DTCs on Protege

My car’s engine light has been on for years with a P0442: evaporative emission system leak detected (small leak). I put some petroleum jelly on the gas cap o ring, turned off the check engine light and it came back on within 40 miles or so. I tried this a few times (with the same results) and decided it wasn’t a big enough deal to mess with so I decided to just accept the engine light being on all the time. It’s been over a year since I’ve hooked the scan tool up so I decided the other day that it would be a good idea to see if any new codes had been sent and sure enough, there was a P0421 and a P0400, in addition to the P0442. I did some research on the P0400 and cleaned out the nipple for the EGR valve, turned of the MIL and drove about fifty miles - light came back on and the only codes that it was showing was P0442 and P0421. Since the car has 179k miles on it I’ve decided to replace both oxygen sensors to see if that will take care of the P0421 and because they probably are due to be changed anyways. Thoughts?

Alright, for the P0442. I’d like to take care of it also so that I’ll know if any new codes come up. Here’s the freeze frame data:

engine speed: 2032 rpm

calc load: 58%

coolant: 196 F

STFTRM1: 1.6%

LTFTRM1: -0.8%

vehicle speed: 39 mph

fuel system 1: closed

fuel system 2: N/A

Any ideas?

1998 Mazda Protege 1.5 LX

How about replacing the gas cap? Clearly that thing doesn’t not make a good seal.
The P0421 could be one of your O2 sensors but first inspect for exhaust leaks.

I thought that the results from the petroleum jelly test was an indicator that the gas cap wasn’t the problem. That’s why i didn’t replace it.
Is a vacuum guage the only way to check for exhaust leaks? because I don’t have one yet.

Never mind about how to check for exhaust leaks… I just googled it.


p0442 could be a lot of things

cap not sealing well (very common)
fuel filler neck assembly (seen this a few times)
fuel sender seal leaking (seen this more than a few times)

If the cap doesn’t fix it, go visit the pros. They will use an evap/smoke machine to determine if you have a leak, the size and location of the leak. Not a big deal for a competent mechanic working n a decently equipped shop.

Your fuel trim values seem pretty good. I’m thinking the problem may not be the O2 sensors. If you replace them, be sure to keep the originals. If the problem recurs, then you can put the old ones back in. The original O2 sensors are often better than the replacements, especially if they are non OEM replacements. I’d be inclined before replacing the O2 sensors to verify the fuel pressure at the rail, that it responds correctly to intake vacuum, and do a visual inspect for exhaust leaks.

So in order to verify that the fuel pressure responds correctly to the intake vacuum, I just (w/ fuel pressure guage connected to the fuel rail) rev the engine? How many rpms? and how much should the fuel pressure increase or decrease?

AKE SURE THE GAS CAP IS ON AT LEAST 3 CLICKES CLEAR THE CODE AND SEE WHAT HAPPENS. sorry about yelling just say my cap lock was on

It won’t help. I’ve tried that plus packed it with vaseline and the CEL still comes back on. The o ring doesn’t have anything wrong with it either and the cap isn’t cracked but for eight bucks I can get an OEM cap so I’ll likely give that a go just to be certain. Thanks for the tip though.

Any chance that the P0442 and the P0421 are related?

The fuel pressure remains constant no matter how much you rev the engine. The fuel pressure regulator controls the fuel pressure. At idle when the manifold vacuum is high the fuel pressure regulator allows more fuel to return to the gas tank via the return hose. When on the accelerator and the manifold vacuum is low, the fuel pressure regulator restricts the fuel from returning to the gas tank so more is forced thru the injectors.

Code P0421=Warm Up Catalyst Efficiency Below Threshold (Bank1)

Code P0442=Evaporative Emissions Control System Leak Detected (Small Leak)

These codes are not related to each other. They’re two seperate issues.


@mystic … as mentioned by @Tester above, verifying that the fuel pressure regulator responds to manifold vacuum doesn’t involve changing the rpms. Here’s how I understands it works on my Corolla at least. The injector has a certain pressure head at its input, controlled by the fuel pressure regulator. The amount of fuel injected by an injector is controlled by the pulse width of the electrical signal from the ECM. The more air the ECM measures going into the engine, the more gas needed, and therefore, the longer the pulse width. That’s how the ECM meters out the gas to match the air volume. The problem the ECM has is that the amount of gas injected by the injector (for a given pulse width) depends on the pressure difference between the input head, and the injector output, which is in the intake manifold. At idle, there is a high vacuum in the intake manifold, which tends to suck more gasoline out with each pulse, compared to if the intake manifold was at closer to ambient pressure (less vacuum, for example if the driver is stepping on the gas pedal). To compensate for this, the fuel pressure regulator drops the fuel pressure at the head of the injector when the intake manifold vacuum is high. Likewise, when the manifold vacuum is low (less vacuum), the fpr increases the fuel pressure.

How much this varies as a function of manifold vacuum varies from car to car. You’ll need the shop manual for you car or something equivalent (like All Data or a Motor manual) to find this spec, along w/the procedure on how to do it. Be sure to follow the safety recommendations in the shop manual anytime working w/gasoline.

Seriously appreciate it guys. I’m going to test the front O2 sensor first since I’ve got a meter and it’s a piece of cake to get to the electrical connector then move on to checking the fuel pressure and the exhaust system if the sensor checks out. I’ll keep you posted on the results.

About the exhaust. Am I correct to assume that the only section that would matter here is the exhaust manifold down to the front pipe? which is where the second and last O2 sensor is connected. Everything else downstream from the last sensor wouldn’t cause the computer to throw a P0442 code would it? I do plan to inspect the whole exhaust system but am just trying to get a better understanding here.

I believe code P0442 has to do with the fuel system, not the exhaust or O2 sensors.

Oops, I meant P0421

I think you are correct about the exhaust system @mystic. If an exhaust leak is the source of the problem, it would most likely be upstream of the final O2 sensor. I suppose if it were downstream but only a few inches away, it could affect it there too. Or if there was a super large leak, that might affect how the flow profile past an upstream sensor. I doubt you have a super large leak, you’d usually hear it first.

You can do a little experiment if you like. Hold a piece of paper near the tailpipe-- I use a dollar bill – and let the exhaust gas blow on it; you’ll see the paper mostly is blown in a direction away from the tailpipe in bursts, but between bursts it can briefly moves towards the tail pipe. So there’s a brief period in each cycle where some reverse suction is created in the exhaust. That’s how outside air can get sucked into the exhaust stream and confuse the O2 sensor. The effect is more pronounce the further upstream the leak is. The paper trick can be used to probe for small leaks in the upstream exhaust pipes too. (I’ve used the paper trick at the tailpipe as a visual test for verifying that an engine is missing. The paper will move backwards more than normal with each miss.)

There’s u-tube vdo’s on the internet for suggestions using a propane torch on simple ways to test 02 sensors. Not sure how accurate they are, but at least you can test that the sensor hasn’t completely failed. The vdo I looked at was for testing the narrow band type of sensor which is heated by the exhaust stream and not by an electrical heater element in the sensor. Testing the wide-band electrically-heated type is probably more complicated.

I found out yesterday that I could use my scan tool to test my 02 sensors. So after work last night when I got home and the car was fully warmed up and running I plugged it in and found that at idle the front 02 sensor was all over the place. I got readings ranging from .050v up to .800v. And the rear oxygen sensor gave readings of .055 up to .575v. Don’t know if it makes a difference but the coolant temp was 208F and the idle was jumping around from 803, 808, 816, 822, 831, and 833 rpms.

Should I rule out the oxygen sensors and move on?

What was the average reading of the front sensor?

Does your scan tool read that?

Did it have plenty of crossover counts (low to high and high to low)?

Were the high to low to high transitions pretty quick?

Or pretty slow?

I don’t think that it does give average readings for the front O2 sensor. I’ll have to get back to you on that. It’s an Actron CP9575.

I do think that the front sensor did have plenty of crossover counts if you’re talking about what I think you’re talking about. I copied down most of the voltage readings: .050 .030 .070 .275 .265 .220 .130 .390 .425 .725 .710 .760 .800 .81 .820

I’ll be back with more info in a bit.

It sounds like your front sensor is NOT lazy. That’s good.

Was there some sort of misfire, blown headgasket, etc. which may have damaged the cat?

That is assuming the cat is indeed faulty . . .

The high to low to high transitions are quick on the front sensor and even quicker on the rear sensor.

Front sensor @ idle:

.020 to .720 (about 3 seconds)

.765 to .035 (about 3 seconds

Rear sensor @ idle:

.060 to .810 (about 2 seconds)

.830 to .070 ( about 1 second)

I also measured the resistance of the heaters on both sensors.

front sensor: 7.6 ohms (engine cold) 9.6 ohms (engine hot)

rear sensor: 7.0 ohms (engine cold) 8.0 ohms (engine hot)

The official Mazda shop manual for this car says that the resistance of the oxygen sensor’s heaters should be approx. 6 ohms @ 68F. It was probably about 75F this morning when I took the “cold” measurements and the car hadn’t been driven since last night.
And the ‘‘hot’’ measurements were taken with the engine fully warmed up.

To confuse things the Chilton says that the sensor resistance should be 11 - 17 ohms and doesn’t give a temperature.