I’m working on a new to me 2004 civic hybrid 5 speed with 240k. It had been parked about 15 months due to a bad clutch. I picked it up next to nothing and towed it home. And I installed a new clutch kit, charged the 12v battery and the car runs great at idle. But on my first drive it had a rough acceleration and flashing check engine light, I returned home and plugged my scanner into it. Had po147,p1130, po301 and po304 I was able to test the o2 sensor with my scanner and sensor 3 had a heater fault. Replaced both down stream 02 sensor with NTK sensor as that’s what was in it (and what Honda uses as well I was told)

cleared all codes started it back up and took it for a drive everything seemed great, then once the car got up to operating temps it started having a rough acceleration give a p0420 code and when you floored it it would flash the cel and give a cylinder 4 misfire

parked the car and let it cool down. Then took it for a drive, again once it got to operating temps it started having a rough accelration and giving the p0420 and cylinder 4 misfire.

so currently I get a P0420 code and the misfire only shows when i floor it

So a P0420 with is a Catalyst System Efficiency Below Threshold

are my cats plugged/bad or is there something else I’m over looking?

I’m at a dead end with this car. Any help would be appreciated before I put a for sale sign on it

 

Your username is 100% appropriate to the car. I have an '03. Pretty sure it's broken its CVT belt. You should buy a lottery ticket for not having a failed hybrid battery.

P0420 means either the upper cat is operating below the efficiency threshold, or it's clogged.

Flashing CEL means ACTIVE misfire. Driving should be avoided.

Unless you KNOW for certain they've been replaced within the last 30K miles, I recommend you start by replacing all 8 plugs (yes, 8) with the NGK or DENSO OEM equivalent - long life iridium. They likely weren't done. They may be as much as $15 each, but rockauto usually carries them for a bit less if you're willing to wait.

If the plugs aren't the fix, and you're still getting misfires, buy 1-2 of the OEM equivalent (Hitachi?) coil packs. Don't buy the cheap ones. They may not work at all, or they won't last long. Swap them out starting with the #4 cylinder and go from there.

If both plugs and coils don't correct it, you may have a plugged cat. It's very common on the HCH1 to get P0420 codes due to efficiency issues.

The fact that the problem starts when it gets hot could be either the ignition or exhaust issue, but I would expect faulty ignition components to be the likely cause.

You can also pull the cat to see if the mesh is broken.

 

I don’t know the history of the car beyond what car fax shows. I’ll look into doing the plugs I was kinda hoping to not spend more money lol. Wanted to get it running and driving good for under $500

now I haven’t googled it but you seem like a wealth of knowledge, changing the plugs on these cars simple or is there some over complicated process? Like the ford Tritons v8? Lol.


ive thought about pulling the cats but the broken studs in the head nightmare has been keeping me at bay on that idea for now.

But I think first I’m going to inspect the coils and then swap 4 to 1 just for ***** and giggles


the car needs tires and a couple ball joints so I have some stuff to think over I’m currently into the car for a total of 350 as of now, so idk if I want to keep going into it. 120 for plugs and I’d suspect coils are 50 or more each. And then it’s 500 for aftermarket cats not to mention new tires I might be 2000 into a 1000 dollar car all done and said.

🤦🏼‍♂️

 

So today I pulled all the spark plugs out. The back side has autolite xp 3924 and the front side has autolite xp 3923

plugs from cylinder 1-3 looked great but cylinder 4 plugs were dark all were dry. I did switch the coil packs from 1-4 and took it for a drive. I was able to drive it about twice as far before the misfire and p0420 code returned this time the misfire moved from cylinder 4 to cylinder 1. So I am going to swap them from 1-3 and see if it moves which I think it will... then I will order new plugs and 2 new coils


Left is spark plug from cylinder 4 Right is spark plug from cylinder 1

 

I'd get the NGK 3107 plugs.Not sure why they had different front and back, and the 3923 isn't listed on Rockauto's site as a valid option. I recommend the NGK (or DENSO on HCH2 as both were used from the factory) as I've seen original plugs pushing 200K miles without issues. If you don't want to deal with plugs again for the next 120K miles, this is what you get, IMHO.

You've definitely confirmed an ignition issue, so hopefully the P0420 is only an underperforming cat and not clogged. Your power problems may be due to the misfires. Flashing CEL almost always results in substantial power loss. P030X codes are first reported before they are detectable by the seat of the pants. Flashing CEL means, "holy ****, this b!tch is misfiring all over the place!" IIRC, the HCH1 had a recall due to excessive P0420. They determined the criteria for the code was too tight. The recall reprogrammed the computer to use the correct efficiency threshold. P0420 is generally only an issue if you need to pass emissions or there is clogging. In most cases, your best bet is to buy a bolt-on and DIY. If your cat is stock, it is likely worth more to a cat salvager than the replacement costs. I'm personally aware of Honda Insight owners who have had success with adding 1/2 gallon of acetone to 1/2 tank of gas and then topping off the tank with gas to mix it (don't get acetone anywhere but INSIDE the tank). Driving out the whole tank may help to clean the cat effectively. Worst case is it does nothing at all. This is an old trick that's been used for decades using "lacquer thinner." Lacquer thinner has a variable composition, one of which is up to 100% acetone, so I remove the variables and recommend acetone.

Again, nab yourself a lottery ticket for every day that IMA light stays out...

 

We shall see what happens next week! With the ngk plugs and2 new oem coils. Rock auto saved the day with low prices yet again



if I keep up buy new parts to get it running correctly the person who buys it from me is gonna have a new car lol

 

Dude... you just described every process I've gone through selling a vehicle... last was a 2006 HCH2... bought it for $4000... put about $1800 into it (A/C compressor, motor mounts, cat, plugs/coils, new tires)... sold it for $4800.

Good luck!

 

Sometimes they don’t work out. I’m into this one so far for 650ish dollars total. But I don’t think it’s worth much more than 1200-1500 cause the paint on the hood and fenders are toast and it looks pretty bad. But I do have a paint booth so idk the thought of giving the front end a new paint job is floating around still

 

Little update: I got my package from rock auto. I ordered 2 coils and 8 spark plugs. They sent me a fuel pump a fuel pump strainer and 8 wrong plugs. So I shipped it all back so it’ll be another week or so before I get the correct parts hopefully. So Progress on the Honda is on pause. Luckily it’s a project so I’m not on a time crunch.


will update once the parts come in and I get them installed.

 

WOW! Sorry to hear it.