Bleakened

I haven't had a catalytic converter in over 2 years and just now I get a code saying it's broken/inefficient.

I reset the ECU and it hasn't come back since. Anybody know why it came about?

I have a 98 DX Hatch with a stock Y7 but aftermarket full exhaust.

Former User

Do you have two O2 sensors installed?

Bleakened

Yeah, both are installed and fairly new (8 months or so)

Former User

A bad cat (P0420) is sensed by the ECU through signals from the two O2 sensors. One or both O2 sensors are not working properly or the ECU is bad.

Bleakened

Wouldn't a code come up for a bad O2? It has in the past and it even told me which one.

Also, how could I check the O2's for proper function? I have a bluetooth scanner and app on my tablet for it. What do I look for? Voltage ranges?

Former User

A bad O2 sensor does not always throw a code.

What O2 sensors are installed?

grumblemarc

iTrader: (4)

WHy would it read as a bad O2? The second sensor looks for the efficiency of the cat that you don't have. How you weren't throwing codes earlier I don't know. What brand O2 sensors?

Bleakened

It's the Bosch brand I believe

B serious

iTrader: (1)

The 2nd O2 sensor's job is to check if the cat is working. Yours is not present and the secondary O2 sensor can see that. So...not a bad sensor. The sensor is doing its job.

You can either reinstall a cat (yeah...I know you're not gonna do that), or install a spark plug defouler on your 2nd O2 sensor. The 2nd O2. The 2nd one. The one downstream.

I said 2nd one because if you do the defouler trick to the primary O2 sensor, you'll have a lot more issues.

Google "spark plug defouler O2 sensor" or something.

Bleakened

I am actually going to be putting a cat back in it. (Moving to PA and have to pass inspection.) Will I see MPG gains when I do this?

Former User

While this^ is currently true, it hasn't been true for 8 months, if I understood what the OP has said.

B serious

iTrader: (1)

It's a monitor. The Y7 probably runs clean enough that the waveform the secondary sensor sees during closed loop is USUALLY within a tolerance and the light never went on. Rare...but not unheard of on extremely low emissions engines.

Things that could have changed:
-new O2 sensor is of a different brand with different acceptable internal resistance standards during manufacturing.
-new O2 sensor is more sensitive/less fouled than the old one.
-driving habits changed
-engine condition changed.
-an exhaust leak prior to the 2nd O2 sensor is now sealed.

It was probably by fluke or some stroke of luck that the light didn't come on before. Either way...if he was lucky before, you can't blame the new sensor for actually doing its job.

The sensor may still be faulty (if the voltage reading is always high), but you'd be groping in the dark for that theory at this point since the cat is NOT there...the O2 is technically doing its job.

Former User

Help us out here OP. 8 months no CEL and no P0420, right? Was a recent change made? Was this CEL/code associated with some work done or some road event?

Where are the 2 sensors installed into the exhaust system?

Bleakened

It came after I installed my own ebay fog lights but never touched anything besides the wires that it came with.

I have the upstream and downstream O2's installed in proper places. First one is on the 3rd cylinder header pipe I believe and 2nd one is down under just past the test pipe (cat delete). Been this way since I got the exhaust and installed the O2's. No CEL till now.

Also, still no CEL came back yet as of me writing this post.

Former User

The primary O2 sensor should be located where all the header pipes converge. Your O2 sensor may be primarily monitoring exhaust from one cylinder.

B serious

iTrader: (1)

He has an aftermarket header. Its not uncommon to see the primary sensor in only on tube of the header on a D series.

Former User

I trust the logical Honda design much more than that of a manufacturer of cheap parts.

B serious

iTrader: (1)

Lol true...but I think the O2 location is offset to 2 runners even on the factory Y8 header.

If only Honda would have made the primary O2 sensor sit in the collector like they do on a B series....

The primary can't throw a cat efficiency CEL anyway.

Former User

The primary sensor signal in conjunction with that of the secondary sensor is required to monitor the cat. It's part of the equation. [I enjoy our debates ]

B serious

iTrader: (1)

^you right, you welcome.

Former User

Just wanted you to know I wasn't being sarcastic. You're clearly extremely knowledgeable and all of your posts are highly informative to members at all levels.

B serious

iTrader: (1)

You too sir. No offense taken.