Check-engine light...Code P0420
 

Hey guys,

My check-engine light came on two days ago. Did a diagnostics myself that plugs into the computer. I got a code P0420, something about my catylast level very low. Does this mean my O2 sensor is bad? or is it something else?

For me to fix it and find out exactly what is wrong, a shop wants to charge me $90 to find out then + the parts to fix it. Can you help me skip this $90 step...

Re: Check-engine light...Code P0420
 

Reset the code and see if it comes back. If it does, you can buy a new o2 sensor and install it.

Re: Check-engine light...Code P0420
 

How many miles on the car? If it's an o2 problem, it could be the front or the rear sensor that's bad. If I had to take a guess, I'd say you probably need a new cat.

Re: Check-engine light...Code P0420
 

I have 105k and some change. Is it pricey to have the sensors replaced?

Re: Check-engine light...Code P0420
 

If your O2 sensors were bad, you wouldnt have a P0420 code. The ECU expects a lazier wave pattern from the rear O2, if its the same wave as the front O2, the cat is not doing its job and throws a code P0420. There are a different set of codes for the O2 sensors.

Re: Check-engine light...Code P0420
 

cat is bad

Re: Check-engine light...Code P0420
 

Re: Check-engine light...Code P0420
 

cat is bad. pull it off the car and shine a flash light thought it. if you can't see light its stopped up (of if you see very little light).

Re: Check-engine light...Code P0420
 

Ok, so does everyone agree that its not the sensors, its the catalytic converter? I dont have the tools to take it a part myself. I would have to have a shop do it all.

Re: Check-engine light...Code P0420
 

Yeah, it's a cat. It's really simple to take off, just 3 (?) bolts on each end.

Re: Check-engine light...Code P0420
 

Alright then, looks like I have a small weekend project:thumbup: Once I swap a new Cat in place, how would I go about reseting the check-engine light? or would it reset itself?

Re: Check-engine light...Code P0420
 

It's simple, either pull the clock fuse for 10 mins or un hook the battery for 10. Or autozone.

If you do either of the first two, make sure you have your radio code.

Re: Check-engine light...Code P0420
 

Re: Check-engine light...Code P0420
 

Don't take it to autozone to reset a cel. i'm a manager for the zone. we can actually get fined like 10k for clearing a cel. emmisions bs. But the other two work great.

Re: Check-engine light...Code P0420
 

I'm going to take out the clock fuse or unplug the battery for 10min this evening first (after work), then see if the CE light comes back on, which I think it will...I just want to spend as little as I can. The light came on when we had Icy conditions over the holiday break, so maybe that caused it...who knows. If it does come back on a new cat is going to be swapped in.

Re: Check-engine light...Code P0420
 

fair enough :P

Re: Check-engine light...Code P0420
 

Well, I unplugged the clock fuse. Started the car after ten minutes and the fuse back in place. I drove it about 10 miles and so far no CEL. If the Cat is back, wouldn't the CEL come back on right away?

Re: Check-engine light...Code P0420
 

If you have an exhaust leak before the cat, you'll pull a p0420
If you're running lean, you'll pull a p0420
If you're running rich, you'll pull a p0420
If your EGR is clogged, you'll sometimes pull a p0420.
If your car is automatic, the exhaust manifold might be broken which would cause a p0420.

Just a reminder.

Re: Check-engine light...Code P0420
 

The car has to meet certain enable criteria. Honda calls it a "drive cycle" Driving conditions have to be right before the ECU will start running the cat moniter. The cat also has to fail during two consecutive drive cycles. The light may not come on for a few days.

Re: Check-engine light...Code P0420
 

Re: Check-engine light...Code P0420
 

Exhaust leak before the cat-code P0171
Running lean-code P0171
Running rich-code P0172
EGR plugged-code P0401
A P0420 is Catalyst system efficiency below threshold

Re: Check-engine light...Code P0420
 

Exhaust leak before the cat will decrease catalyst efficiency.
Running lean will decrease catalyst efficiency.
Running rich will decrease catalyst efficiency.
EGR Plugged is open for debate, it's happened to me once and it was on a Ford.

At least once a week, someone will roll in with a P0420 saying that they just got a new cat because Autozone told them to change it and I'll have to find the problem. Periodically the problem will be an incorrect cat, but usually it's something else.

Re: Check-engine light...Code P0420
 

Wow, ok then...what do you think I should do? I definately put the order on the Cat on hold. The first thing I think I might do is check for a leak in the exhaust then?

...and my car is the 4 spd automanual.

Re: Check-engine light...Code P0420
 

its usually a cat thats bad 9 times out of 10. but here you can try this.
heres something that might help you. this is right of of the honda service news.


Before Checking a Catalytic
Converter DTC
Before you troubleshoot an OBD II vehicle that stores
DTC P0420 (67) (catalyst system efficiency below
threshold), run these quick checks:
1. Check for a leak in the exhaust system. If you find
one, repair it, clear the DTC, and test-drive the
vehicle.
• If the DTC doesn’t come back, return the vehicle
to the customer.
• If the DTC returns, go to step 2.
2. Connect the PGM Tester, and test-drive the vehicle
while an assistant monitors the voltage signal from
the secondary oxygen sensor (HO2S S2). After the
catalyst reaches operating temperature, the HO2S S2
voltage should stay between 0.5 and 0.8 V at steady
cruising speed. During deceleration, the voltage
should be steady at 0.1 V or less.
• If the voltage readings are OK, clear the DTC, and
return the vehicle to the customer.
• At cruising speed, if the voltage fluctuates or stays
below 5 V, go to step 3.
3. Measure the inlet and outlet external temperatures of
the catalytic converter with a thermometer capable
of reading up to 500°F.
• If the outlet temperature is more than 100°F hotter
than the inlet temperature, the converter is OK;
clear the DTC, and return the vehicle to the
customer.
• If the outlet temperature is less than 100°F hotter
than the inlet temperature, replace the converter

Re: Check-engine light...Code P0420
 

A Worn out engine will damage a cat! So does that mean he should replace his engine? Fact is, the ECU tested the cat and it failed. And if the car is running too rich or too lean there would have been other codes stored, In my experience as a Honda master tech, I know this car needs a new cat and I would check the coil and igniter as the probable cause of premature cat failure. Hey, maybe the cat failed because its a crap cat! Seen that too.