P0420

Hatmouse

Hatmouse

Well-Known Member
Joined
Jun 30, 2005
Messages
4,791
Location
Bay area, California
Car(s)
'03 Civic Si
So, my car is throwing up a P0420 code.

Brief history: This isn't the first time it's happened. A year and a half ago, it threw the code, I took it to the mechanic and he said "It's probably nothing, I'll turn it off now and if it comes on again bring it in and we'll see".

For six months nothing, then it happened again. The mechanic still was of the opinion that it was probably nothing, and turned it off.

It's happening now, a bit more often, and the last time it happened before today, it turned itself off.

I am officially tired of P0420. Should I try replacing the O2 sensor? Or is it the cat on the fritz?

Also, the car passed emissions just fine a couple of months ago.
 
So just the light? If it is the downdraft cat and you aren't noticing any mechanical issues you can ignore it.
 
The car itself drives as it has always, in an enthusiastic manner.

Other than the light itself coming on, (and if I turn it off, it stays off for a couple of months) I can't tell anything.
 
How many miles does your car have? It could be your downdraft O2 sensor is going out.
 
It's about time for you to change that sensor, it will go 'out of range' more and more frequently as time passes now.
 
Yep...just need to get me a floor jack and some jack stands.

So there's two O2 sensors; which should be the one I change first? Or should I just change them both out?
 
Hatmouse said:
Yep...just need to get me a floor jack and some jack stands.

So there's two O2 sensors; which should be the one I change first? Or should I just change them both out?
Click to expand...

I would say both as they tend to go out at once. However if you can only replace one replace the downdraft. That you aren't suffering any noticeable problems besides the light suggests that one is the culprit for the code.
 
It's the rear-most oxygen sensor that's reporting the problem - but you should change them both out if you have the cash as the other one will probably go soon. Oxygen sensors do eventually "burn out".
 
Sounds good, I guess I'll do that.

So I could do the work myself which would require buying the means to lift the car, or I could get a shop to do it. If a shop does it, any idea how much labour would cost?
 
Provided it is easy to access (I don't remember if it is or isn't on an OBD-II Civic) and there are no complications like the sensor being carbon- or rust-welded in place, it should take less than an hour to change. If you replace both, perhaps the full hour if there are no complications. Usually they're pretty easy.
 
Per a picture from a google search, it's apparently here:

o2.jpg


I assume all it involves is:

1. Lifting and jacking car
2. Unplugging and unscrewing the old ones
3. Screwing in, and plugging into, the new ones
4. Lower the car?

Disclaimer: the only maintenance I've ever done myself was to change my air filter a year ago.
 
Pretty much. The new ones should come with antiseize already applied. You might want to get a torque wrench (cheap at Harbor Freight) and the proper socket (same) to do the job right, but that's pretty much the entire task you have described.

Just for the dataset, HO2S like these (Heated Oxygen Sensors - the kind with 3-4 or more wires) have a rated average lifespan of about 100,000km or 60,000 miles though some are rated to 160,000km or 100,000 miles. The older *unheated* 1 or 2 wire units have a lifespan of about 30-50,000 miles, shorter because they lack the HO2S's self-cleaning function (no built in heater to burn crap off when the sensor is cold.) Many times you will find the sensors running for much longer than their rated lifespans, but if it makes it to the design lifespan (whatever that might be,) there's really nothing to complain about - they're consumables with a finite lifespan.
 
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I would suggest spraying them with either Deep Creep or PB Blaster a day before taking them out. It should help loosen them. If you are using a ratchet you'll want an O2 sensor socket. It is a socket with a cut running along the side for the wires to go through.
 
Autozone rents out the crows foot socket for a 25 dollar deposit, majority of oxygen sensors are 22mm. Just to warn you, the downstream oxygen sensor might be the issue, but it's not necessarily the problem. An ignition misfire, coolant temperature sensor, or failed converter could all trip the code. Also replace the oxygen sensor when the car is warm, the heat in the exhaust will help when removing it.
 
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