down about a 1/2" or let up completely and the tach needle should barely move. If it moves 200 rpm or more, the clutch is not engaging any you are getting less gas mileage at highway speeds than you should.

The good news is that the clutch solenoid is external on the transmission so it is an easy fix. The hard part is finding a mechanic that will fix it instead of wanting to replace the whole transmission. While you are at it, check the condition of the trans fluid. It should be a bright red. If it is dark red, brown or (worse case) black, then you need to get it changed as well, but only use the Honda ATF, nothing else.

Yes, it is the P740, glad you figured that out.

Here’s the deal. My car has 251k miles and I believe it’s the original transmission. Transmission fluid was changed in July with Honda ATF, it was dark when we changed it. I had it in the auto shop for an oil change in late October and they said it was dark. Could it be that dark again so fast?

They also say the same thing about my power steering, although I believe that, I don’t think it’s ever been changed.

What could happen to my transmission if it is indeed the clutch solenoid going in and out working periodically or what if it stops working at all?

First, whether or not the fluid is dark again depends on how you changed it back in July. If you just used a drain plug or pulled the pan off of the transmission then you only get about 40-60% of the old fluid out. So it was sort of dirty again immediately. If this was the first time the transmission was serviced then it wouldn’t surprise me if the fluid could use another change. (And its a small miracle that its gone this far).

Power steering fluid really ought to be changed once in a while, but you can just empty out the reservoir with a turkey baster or something once in a while & leave it at that.

I would service it again. I had a similar code once that went away after a pan & filter service. However, your code is about electrical stuff & fresh fluid is unlikely to help. So when it persists you might see about having someone change it.

The only really bad thing that’s possible if you just ignore that code would be overheating the transmission - especially if you do a lot of highway driving. In fact, if you do that could also contribute to short fluid life. Heat is the enemy of the automatic transmission.

okay. Now it makes sense it’s still dark. My dad drained it and he ended up cursing the thing out and just pulling the plug, because he couldn’t suck out the transmission fluid. Apparently he lost his powered auto-drainer somewhere. I’ll probably change that it again this month…not sure if the transmission fluid was changed before. Last time it was brown, but not black. So maybe it was just past only a little overdue?


My tach seems to be pretty steady. At 60MPH on the interstate it was right around 2500 steady. Is there any other way of testing it or could it possibly be anything else?

Also, do you know of any guides to replacing the clutch solenoid? I’m tired of my check engine light being on.

Another way to find out whether the TCC itself is actually operating is to get it up to cruising, highway speed under very steady throttle. Wait until the car has settled in to an easy cruise. Then tap the brake. If the TCC was on that will release it and your rpms should jump by 2-300 or so. If you then go back the steady throttle/speed it should drop back down 2-300 as the TCC kicks in again.

You don’t even know if its actually the solenoid itself that is the problem. The code refers to the circuit. The circuit involves its wiring, wiring harness & connections at the PCM.

As for replacing it, I don’t know this trans, but if this is an external bit that isn’t too crazy to replace as keith mentioned then I’d be really surprised if a basic internet search didn’t turn up more than one person explaining how to do it.

You can’t suck out the transmission fluid, you have to pull the plug, which is pretty easy to do. But, you only get 2.5 qts of fluid out, the capacity is 6.5 qts. If the fluid was brown, then the fluid changes have been neglected. A fluid change every 30k miles only replaces about 40% of the fluid, but that is enough to keep the fluid fresh enough for the transmission to last the life of the car.

To play catchup, you have to do a couple of fluid changes about a week apart. If you use Dexron instead of Honda ATF, it affects the shift quality. I don’t know if it could cause the check engine light. You have to get the Honda ATF from a dealer, but it isn’t much more than ATF at a parts store, it’s all getting pricy, along with the cost of oil.

I just checked the FSM for a 97 accord, same generation as the 96, the transmissions are alike. The first thing listed in troubleshooting is to repair any other codes first. An unstable idle can trigger this code.

There are two solenoids and they must be replaced as an assembly. You can see them from the front of the car with the hood open. They look like two small cans, about 1" in dia. and about 1.5" tall, side by side. There is a connector and a couple of bolts. EZ to replace, but I do NOT recommend that you try this just yet.

Do a couple of fluid changes over the next couple of weeks. Clear the codes either with a code reader of by opening the underhood fuse box and pulling the radio fuse for 10 seconds. I recommend the code reader because the next thing to do is get up to 50 mph, warm engine, hold steady for a few minutes, then stop, shut down, plug in the code reader and see if the P0740 returns, even if the CEL is not on. If it doesn’t appear, then the solenoids are probably good and the code is related to the other code.

It’s funny you mention going 50mph. Last time I cleared the codes it came back while driving on the interstate doing 60MPH. Does this confirm absolutely it’s the solenoids?

Before that the code was off for a couple of days. The delay made me think that the problem wasn’t that serious.

I finally did something I should have done a long time ago, look under the hood. I noticed one of the wires going to my transmission from my batter has lost it’s housing, is exposed and frayed and part of it isn’t connecting.

There’s a connector to my clutch solenoid. The two cylinder shaped things to the left of my engine block, right?, one of the connectors is white and powdery, looks like it must be corroded.

Can anyone link me to a diagram with all the parts labeled? I tried Google searching for one to no avail.

Have you tried a Haynes Repair manual from the local parts store?

The two cylinder shaped things to the left of the engine block are the shift solenoids. You should fix any wiring problems. The white powdery substance, was that inside the connector or on the outside?

Remember, the other codes HAVE to be addressed FIRST according to the factory service manual.

see attached

From your diagram what I was locking at was the lock-up solenoid(looking down with the hood open). The white powder was just a little bit and on the outside of the connector.

I’ll look in the Haynes for the clutch solenoid, but usually the pictures suck.

The lock up solenoid is commonly called the clutch solenoid. The circuit is called the Lock up clutch. BTW, powder on the outside is usually not a problem. Have you checked the fluid yet?

Where’s the circuit for the torque converter solenoid?

I checked 4 connectors right under the hood, ones I could easily access and found all the white plastic ends black. I guess this confirms the open circuit and that it’s overheating the fire.

Heres the deal. The torque converter has a sort of clutch in it, this is the lock up circuit. The lock up solenoids open a valve that drains pressure from the torque converter causing it to lock up. This occurs when the car settles into a speed in 4th gear. The rest of the time, the torque converter is pressurized so that it can slip, this allows the car to stop while still in gear and to shift gears.

People use a lot of different names for this circuit. The service manual uses several different names so I can see where it could be a little confusing. The two attachment I sent are directly related to P0740. I cannot attach the whole section of the service manual for the operation of the lock up circuit.

If you do a little googling for a download version of the service manual, you may find one available as a free download. I suspect that most of these downloads probably violate someones copyright, but still they are available.

Here’s what I was talking about. Sometimes pictures speak louder than words. The same connector on the other side isn’t black like in here. I suppose I could reckon the open circuit on the wiring is somewhere on the passenger side of the car?

You got in too close when you took the picture, it is out of focus. All the resolution in the world will not compensate for focus. When you want a close up, do not use the zoom feature, it moves the closest focus much further out that the zoom brings in. It also help to see some of the surrounding area to figure out exactly what you are looking at.