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P0420 Code - Light went on, light went out..... now what?

So, after doing a google search, found out that the P0420 code is one of the most searched for on the interwebby thing.

I have a 2007 Suzuki SX4, base model hatchback, AWD, and mostly love it. I normally fill up at Costco when I’m in town, but the other day, had to fillup somewhere else. A couple days later, the car threw the P0420 code.

My mechanic… who comes highly recommended from the Car Talk folks, suggested doing further diagnostics to find out what’s really wrong (as I understand from my research that the P0420 code can mean a variety of things).

So, I was supposed to take the car in on Thursday, but had to cancel at the last minute. On Friday night I filled up at Costco (as normal), and on Saturday morning, the light went out.

Question… I love my mechanic, but they’re kinda pricey. Should I still do the diagnostics anyway to find out what’s wrong? Or should I assume that everything’s “back to normal”?

When I asked someone if a bad tank of gas could cause this, they said, “not likely.”

Oh, and I do keep meticulous records on the MPG. It doesn’t seem like there’s been any significant drop. OTOH, I do think I should be seeing a bit better on the highway, for what it’s worth. Towards that end, I’ve ordered one of the K&N lifetime air filters. Not that that will help the P0420 code, but I like what I read about them.

So… question… should I take it in and find out what’s up? Or… blow it off till the light comes on again?

Drive on and if it reappears one or two times again address it.

Thanks for the advice, madRiver! But one thing I failed to mention… the car does have 161k miles on it. Does that change anything? (as in, you know, parts can fail after that many miles sometimes?)

Not really. If it’s not in the maintenance schedule to be replaced at X mileage or time, then best practice is generally to not worry about it until there’s a problem.

Even if the car had twice the mileage, I’d wait for the code to get thrown at least one more time before I started trying to figure out what it is.

Great! Appreciate the advice. This was something I wasn’t really looking forward to spending money on anyway :slight_smile:

I agree. Most of the time the check engine light goes on because of an issue in the pollution control system, and driving a bit longer with it on will not be a danger to you or your engine. Your guess that different gas might be responsible is possible. If it was me I’d just do as @shadowfax suggested.

I agree with Shadowfax’s suggestion, however what’s likely happening is that your cat converter is on the cusp of failing to continue doing its job adequately. The “on then off” P0420 is a common symptom of this condition.

Follow Shadow’s advice… but start saving for a new converter.

Worry about it when the time comes (steady light). Until then…drive on.

K&N filter will not alter MPG’s, but it will let more dirt into the engine.
When you get your new filter shine a light through it; and you’ll likely see pinholes.

If filter systems like K&N are so great why don’t they come on new cars? Besides why put that kind of money in a filter for a 9 year old car with 161000 miles.

P0420 is very often bad sensors. Many mechanics want to replace the very expensive cat AND the sensors “while they are at it.”

If you can find one, a very good mechanic can look at the sensors with a good scanner, and tell you exactly whether one or more of them needs to be replaced.

Any mechanic who tells you P0420 always means a bad cat, is a bad mechanic. There are so many of them.

If you can’t find a good mechanic, which is quite possible, do not replace the cat until you replace the sensors first and drive the car a while.

At least one really good mechanic on this board was furious the last time I said this. But, since so many mechanics want to simply replace the whole thing, cat and all the sensors, there is no big loss to try the sensors first and drive the car a while.

Somewhere on this URL is a place called Mechanix Files or something like that, with recommendations for mechanics by satisfied customers. Try that if you need help finding a good mechanic. You want someone who is believable when he (she?) says he can tell with his scanner what the problem is. That is the best of all possible choices, in my opinion.

For cat efficiency, the sensors report the amount of crud coming into the cat, then report the amount of crud coming out of the cat. There must be a substantial difference or it reports a failure. Clearly, if either sensor gives a false report, because of being in a bad condition, it will falsely report bad efficiency.

Some people have reported that taking off the cat (when it is very rusty this may be a horrid or impossible task) and soaking it over night in dish-soapy water will clean it up if indeed it is contaminated. One poster on this board said when he did that, little pieces of white things came floating out.

There would be one valid reason to replace the whole smear. As the good mechanic on this board said, in some cases the cat is so rusted you simply cannot get the sensors out so you have to replace everything. That is why it is so important to find a really good mechanic you can trust.


“in some cases the cat is so rusted you simply cannot get the sensors out”

Rust is a fact of life in some areas

What does that have to do with somebody’s skill level or trustworthiness . . . ?

I appreciate all the comprehensive info! As for the K&N filter, guess I’ll be returning it, unused. All the reviews I read were extremely positive, and I just thought it sounded like a good way to save money on buying the replaceable air filters…

Having said that, my main complaint on the Suzuki is that it does shift from 3rd to 4th rather early… which is great when you’re driving around town, but it makes it a bit sluggish getting on the highway. Based on the reviews I read, thought the K&N might help with that, but not worth the price of admission if it’s gonna tear up the engine as a result.

My mechanic, irlandes, wanted to do a thorough diagnostic to find out precisely which part was failing, just as you suggest a good one should. As I said earlier, they were a “5 star” recommendation from the Mechanics Files here at, so that makes me actually feel better about them.

Hopefully, I can avoid a major repair for a while yet. Will keep everyone posted. Thanks again!

P0420 Code
Last Time I Checked, That Particular DTC (Diagnostic Trouble Code) Was The Most Commonly Occurring Code Found On Vehicles With Illuminating CELs (Check Engine Light).

As others have pointed out, that code doesn’t necessarily mean the cat is failing, replace it. On the other hand, a poorly functioning engine could eventually contaminate a catalytic converter and cause its untimely demise.

It would be worth investigating the reason the code is being triggered if it becomes a frequent happening. Sometimes that can be very difficult, especially when intermittent. Some vehicles have been manufactured that require reflashing the ECM to cure a false signal.

I suppose in States that require “smogging” this would be frustrating and problematic, and inconvenient, as well as costly.


@Stegy Frany
Suzuki TSB (Technical Service Bulletin) #TS 04 11307 Which Pertains Only To 2007 SX4 Models Is Interesting.

The bulletin, 1 page with photo, written for Suzuki Technicians, points out that not taking precautions during oil changes, particularly oil filter removal, can contaminate the downstream oxygen sensor internally and possibly set a DTC P0137 AND/OR (Suzuki’s words, not mine) a DTC P0420 Catalyst System Efficiency Below Threshold (Bank-1). Oops.

Suzuki moved the problematic O2 sensor location for the 2008 model-year SX4.

I’m not saying this is the problem, but it is interesting and something to consider. I would think a trashed sensor would not cause an intermittent P0420, but I know better than to dismiss anything automotive at the first chance. I’ve seen some strange stuff apply to vehicles, or any machines, for that matter.



That’s really interesting you mentioned that. In doing research on the P0420 code, I decided I would google for videos about replacing the Oxygen sensor… to see if it was something I could do myself. In one video I found, they actually mentioned this problem (oil dripping onto the sensor), but since it was one of those “home amateur” kind of videos, they did not mention that there was a TSB about it. Fortunately (knock on wood anyway), my car does not seem to leak oil… but I will definitely mention it to my mechanic now that you let me know!!!

@Stegy Frany
"Fortunately (knock on wood anyway), my car does not seem to leak oil…"

The service bulletin isn’t about oil leaks. It’s about changing the oil filter and having oil fall onto the sensor below. The problems outlined in the bulletin caused Suzuki to move the sensor location on the 08s.

The bulletin photo shows a tech protecting the sensor with a shop towel prior to an oil filter change.


I would wait and see if it comes on again but send that K&N filter back. What in the world do you want that for?

Well, as I said earlier, all the reviews I read were extremely enthusiastic, and also seemed to correlate to the one main issue I have with the car, which is somewhat sluggish acceleration after it shifts into 4th gear. Like I also said earlier, I apparently did not do adequate research on these filters, because I had not heard about the dust and dirt issues. I primarily just thought it was a great way to save money on not having to buy disposable air filters, the concept of which, at least, is an appealing idea. Anyway, no harm done. It arrived yesterday, I did not install it, so I can send it back…