I just got a CEL last night - P0420, catalyst system efficiency below threshold (bank 1).

VERY cold here, in the high single digits. Don't know if that's a factor or not.

Car has 80K miles on it, I replaced the OEM catalytic converter with a highflow cat a few months ago, no CEL was ever thrown until last night.

Scanned the code, then scanned the O2 sensors. The primary shows a nice tight sine curve, like it did a few months ago. The secondary fluctuates much more wildly, as usual, but this time the valleys are much lower than the peaks are high.

Don't have actual number ranges, it was so cold last night I literally could not hold the scanner steady enough to see the numbers clearly.

Does it sound as if I need a new secondary O2 sensor?

Looks that way--go with a Denso. Stay away from Bosch or other generics.

Thanks, why do you say it needs replacing? Irregular voltage output curve?

I've heard about Bosch and other generics, this place I regularly buy parts from has only NGK for the secondary even though they sell NGK and Denso primaries.

The NGK is substantially cheaper than the OEM secondary even from the online dealers that sell at ~ 20% discount. About $100 cheaper.

Is the NGK OK as well in your opinion?

Yes, the curve is indicating a problem. It's either with the sensor (cheaper alternative) or a problem is developing with the cat.

I have no experience with NGK oxygen sensors, but I would not hesitate to use them. I view most Denso and NGK parts as equal quality.

Lord, I hope it's not the (almost brand-new) cat!

Ever heard of this place - MUCH cheaper for the secondary, MORE expensive for the primary, wonder why...

2002 ACURA RSX Oxygen Sensor

No, I haven't heard of them but that doesn't mean much. Check the price at RockAuto.com. I use them often and can give you a 5% discount code.

Thanks for the offer - I checked rockauto and the $45 secondaries they carry are Delphi and Standard Motor Products parts. Probably likewise for the el cheapo I found at autopart.com. They may not even be heated at that price.

I can get an NGK secondary for $139 at clubrsx.com.

Club RSX.com: NGK Secondary O2 Sensor

I know it's OEM quality (NGK) and that seems to be about the best price for OEM style, direct plug-in and heated.

Rock carries the Denso, but it's $180.

Here's the deal. Some Standard parts are actually OEM in disguise for a MUCH cheaper price. You have to check it out closely but I have gotten some excellent buys that way. I bought a Standard MAP Sensor that was a Denso for a fraction of the price. Many of the TEC coils at RockAuto.com are actually Honda OEM parts for 40% of what Honda is asking.

Thanks for the tip - any way to know without ordering the part and waiting to see what arrives?

TEC uses terms like OEM. My understanding is that Standard doesn't really manufacture parts--they just put them in their box. Everything I have bought from Standard has been Denso. RockAuto's pictures also provide tips--the picture of TEC coils I've bought are exactly the same as those from Honda.

Well, for ~ $45, I can hardly go wrong!

Thanks again for the diagnosis and the parts purchasing tips!

One last question - how can I tell if it's direct fit, plug and play?

If it doesn't say generic, it should be P&P.

Do you want the discount code?

Sure, I'll give it a shot - thanks for the code!

Remember, keep all lubricants (like WD40), brake cleaner, etc. from the sensor's pig tail. Otherwise, it will throw a code forever.

In fact, you may want to take a set of jeweler's files and carefully clean the connectors to the old sensor before buying a new one. Slight contamination may be playing with the curve and throwing a code.

Yes, thanks for the tip.

I sprayed some PB Blaster around the threads of both sensors when I R&R'd them and the cat, I think I did it carefully enough as I haven't had a code until now.

Have to be careful with the anti-seize too, usually new sensors are pre-greased.

Apparently road grime and rock salt don't interfere or I'd be SOL!

Let me know how the new sensor works out.

Interestingly, the upstream sensor from Standard is MORE expensive than other sources...

Yes, it's hard to tell what factors go into setting their price. Again, I've never had any problems with Standard products.

There was also a bulletin released by the Wisconsin Vehicle Inspection Program regarding the P0420 code. They listed the following as possible causes for a faulty code being thrown:

• Damaged pre-cat exhaust manifold/catalytic converter/pre-cat exhaust pipe; this can cause too much air to enter the system, interrupting the O2 sensor reading. Modifications to the OEM exhaust system can also throw this code.
• An oxygen sensor; a sensor sending bad info can set the code
• High fuel pressure; this causes the converter to overload with hydrocarbons. Check the pressure both at idle and under load, also check the regulator, injectors & o-rings for any leaks.
• Coolant temperature sensor; this can cause the vehicle to remain in open loop.
• Cylinder misfire; no combustion taking place in the cylinder can cause a large amount of unburned oxygen to pass through the system.

Thanks for that, I actually saw that elsewhere around here.

Based on the fact that I just replaced (within the last six months) the entire exhaust, from head to tailpipe (EXCEPT for the O2 sensors), and had no issues until just now... and for $45... I took an educated leap and will try replacing the sensor.