My check engine light just came on . I took it to a local auto parts store and they read the code . It was P0420 . The guy said I had a faulty catalytic converter . The car has 117000 miles on it . It is the es model with the 1.8 engine . I think the car has 2 converters . How can you tell which one needs to be replaced ? I called some muffler shops to get a price and they asked me which converter . Thanks for any help , Tom

 

I have a '99 Protege 1.8L LX version (Canadian close equivalent to the SE)

Code 0420 is Cat. Converter efficiency. It is an implied fault, deduced but not measured by the ECM, by comparing the ratio of the front and rear O2 sensors located at the manifold end and tailpipe end of the first Cat. converter (the Cat. nearest the exhaust manifold).
The number of inversions (meaning lean voltage to rich voltage changes) of the front and rear O2 sensors is counted and the ratio of the two values is calculated. If the rear to front ratio gets excessive the ECU makes the assumption that the 1st Cat. converter is not doing its job.
BUT:
This code can arise from several causes:
1. First Cat.Converter is kaput.
2. Front or rear O2 sensor loose (by loose, it can be either physically loose, or it can
be poor electrical connections).
3. Leak in exhaust system between engine manifold and rear O2 sensor.
4. Front O2 sensor kaput.

117, 000 Miles sounds far too soon for a Cat. Converter death.
I recommend doing a visual/ acoustic check first, then remove and clean the O2 sensor connector contacts, reset the CEL (by disconnecting the battery negative, leave a door open with the courtesy light on for about a minute, then reconnect the battery). The CEL should now be out. Give the car a trial run to see if the CEL reappears.
If the CEL comes back, go to a competent Mazda service agent for a full OBDII diagnostic, and be sure to ask for a hard copy printout listing ALL OF THE DT CODES, not just the foreground ones.
Keep us posted on how you make out.

 

I got this car from my stepson . He bought it new . I changed the engine oil & filter and the air filter. I took it out for a test drive after about 10 miles the cel came on . It has never had a problem before . He did replace the intake hose last year because of a crack . Thanks for the advice about the 02 sensor wires . I'll check those .

 

I've got a 2000 Protege with the 1.8 and 138,500 miles. I had the P0420 code for quite a while. It used to be intermittent, but lately it would not go out, and come right back after I turn it off. I've replaced both O2 (rear then front) sensors, each of which only helped temporarily. I also just started getting a P0171 in addition. This weekend I replaced my converter (the main one), and the P0420 has disappeared, but I'm still getting the P0171. My MAF sensor is giving good data, so I'm off to look for intake leaks... I'm really hoping I don't have to get at the intake manifold.

But on the cat replacement, I bought a Eastern Auto model from Rock auto for about $200. Don't forget new gaskets on each end (another $10 or so). It was quite a job - I had to remove the front O2 sensor, the exhaust manifold heat shield, 3 nuts underneath and 5 nuts on top. One of the bottom nuts was ridiculously hard to remove. There was another bracket on the left side, which was cracked so I didn't have to unbolt it. Then to get it out I also had to remove the exhaust manifold. The replacement cat had through holes only, where the OEM had threaded studs, so I also had to buy new nuts & bolts and grind away some of the the weld on the new converter where it came too close to the holes on the flange. I also had to put some tiewraps on the harness connecting to the lower O2 sensor because the new cat didn't have the hanger brackets like the old one. Overall this job took me about 7 hours of work - I consider myself somewhat of a novice at DIY autowork... but maybe not after that job. On the plus side, the heatshield on the old converter had rusted away from one of its bolts, and was making a nasty rattling - this took care of that.

So, overall, if a P0420 is your only code, you may want to believe the message and try to replace your converter. But you better have a full weekend.

 

"I also just started getting a P0171 in addition."
Unclip your air filter and give the intake duct a good flexing while looking for cracks. It's a prime candidate for P0171's. A temporary fix with duct tape works fine. A replacement costs about $40 -$50, but the damn things go so often that they are often on 1 month back order.
The last time I replaced mine I also used stainless plumbing type 3 -3 3/4 in. pipe clips too because the original clips used stovepipe hardware and were rusted and a pain in the *** to slacken off.

 

This afternoon I sprayed different areas of the air system with carb cleaner to see if I could find my leak... the plastic air duct didn't show an effect, and it feels sound (although I haven't disconnected the air filter & flexed as you suggest) The spot that seemed to cause an effect was the center of the area where the intake manifold hits the engine block. I may recheck the air duct as you suggest, but I think I'm in for another long weekend to replace a $6 gasket.

The only odd thing is that I thought the "enrichment method" would cause an increase in RPM, but when I hit that area with the carb cleaner, the engine would shudder a little and drop about 50 RPM.

BTW... do this outside - the cleaner boils off the hot manifold and makes quite an odor.

 

The duct flex is poor. When the engine is cold, the duct is rigid and is highly stressed because the engine is torqued on its mounts, while the duct is anchored to the chassis-mounted air filter assembly and it gets tugged and twisted back and forth with engine torque load changes. As the engine warms up, heat is conducted primarily into the duct interface with the engine, making the duct more supple in that area, but the remainder of the duct remains cold and rigid. The result is that all the flexing and twisting is concentrated in the first 2 or 3 sections of the bellows nearest the engine. That's a good place to look for leaks for a start.
In any case, unclipping the air filter so you can really flex that duct to locate any cracks is an absolute must.

I prefer stethoscoping to squirting on modern engines because the MAF, front O2 sensor, ECU, injectors, and ignition advance loops all have their own transient response times, so hitting the air intake of a fuel injected engine with a burst of carb cleaner (fuel) can have unexpected results; for example, its volatility alone would cause a rapid drop in the inducted "air" temperature that a carburetor engine would take in its stride; not so with a closed-loop fuel injected type.

Another item worth checking is the EGR mounting flange; that 10mm. mounting hardware is hard to get at: also there's a gasket there which often gets lost if the EGR assembly has ever been removed.
If the EGR has ever had servicing, it's just possible that the inaccessible mounting hardware means it might not have been securely torqued down, and air is leaking in at the flange.

 

I took a second look at the duct, and found nothing cracked open - I can see some creases in between the bends, but nothing broken. I suspect it's got about a year left in it. I'll recheck the EGR area.

My plan is to do a bit of dismantling - take off the fuel injector rail and the oil tubes, and then get the suspect area with some Permatex Spray n Seal. We'll see if that does takes care of it. I also got a quote for $275 in labor to have a local shop replace the intake air gasket, which doesn't seem too bad.

Another interesting thing - I've got no check engine light on now, but my scanner is telling me I've got the P0171 still.

 

That's normal. If a code P0171 is thrown, and the problem which caused it goes away and stays away for 2 consecutive drive cyles, the CEL goes out, but the P0171 stays latched in memory.
You can try disconnecting the battery negative while leaving the courtesy light on for 30 seconds or so; this should reset the PCM memory.
It makes me very suspicious that the CEL went off after you checked the air intake duct.
Did you unclip the air filter and flex the duct when you were checking it? If not you might have missed a crack. They usually open up with the engine loaded since it then tilts on its mounts, stretching and twisting the duct.

 

Ran into the back of a Jeep that decided not to merge after all. Between the spare tire that crumpled my hood, and the lower edge of the bumper that pushed my radiator back, my Protege is totaled. Shouldn't off road vehicles stay off the road?

So no more troubleshooting. Thanks for the advice though.

I'm gonna miss that car... fun to drive, economical, and never let me down in 8 1/2 years. But I was getting it ready to sell anyway. 2010 3, here I come.