Vladi17

Okay, this is my least favorite number. This is my third time recieving this code within 9 months. The first time I replaced the o2 sensor before the monifold. I then got the same error three months after and replaced my other oxygen sensor along with the previous one. Therefore I have had two new o2 sensors in my car for the last 3 months. They are the bosch universal ones. I read after installing that those aren't the best to get, but they are the cheapest. Anyway, I am now wondering if my cat is bad and what I should do about these stinkin o2 sensors. Please help me fix this problem so it won't come back again.

Thanks, Andrew

98CoupeV6

Why are you replacing O2 sensors? P0420 is a catalyst efficiency code.

Edit: In my AV6, anyway. What engine do you have?

Vladi17

The code states that "Catalyst System Efficiency Below Threshold", I was told this was a problem with the o2 sensors from reading other forums. Problem seemed to go away after I replaced the sensors but then came right back months later. What should I do??

Oh yeah, its also automatic. Same year as yours too (1998). Do I need to replace the cat converter? Tranny is also starting to slip a bit, errrr

98CoupeV6

Well Andrew, I think the only way that P0420 would come on and an O2 sensor would be the cause would be if there was another trouble code such as P137 or P141 indicating an O2 sensor problem, which would trick the computer into thinking that the catalytic converter wasn't doing its job and thus setting off P0420. My PO420 has been on for the past few months and I'm 95% sure it's the cat. I really need to replace that.

DTC P0420: Catalyst System Efficiency Below Threshold

1) Reset the ECM/PCM.
2) Start the engine. Hold the engine at 3,000 rpm with no load (in Park or neutral) until the radiator fan comes on.
3) Except F23A4 SULEV engine: Test-drive at 40 - 55 mph (64 - 88 km/h) for approx. 2 minutes . Then decelerate for at least 3 seconds with the throttle completely closed. Then reduce the vehicle speed to 35 mph (56 km/h ), and try to hold it until the scan tool indicates FAIL or PASS as the TWC test result. F23A4 SULEV engine: Test-drive at 50 - 55 mph (80 - 88 km/h) for approx. 2 minutes . Then decelerate for at least 4 seconds with the throttle completely closed. Then accelerate to 55 mph (88 km/h ), and try to hold it until the scan tool indicates FAIL or PASS as the TWC test result. Is the test result FAIL? YES - Check the three way catalytic converter (TWC). If necessary, replace the TWC. NO - Intermittent failure, system is OK at this time.

So if P0420 comes on and it's the only code, either replace the TWC or have your Honda dealer (or someone else) test the TWC to see if it's bad or not. If there are multiple DTCs, troubleshoot those first. I only have P0420 I'll copy this in the DTC thread.

Vladi17

Sorry, I'm a little confused by your reply. Not sure what the Three-Way Converter (TWC) is. Also not sure what F23A4 means, could you please go a little bit more into details about what all this means. Also, I don't have a scan tool, I went to autozone for the reading. Should I buy one and have it plugged in while I drive?? I'm a little confused. Thanks for your quick response and all your help.

skabone69

sounds like you should try the o2 defouler trick.

Vladi17

Could you be a little more specific???

98CoupeV6

The TWC is the Catalytic Converter. F23A4 is an Accord ULEV engine.

The scan tool referred to is a Honda tool that allows them to scan your entire ECU and get all sorts of data. I believe you can purchase one but they're very expensive.

Vladi17

Okay, I think I'm gonna replace the cat converter, seems to me that has to be the problem since I replaced the both 02 sensors. Found one for around $140 (3.0 V6 Direct-Fit), if you know of cheaper please let me know.

98CoupeV6

Yeah if I were you I'd either go OEM or Magnaflow. Magnaflows do not set off the check engine light. I have no idea who makes those $140 ebay cats, I almost bought one but I've never heard any opinions on them or even who manufactures them.