Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

P0420 Code -- 96' Honda accord -- 253k miles

P0420 Catalyst System below efficiency – bank 1

I’ve had this code for a couple months trying to trouble shoot it. My gas mileage has been affected, as I’m getting about 23/24 MPG city/highway. I replaced the upstream 02 sensor with am aftermarket Bosch variety( I’m not sure why a non-OEM one would make a difference, as long it’s working correctly).

My car has 253k miles, the cat doesn’t rattle and I don’t smell any rotten eggs or anything, but it has to be clogged or the 02 sensor I got was defective. My engine runs fine and I don’t have any problems accelerating or decelerating. It may still be the 02 sensor since the gas mileage decrease is so significant, or it could be my converter is really clogged up.

Any input is greatly appreciated.

You need to do some more research on what this code means. For starters, it is largely generated by the downstream O2 sensor, not the upstream one. So if the upstream replacement was aimed at the code then you went after the wrong one.

Second, this code isn’t generated by having the cat fall apart and get blocked up. In fact, all it is generated by is the downstream O2 sensor showing too much switching activity - which is interpreted by the computer as the converter leaving too much gunk in the exhaust.

If there has been a real and verifiable change in gas mileage (please tell me you don’t use your gas gauge to figure it out) was it after the upstream sensor was replaced? It not likely to have anything to do with a P0420, so you should probably look elsewhere. Cooling system service/thermostat change, plugs & wires, brakes, alignment, tires…

You replaced the wrong oxygen sensor…It’s the down-stream (rear) sensor that throws this code “Converter efficiency below threshold”…

At 253K miles, EVERYTHING is operating below efficiency thresholds…it’s cheaper to buy a new car than to restore this one…

Do either of you own a Honda accord?

Is that relevant? (Correct Ans: No - not in the least).

Get or find someone with a scanner or scope that can look at real time data coming from the O2 sensors.
Stop guessing.

I recently bought one of these:

I currently don’t have a problem with my car, but when I or family/friends do have a problem I’ll be ready.

p.s. I’ve owned 3 Accords: '81, '85, '88

I borrowed a Actron CP9580 from my local auto parts megastore and found this LIVE! data.

Abslt TPs(Throttle Position Sensor): 9.4
MAP: 28.5-28.9
Coolant: 190-196*
IAT – 120 – 127
LT Ftrm 1 – 7.8

OS2S11: .700
OS2S12: .715

Work update

I got under my car and replaced the downstream 02 sensor, not because I was guessing but because in the process I unfortunately broke it trying to remove it. When installing the new one it was quite a bugger, as the threads aren’t as great as they were originally but I did triumph in getting it in there. I cleared the P0131 code that came on after I unplugged it and I’m waiting for the P0420 code to return? I hope not, but based on the output on those 02 sensors it looks like it will come back on soon.

Both sensors are new!

HH, you are to be commended for your perseverance…Even if the CEL comes back on squawking P0420, at least now you can install the new converter knowing you covered all the bases…

HH: How do those readings from the scanner compare to what you are supposed to see? Have you found someone able to explain them?

I’ve never had experience with these readings, but if the converter were working, wouldn’t you expect there be a noticeable difference between the upsteam and downstream O2 sensors? If so, this seems like the readings you got are evidence the catylitic converter has failed. This wouldn’t be very surprising at 250k. Does the converter seem to get hotter than expected after you’ve run the car for while?

One other test you could do is an emissions test. Have you done that?

I’d be very surprised if your fuel mileage increases after changing those O2 sensors. It could be running a bit rich for any one of a number of reasons and the first thing I always do is weed out the possibility of a mechanical fault. That means run a compression test; especially at a quarter million miles.

This document shows what kind of data to look for from the O2 sensor:

Yea, this latest live data definitely points to the cat failing. Although it seems like my gas mileage has only really dropped off in the last 2 months or so.

I also noticed when I first checked the data the whatchamacallit loop was open with the engine off, and when I turned on the engine on it was closed loop. This rules out the thermostat, right?

…I hate chasing down complicated stuff like this.

Come to think of it, I was running 89 octane gas up until a couple months ago. The switch probably exacerbated whatever problem I’m having. I’d probably be smart to put 91 octane in there, I don’t really have $600 to drop on a new OEM cat.

So, if I can’t fix the cat, I’ll have to settle for delaying it’s eventual demise.

Regular gas should be sufficient. Changing grades is not going to do anything to solve the problem and is most likely extra money that need not be spent.

You can find a “direct replacement” after-market CAT for MUCH less than $600…

If the converter is not clogged then it should have little or no effect on your fuel mileage. If the converter is clogged then it can and that problem is a symptom, not the cause, of lowered mileage.

On the offchance there may be several problems here, have you checked the tire pressure on your car? One low tire can kill fuel economy.

I will still reiterate that checking compression would be a must do, in spite of a subjective opinion that it runs fine. Degraded performance is something that often comes on gradually and the driver never notices it. Point being that lowered compression can lead to a clogged cat over time; assuming that particular problem exists.
I’m a firm believer in making sure the basic building block is solid before throwing parts at it.

HondaHonda"I replaced the upstream 02 sensor with an aftermarket Bosch variety (I'm not sure why a non-OEM one would make a difference, as long it's working correctly) ..."
Because Bosch primary O2 sensors do not always work correctly on Honda Accords of that era. Only OEM Denso O2 sensors are guaranteed to work. Don't ask me why. Just a friendly warning.

Before replacing the TWC, ask the service manager at Honda to look up any recent TSBs on the P0420 DTC on '06 Accords. (He won’t do this unless you ask him, but he will do it.) Sometimes all that is required is a re-flash of the ECM. See PROBABLE CAUSE/CORRECTIVE ACTION in the sample TSB below.

Honda<font color=black"> Service Bulletin 08-015

A re-flash usually just re-programs more allowable slop into the sensor readings to accommodate component aging. This usually won’t be free, you might pay between $90-$135. Just be glad you’re not driving a Beemer, where they can get $350 for a re-flash.

Your LT FTRM is a little high (assuming you measured it at speed and you mean a “+” sign in front of that number). You might want to compare it to the ST FTRM at idle and based on a comparison of those two readings look for a vacuum leak or a sticking MAF sensor. Don’t leap for the TWC too soon.

BTW, I hope you did not run out of gas on that Accord at speed on the highway. That could kill the TWC. Also splashing cold water on a hot cat. That shield is not put there just to keep the grass from burning.

Good luck.

I haven’t ran out of gas, but I’ve driven it down to 2 gallons several times. Is that bad?

New(and more) live data!

Engine Speed(idle): 711
MAP: 8.9
IAT (Down arrow) : 122/124
IGN advnc: 14.0 -15.5
ST FTRM(%) 1.6 to 2.6 up to 3.3
LT FTRM(%) 8.6
ST FTRM12: negative .80
OB2Stat: CA

What’s the TWC and what’s a TSB? Sorry not familiar with the lingo.

I hope it’s not the ECU. I’m going to check the air filter and see how that is doing. Back in a flash!(Of the ecu?)

TWC 3 way cat, TSB technical service bulletin.

I’m about ready to give up chasing the code. I don’t think it’s clogged my converter, and if it does in the future I’ll just take my rubber mallet to it! Mwahaha.

I’m really annoyed to as why it failed though. It’s running rich for sure, because the tail pipe has black soot all inside and at the tip. My exhaust manifold has a small leak, but I don’t see how the tiny leak could throw off my air/fuel mixture so much it runs rich constantly(gasket is black in two small, specific spots). I’ve tightened the bolts all around the leak and the missing bolt a couple times. Not sure how tight I can tighten these?

I checked the air filter, it was white and clean as fresh linens. I suppose next I could check the EGR Valve and see what that looks like. It’s easy to get to.