That code points to a "Powertrain Control Module (PCM) Power Control Circuit/Internal Circuit Malfunction".

Troubleshooting steps from the workshop manual 11-162 and 11-163:

DTC P0685: PCM Power Control Circuit/Internal Circuit Malfunction

NOTE:
• Before you troubleshoot, record all freeze data and any on-board snapshot, and review the general troubleshooting information (see page 11-3).
• If the problem doesn't return after you clear the DTC, or it this DTC is stored intermittently, check for loose terminals at the IGP line connectors before replacing the PCM.
1.Turn the ignition switch to ON (II).
2. Clear the DTC with the HDS.
3. Start the engine, and let it idle for 30 seconds.
4. Turn the ignition switch to LOCK (0).
5. Start the engine, and let it idle for 30 seconds.
6. Turn the ignition switch to LOCK (0).
7. Turn the ignition switch to ON (II).
8. Check for Pending or Confirmed DTCs with the HDS. Is DTC P0685 indicated?
-- YES- Go to step 9.
-- NO- Intermittent failure, the system is OK at this time.
9. Update the PCM if it does not have the latest software (see page 11-246), or substitute a known-good PCM (see page 11-7).
10. Start the engine, and let it idle for 30 seconds.
11. Turn the ignition switch to LOCK (0).
12. Start the engine, and let it idle for 30 seconds.
13. Turn the ignition switch to LOCK (0).
14. Turn the ignition switch to ON (II).
15. Check for Pending or Confirmed DTCs with the HDS. Is DTC P0685 indicated?
-- YES- If the PCM was updated, substitute a known-good PCM (see page 11-7), then go to step 10. If the PCM was substituted, go to step 1.
-- NO- If the PCM was updated, troubleshooting is complete. If the PCM was substituted, replace the original PCM (see page 11-247). If any other Pending or Confirmed DTCs are indicated, go to the indicated DTCs troubleshooting.

Lots of stuff!

When I see module faults like this, a first step for me anyway is to make sure all the battery post and battery cable connections are clean and snug. Intermittent voltage supply is a too-easy and too-regular result of poor connections. Look hard at the battery ground cable especially at the connection at the radiator sheet/bulkhead. If there's any corrosion in the copper inside the insulation there or at the battery clamp end, invest in a new cable. That radiator sheet connection needs to be disassembled, cleaned with a wire brush at both the lug and the bolt regardless of whether you replace the ground cable.

If recent work on the car has included stuff like a timing belt replacement, look closely at the wire jumper that bridges around the motor mount at the belts end of the engine. It's easy to forget. Similarly there's a ground cable from a bracket at the transmission end in the driver's wheel well area, running to that same front radiator sheet/bulkhead but lower down.

Go back through work recently done on the car and see if a step was missed especially if the electrics were involved.

PCM failure seems to be quite rare. Most PCM faults are caused by external issues, so best to make sure all the basic maintenance and housekeeping chores are taken care of before you admit defeat and shop for a new $$$ PCM. Changing critical relays is never a bad troubleshooting step, so you are going in the correct direction.

 

Dr Bob -
This is great, thank you for the thorough reply and suggestions. I am hoping to dive into this tonight. I may just replace the battery cables, terminals and ground wire. They appear to be ok but its hard to see if there is any corrosion in the lines that may be impacting the connection.

Do you think a failing transmission pressure switch could potentially be a culprit? I don't drive my daughter's car but when i hopped in the other day I felt like maybe there was a little lag between shift cycles. It may have been me just not use to driving her Pilot but wondered if there was anything there? I know it's probably a long shot since they tend to throw their own DTC's but figured i would ask.

I will report back on the outcome. If I don't have any luck, I guess I am off to the local Honda dealer to play car repair roulette. If updating the PCM software and reinitializing isn't too expensive I may go that route first to see how it does. I hate the fact that it's an intermittent issue as i don't want her to end up on the side of the road with a dead vehicle.
- CoPilot

 

I have several different makes of cars in the fleet here, and participate at some level in user forums for each of them. I'm too regularly surprised when folks chasing some symptom immediately jump to the most expensive (and least likely) conclusions in their hunt. Consider "It worked fine until" pick your symptom. These days, the onboard diagnostic (OBD) systems are incredibly good at detecting sensor and module faults, but have no way to remind you about normal maintenance or external problems except to send up a "Help!" flag like this. So the first steps of almost any trouble diagnosis will include bringing all the maintenance current, including in this case the related electrical supply stuff and the battery itself.

Do the housekeeping stuff like cleaning and inspecting the battery, cables and connections. Those need to be part of a regular maintenance protocol anyway. That will also eliminate low voltage as a possible cause of the error code you reported.

If the transmission doesn't seem to be shifting right, start with making sure your fluid changes are up to date. The group wisdom has the fluid changed at 25k mileage intervals. Each change is three drain-and-fill efforts, as draining the fluid doesn't actually drain all the fluid. Three, separated with a little driving, gets over 90% of the old fluid replaced.

A failing transmission pressure switch will set a specific trouble code in the PCM. No stored code for that means the switches are almost certainly OK. But if you are behind on fluid changes (as many are...) the first step in the diagnostic process is to get those current. The service intervals for "severe service" tend be about half of "normal" intervals. The descriptions of severe include stop and go driving (read: rush hour commuting), sitting at idle a lot (read: picking up kids at school, any drive-up window, etc.), and hot or cold weather (read: summer or winter driving). Add winter snow loads with relatively low cooling airflow and longer warm-up times, and it's not at all hard to get into the shorter intervals.

 

UPDATE:
I replaced the relays, pulled the battery and cleaned the terminals, cables and ground connection, checked every ground connection in engine area that i could see for any signs of corrosion or loose connections. I drove the car and it seemed fine but realizing this is an intermittent issue I figured I would take it to Advance Auto to have them do a deeper battery and alternator test versus my multimeter and load test. They also scanned for codes but none were present which makes sense since they were gone this morning and would have been erased when i pulled the battery.

So it looks like her alternator was fine but her 36 month battery which was about 15 months old failed when we did the test at Advance Auto. The cold cranking amps were fine but the battery's reserve capacity was deficient. It was under warranty so i swapped it for a new battery and I am hoping this resolves the issue. I will keep the car and run it for a week or two to try to recreate the problem before handing it back to #1 daughter. If something crops up again I will update this post in case someone has a similar issue and is looking for help.

Dr Bob, thanks for the assistance. Regarding the shifting, I think that was just me get reacquainted to how the Pilot CVT shifts. I do try to do a drain and fill every other oil change and generally speaking the car is pretty well maintained but it is harder now that #1 daughter has flown the nest and I only get to putter with it when she visits.

 

Sounds like a good plan. And good on you for the every other oil change ATF schedule. Mine has been on that for years now. So far so good.

On the shifting: This is NOT a CVT. It's a 5 speed auto with overdrive and locking torque converter clutches. You should be able to feel definite shifts, and be able to see noticeable RPM changes as it advances through gears. Take a look at the ATF level and color on the dipstick, and give the fluid on the stick a sniff test to makes sure something hasn't happened. Remember that Honda's fluid level check procedure is a little unique. Stop the warm engine after driving, Check the level on the stick a minute after stopping the engine. There's a time window that I'd need to look up, but I know one minute is in that window.

 

Wanted to provide an update on this. Thought the issue was resolved only to have it happen again after four months. I took it to the dealership for a diagnostic check since I didn't want to randomly start replacing any more electrical components without understanding the root cause. As it turns out, they indicated that the positive battery cable/starter cable was faulty and not making good contact which was causing the intermittent loss of power and the CEL and code. Shout out to the dealer who originally quoted me a complete harness replacement but when i picked up the vehicle they only charged me for the diagnostic and a replacement of the positive terminal head saving me a couple of hundred. I am hoping that this resolves the issue. My take-away lesson from this experience is to make sure there is a tight connection at the battery terminal. I had cleaned the battery and tightened the connections but apparently it was faulty and not tight enough. Thanks Dr Bob and Piloteers!

 

Glad you were able to solve the issues finally. And glad it was something as simple as the battery cable and connections.


For those playing along at home, the positive battery cable is a mini-octopus with tentacles reaching out to the starter motor, and the under-hood fuse and relay boxes. At the battery post, the clamp is hard-crimped to one cable/tentacle, plus has a stud to accept lugged cables to the other 'clients'. Those connections at the post are "protected" by a plastic cover, and tend to get ignored when servicing the car. In fact, the cover tends to harbor the corrosive fumes generated when the battery charges, so festering corrosion in there can be a hidden cancer that appears to cause a long list of phantom symptoms.

Replacing the cable clamp with an emergency-fix clamp is a temporary solution at best, unless the cables are -all- re-lugged to get the clamping back into good clean copper. Those corrosive fumes start corroding the exposed copper inside the cable insulation, so the corroded copper needs to be excised and new lugs get installed. Whew! Better to just buy a new factory battery cable, and invest the time and effort installing it correctly.

I started a thread on battery care, including the cables and clamps and the ground wires. It got derailed by a resident troll, unfortunately. Regardless, some of the suggestions there are still valid. One of those suggestions is to clean the terminals, and if the copper in the lugs is still good (not too brown, and no green) then coat them with Vaseline. This includes the connections under the positive clamp cover too, plus both ends of the battery ground cable if the copper is still good. Vaseline melts and flows into the copper cables as well as filling and protecting the clamps around the cleaned battery posts, so has a specific advantage over almost all $$ battery terminal protectant coatings that try to seal over the connections. A reliable alternative for the post connections is the felt washers soaked in ATF, although those don't do the same job protecting the outer surfaces of the clamp and do nothing for the cables themselves. Vaseline is cheap (generic is at the dollar stores for...) and quite effective for the duty, plus doesn't stain clothes if you gets some on yours.

 

And to provide a further update, the problem re-appeared with my daughter driving on the highway and she lost power. Fortunately she was able to get off and make her way home though it did occur two additional times. Back to square one on this issue. Any thoughts or suggestions appreciated, I guess back to the dealer for another evaluation. Ugh

 

Back to the details in post 2 above, look hard at the ground cables (3). One is from the battery negative post to the front bulkhead. The second bridges the right-side motor mount, the one below the PCM at the belts end of the engine. Third is the main drive unit ground from a purpose bracket visible/accessible in the left front wheelwell. It's a non-insulated fine-strand copper able, and it connects to a bolt in the bottom of the frame horn almost directly above and forward of the end you can see. If the that copper shows corrosion, it needs to be replaced. At minimum it needs the lugs and bolts cleaned until bright.

Take a hard look at the work the dealer did for you on the positive battery post. The factory cable has one leg crimped in the original clamp, plus more connections via lug and a stud/nut. From your description, it's possible the dealer used an 'emergency battery terminal', one that has the main copper cable just clamped under a narrow strap and a couple bolts. These are temporary at best, as they leave the copper exposed to more corrosion. Solution is a new factory cable and an hour of careful work removing the old and installing the new. Aftermarket cables are a poor choice, mostly because they don't have enough connection points for all the wires that land there at the battery positive post.

If you get a chance, share a picture of the dealer 'fix' on that positive terminal. I'll get back to that battery terminals thread at some point, and some working pics of various 'fixes' would add a lot.

 

My 2005 Honda pilot does the exact same thing. Did you ever find out what the issue is?

 

Hi, sorry you are experiencing this issue. Short answer, no. We ended up getting her a newer car and the Pilot is in my garage. I will at some point get it back to the dealer to assess. I ended up buying a used PCM in case that is the issue so that the dealer can install it. At this point, i am at a loss but i am wondering if it could be an alternator issue. Alternator check said it was fine but spool valve had a slight leak and wondering if that may have led to this issue.