Hello,

I joined this thread so I could post a question that I have searched and searched for, but have not gotten the specifics I'm looking for.

So I have a 2003 Honda Accord Ex, I4 5MT. I want to first go back one year ago, right when winter was starting (I live in Virginia), my check engine light came on and when I got the diagnostic code read it was P0420. I replaced the gas cap...didn't do anything. I had an odd power issue if I downshifted and slowed myself down via the clutch(CEL would flicker on and off, other dash lights would come on and off, radio would flicker). I took a look at the battery and the terminals were corroded like crazy. So, I cleaned them off, reattached and CEL never came back....until now.

Light came back on a week or so ago and same code, P0420. I have 156,000 miles and last changed the spark plugs around 50,0000 so I replaced those and also got a new battery (last replaced at 80,000). Light still came back after doing all of that.

I have read many posts here and the consensus seems to be to replace the CAT, but I've also read and heard that O2 sensors can go bad, esp with my high # of miles...or both. So, I'm trying the O2 sensor first.

I've ordered an upstream O2 sensor (Bosch 15400 OE Type Fitment) along with an O2 sensor socket that will be here by the weekend through Amazon (~$15 cheaper than any store in my area). My big issue here is that I can't seem to find the exact location (pictures would be wonderful) of the upstream O2 sensor in my car. It's not obviously visual to me, but I haven't done any removing of covers to look farther than lifting the hood. Can I get to it from the top or do I need to get under the car. This is driving me nuts trying to find out the exact location on my car....please help!

Thanks!

-Tom

 

The front O2 sensor is #16 - looks like it's easier to reach from underneath. Make sure you support the car properly before crawling under.

However, the P0420 code would seem more likely to be related to the rear sensor (#17 mounted in the converter). But by 2003 the ECU is pretty good at telling the difference between a bad sensor vs. a bad converter.

 

Jim,

Thanks for the reply. The P0420 code definition lists "Catalyst system efficiency below threshold bank 1." Probable causes are:
1. A/F ratio concern
2. Air leak near HO2S Bank 1
3. Ignition system concern
4. Faulty CAT

I thought the upstream O2 sensor was Bank 1 and downstream Bank 2. Am I reading this wrong? Is P0420 really telling me I have a bad CAT?

Thanks

 

"Bank 1" terminology refers to dual exhaust in a V-type engine. Your 4-cyl has only one bank. Bank 1 sensor 2 is your rear O2 sensor.

Your list of probable causes sounds like it comes from a generic code reader. Honda shop manual (www dot helminc dot com) will give you much more detail for troubleshooting - probably pages & pages of stuff.

Yes, P0420 is supposed to indicate a bad cat. But its the rear O2 sensor that measures the cat's performance, so you'd think a bad rear O2 sensor can contribute to this. Back in 95-97-ish it was more common for a bad rear O2 sensor to cause that code. More recently (more computing power in the ECU) the system became more successful at telling the difference between a bad cat vs a bad O2 sensor.

Lack of control over fuel/air mixture can cause P0420, and the front O2 sensor can be responsible for that. But in that case you should also see P0131, P0132, or P0133 along with it.

Bad rear O2 sensor can also cause P0420 but in that case you should see P0137, P0138, or P0139.

Finally, the O2 sensors actually measure oxygen DIFFERENCE inside vs. outside. So an exhaust leak blowing onto the outside of a sensor will throw off the measurement. Same with oil or crud dripping onto the outside of a sensor.

 

Bank 1 identifies the location of cylinder #1, while bank 2 identifies the cylinders on the opposite bank.

Only the V6 has Bank 1 and Bank 2 sensors (they have 4: Bank 1,Sensor 1, Bank 1, Sensor 2, Bank 2, Sensor 1, Bank 2 Sensor 2)

DTC P0420: Rear Bank Catalyst System Efficiency Below Threshold (Bank 1)
It does not state any one sensor directly. It can be caused by exhaust leaks, deteriorated catalytic converter, etc., etc., and so on…

But I digress...

For our car there was a Technical Service Bulleting regarding updating the ECM/PCM software.

tegger.com/hondafaq/p0420/A03-063.pdf


Service Bulletin A04-063

Applies To: 2003 Accord L4 except SULEV – ALL
2003 Accord L4: MIL Comes On With DTC P0420

SYMPTOM
The MIL is on and DTC P0420 (catalyst system
efficiency below threshold) is set.

PROBABLE CAUSE
The ECM/PCM may incorrectly interpret sensor inputs
and set DTC P0420.

CORRECTIVE ACTION
Update the ECM/PCM software using the Honda
Interface Module (HIM).

Since you have 156k miles, the Federal emission warranty covering the catalytic converter and the PCM for 8 years or 80,000 miles would be over. Expect to possibly pay around $50 - $100 to get the update, unless Honda does Goodwill.

 

One thing though, there is a chance at that many miles, the cat may have deteriorated and may be the cause.

 

Thanks for the info...I'll get under my car this weekend and see what I see underneath.

 

Great info on the bulletin...called the local Honda dealer and he's telling me it's a bad CAT. Obviously this is just going off the # of miles I have on it. Since I've already ordered the O2 sensor and heard they can go bad at higher miles, I'll replace it this weekend and see what things look like underneath. Maybe it's the sensor, CAT, Software or all three. I'll do this methodically and go one by one down the list. I'd like to get to 200K miles before getting a new car, so hopefully I don't sink too much more money into this.

 

A lot of Hondas last far beyond that.

 

I'm looking at potential replacement CATs on Amazon...anyone see any issues with a MagnaFlow 99354HM Universal Catalytic Converter (Non CARB compliant) type? Cost is cheaper than I expected to see at $95. I'm guessing the "Universal" designation means it's not a direct fit and I'll need to do something to get that perfect fit.