2004 Mazda 3 ECT sensor - P0128 code

Tiny
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  • 2004 MAZDA 3

Electrical problem
2004 Mazda 3 4 cyl Two Wheel Drive Manual 58000 miles

Until my engine is warm my car operates perfectly. Once the car is warm and goes from open-loop mode to closed-loop mode the performance becomes inconsistent (hesitation and varying power output). I believe this is due to a sensor and/or wiring problem. I have read that many people have issues with the MAF sensor wiring but I have been unable to find any broken wires and I have chased down the wiring with a multimeter (also cleaned the MAF/IAT and did fuel injection system service). One day I cleaned the PCM wire harnesses and the car drove perfectly for almost two days but since then no amount of cleaning makes a difference.

Just recently, I have been getting the P0128 code which suggests the thermostat or ECT sensor is bad. There is a TSB out for this problem but I recently had my PCM updated and the dealer said this update would have solved the problem if the PCM was the cause. The temperature gauge in my car never moves a millimeter once it is warm so I do not think the thermostat is the problem. I have read that the P0128 code is based on a calculation that includes input from the MAF and IAT sensors which seem to be the more likely suspects. Does anyone have any advise before I start replacing parts?

I have read that this P0128 code does not let the Mazda3 get out of open-loop (start-up) mode. I am not sure if this statement was accurate but the author was confident that this trouble code would impact engine performance. My car has not behaved any differently since the CEL first came on about six weeks ago due to the P0128 code. My problems are intermittent, only happen when accelerating, go away for hours (sometimes day), but always come back. When my car is experiencing the problems the gas mileage is poor (21 vs 28 mpg) and there is excessive carbon build-up on the muffler tip. I am not sure if my problems are related to the P0128 code but I sure hope so.

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Monday, August 24th, 2009 AT 4:47 PM

23 Replies

Tiny
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Treat the symptoms of the hesitation and the code separately. They may or may not be r4elated to each other.

Regarding the hesitation when warm:

Check engine vacuum while cold and then see if it falls off as the engine has run for a while. There are two nipples on the intake manifold near the frame rail. You may have a compression problem. Check compression when hot warmed up. Look at the plugs while they are out and see if they are loaded with carbon.

The short block on this vehicle is warranted by Mazda for 8 years and 100,000 miles. Cats have been known to fail from time to time and can give this symptom, but generally a code po420 is thrown for this.

On the code po128:

I would have thought recalibration would have been right, except you did that. The temp gauge may be incorrect and giving mis-leading info. Try to establish the temp through another source, such as a scanner or a heat temp gun pointed at the t-stat housing if possible. Check with the dealer on a thermostat and thermostat housing that has been updated.

Do not Run regular fuel rather than the required premium fuel to save money or potential carboning may cause this. ( I know you did some cleaning, but it is not always effective)

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Tuesday, August 25th, 2009 AT 5:00 AM
Tiny
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Thank you for your response.

After letting my car sit for a couple hours with a fan on it I performed the vacuum test as described. The initial vacuum reading was 21"Hg. The vacuum pressure did not change once the car was warm but there was a significant drop in vacuum pressure once the A/C was turned on (from 21 to 17.5"Hg). Turn off the A/C and the vacuum pressure goes from 17.5 to 20. A few seconds later the cooling fan will cut off and the pressure will go from 20 to 21"Hg. Hopefully this is unusual and indicates a problem?

The dealer has performed a compression test and all cylinders are fine. They have also checked the fuel pressure twice and it passed both tests with flying colors. I recently checked the spark plugs and there is no unusual carbon buildup.

You are the expert, but I find it hard to believe that the temperature gauge is faulty. If the car is running hot and/or the thermostat is failing there are symptoms other than a flucuating temperature gauge and I have experienced none of them. The temperature gauge behavior is like clockwork. I have absolutely no reason to believe it is faulty (and I have been staring at it for the last few weeks).

My car does not take premium fuel. The owner's manual states 87 octance and that is the only fuel I have filled with. I am also careful about where I get my fuel and 99% of the time it is from BP. The fuel injection cleaning service was performed by the dealer, was comprehensive, but not necessarily effective as you mentioned. Having good fuel pressure does guarantee proper fuel delivery. One Mazda3 owner I read about solved his hesitation problem by removing the fuel injectors and backwashing them. The intake manifold was cleaned relatively thoroughly in fuel system service but I am quite sure there is excessive carbon buildup in the exhaust/emmission systems. I recently purchased this vehicle and I think the problem has been going on for quite some time. When the performance is inconsistent the gas mileage goes down and the carbon buildup on the muffler tip is far more rapid (like the car is operating in "safe mode").

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Tuesday, August 25th, 2009 AT 5:15 PM
Tiny
JACK42
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Do you have access to a scanner that will allow you to monitor the ECT and IAT readings? This will be necessary to follow the troubleshooting chart. As far as the vacuum readings, those all sound normal, vacuum will lessen with engine load (a/c), but your numbers sound pretty good. This may just be a bad ECT, working, but out of range a little which would effect th emixture and time to closed loop, creating the poor mileage and soot you spoke of

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Tuesday, August 25th, 2009 AT 6:20 PM
Tiny
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No, I'm not an expert. I know more than some and less than others. Maybe less than you. Maybe not.
Just trying to help you diagnose and unturn stones.

Having good fuel pressure does guarantee proper fuel delivery is not true. Good fuel pressure does not gaurentee fuel volume.

I agree with Jack's update.

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Tuesday, August 25th, 2009 AT 8:20 PM
Tiny
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Service Writer, I did not mean to offend you. I am quite certain you know more than me based solely on your response. I did not think to check the vacuum pressure and that was a great suggestion. If injectors are clogged fuel is not being properly "delivered" to the cylinders (the common industry term is "volume" but both descriptions are accurate - "delivery" is flow - flow is quantified volumetrically - flow requires pressure - pressure does not guarantee flow).

I also agree with Jack's update. I have thought all along that a sensor was going bad but still operating within a range considered acceptable by the PCM. When I started messing around with the wiring and saw temporary improvements I began to think it was a sensor wiring problem. I guess is still could be but the fingers are now pointing at the ECT sensor. I will check the ECT sensor for continuity and open/short circuit and post the findings.

Thanks for all the help!

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Wednesday, August 26th, 2009 AT 5:05 PM
Tiny
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LOL. It doesn't matter who knows more. I have often seen a group of very qualified techs that have a different view of a problem. Testing can narrow it down but it can still come to theory and an educated guess.

Very well may be a wiring issue. Troubleshooting isn't always black and white. There are conditions that should set codes and don't and conditions that you would not suspect being related to a particular code existing.

I have seen this scenario once before and after replacing the ect and t-stat, it ended up being the reprogramming. I do believe there was an updated stat and housing that was related to this, but again I would check with the dealer if the ect doesn't pan out.

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Thursday, August 27th, 2009 AT 6:12 AM
Tiny
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I do not have time to remove the ECT until this weekend but I just disconnect the plug, cleaned the terminals, and tested for an open circuit using the instructions below. Continuity between ECT sensor A and PCM terminal 2J was good. There was no continutiy between ECT sensor B and PCM terminal 2AX. Also, the check engine light went off after these efforts and I did not disconnect the battery. I thought the ECU waited for 3 consecutive good drive cycles before turning off the MIL.

Does the MIL going out immediately after messing with the ECT suggest anything definitive?

The manual recommends to "repair or replace the wire harness". Not exactly detailed instructions. Any suggestions?


http://www.2carpros.com/forum/automotive_pictures/400602_ect_open_circuit_1.jpg

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Thursday, August 27th, 2009 AT 8:41 PM
Tiny
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I didn't have time this weekend to pull the ECT and check for continuity and after the failed open circuit test I am not sure what to think.

The MIL came back on today, the performance consistency went from acceptable back to horrible, and I'm sure it is the same code. I have performed one test that indicates a wire harness problem. If there are any Mazda3 owners out there please perform the ECT open circuit test included in my last post and respond.

My research suggests a bad thermostat or improper PCM configuration. The TSB for the P0128 code describes the PCM fix process and I am not sure if the PCM update performed by my dealer would have specifically addressed this problem. However, the latest release of this TSB (01-041/07) suggests this problem is related to cold weather so it might be completely unrelated. Can anyone confirm that updating my PCM with the latest software would have accomplished the steps outlined in this TSB?

I am leaning towards replacing the thermostat. I have read that a stuck open thermostat causes a car to run rich resulting in poor gas mileage which I have been experiencing. I have never had a thermostat fail open on me. I would not think this would cause overheating which is probably why I have not noticed anything unusual from my temperature gauge. Has anyone had a thermostat fail open before? Are there any symptoms other than running rich (poor gas mileage) and MIL illumination?

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Monday, August 31st, 2009 AT 4:58 PM
Tiny
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A stuck open thermostat does happen. It causes the engine to take much longer to warm up as there is more volume to heat up. The longer it takes, the more fuel is being used. The Rich condition may cause a performance issue that very well be related. There may be an upgraded t-stat and housing possiblility on this one. A t-stat can stick both ways. A gauge may not read accuratley either.

The TSB does indicate cold temp conditions and since it has been re-programmed, it is not relevant anymore.

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Monday, August 31st, 2009 AT 7:05 PM
Tiny
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Well, I replaced the thermostat and went through a tank a fuel. The check engine light has not come back on but replacing the thermostat did not solve all of my problems. The hesitation/stumbling rarely happens now with the A/C off and only occassionaly with the A/C on. The weather has gotten a lot cooler lately and I think that has something to do with it. I hope we get some more 95-degree days before fall completely sets in.

The performance (acceleration) did not improve. Before the thermostat change my car would alternate between good, okay, and poor acceleration. Now, the behavior is more consistent but, unfortunately, it is between okay and poor. I know this engine has a lot more to offer - especially considering the bad fuel economy. I got only 20 mpg on the last tank which is the worst since I started keeping track. When the engine performance is good I get around 28 mpg. Something is still restricting the engine power output and at the same time hurting the fuel efficiency.

I think I have a lemon on my hands. I just got out of a Prelude with 200HP that got better gas mileage and I wore out the carpet underneath that gas pedal. I still think some sensor is causing the computer to screw up my A/F ratio. If I can't solve this problem and 21 mpg is a permanent condition I would at least like to enjoy the power I feel for the first couple minutes after start-up. That is the only time you would ever know this engine has 160HP. Is there a way to trick the computer into operating in start-up mode permanently?

I'm getting frustrated.

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Tuesday, September 8th, 2009 AT 5:01 PM
Tiny
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Okay, so one of the symptoms were solved, and confirms why I said to treat them separate.

Are there any codes anymore? I'm not expecting, but wanted to ask.

IS there any long cranking times experienced?

Have you tried using any premium fuel?

You may be at a point where you need a scanner capable of reading the data strem.

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Tuesday, September 8th, 2009 AT 7:55 PM
Tiny
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There are no more engine codes.

On one occasion there was a long cranking time. This was a couple months ago and it has not happened since.

I have not tried premium fuel. Unfortunately, I just filled the tank but will try premium on my next fill up. From what I read it shouldn't make a difference but for a couple bucks it is certainly worth a try.

Would you recommend taking it to the dealer for the scanner? What exactly are they looking for? Bad A/F ratio?

I am not looking to make this a tuner car but would a A/F management system be something to consider?

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Thursday, September 10th, 2009 AT 4:26 PM
Tiny
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I don't have much knowledge on the aftermarket kits for performance. A little too Frankensteinish for me, but for those that know them and use them well, I tip my hat to...not my bag of tea.

Certainly something is affecting the a/f ratio. A scanner will provide so much more data to analyze and help draw a theory. The o2 patterns can be observed to see if one is lazy, Various temperatures can be read, Snap shots of the data stream during performance can be taken and observed.
Mode 6 data can be pulled and help support or challenge the theory.

The dealer may be fine where you are. Bottom line is the knowledge, perserverence and the experience of the tech regardless of who employs the tech.

My concern of how the tech was paid will play into it. IF he is given a flat rate of 1 hour and HAS to have the answer, he may guess and hope. There are times that one hour is not enough, do you rely on the guess or be willing to pay more for additional time?

You can talk to some local shops to get a feel for what they can do through here:
http://www.iatn.net/shopfinder/

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Thursday, September 10th, 2009 AT 7:40 PM
Tiny
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I was at half a tank and topped it off with premium today. We will see if it eliminates the hesitation problem which has become much less of an issue. Again, I don't know if the cooler temperatures are helping. And it is becoming more difficult to make a distinction between true hesitation and the "drive-by-wire" system response behavior. Why the hell did Mazda do away with the trusty throttle cable?

The fuel efficiency was a little better at 22.9 mpg. My driving style is considered "aggressive" and most of that 1/2 tank was city miles. I only got 20 mpg out of my last tank and that was an even mix of city and highway miles. Maybe the computer is relearning?

If the fuel efficiency keeps varying from one tank to the next I think the O2 is suspect. I am confident this running rich issue had gone on for a long time and it seems logical that carbon build up would interfere with the emission control system. Your point about the tech and the pay structure has been taken into advisement.

Thanks for your help.

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Monday, September 14th, 2009 AT 6:53 PM
Tiny
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There were no more 90+ degree days. I tried premium fuel and there was no difference. The hesitation is still there but not nearly as noticeable. I have not found a tech a trust to analyze the A/F ratio issue. A previously solved vibration issue has re-emerged.

I have replaced the tires, rotors, a bent rim, two motor mounts, a tranny mount - had an alignment and had the tires rebalanced per a TSB. But every morning there is still a slight vibration in the steering wheel for the first several minutes of driving and there is a intermittent vibration in the gas pedal that never goes away. I researched gas pedal vibration and there does not seem to be a common cause. Sometimes the car drives perfectly smooth with no gas pedal feedback. What could cause this intermittent behavior? Could the computer not be properly controlling the timing resulting in a misfire?

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Monday, September 28th, 2009 AT 6:27 PM
Tiny
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Due to temperature? May be a case of the engine temperature is cold and adjustments are made until it reaches temperature. Faulty t-stat, CTS for example may be causing a rich condition until it heats up.

Is the intermittancy related to temp? That is does this seems consistent with starting off from a ambient temp. Start?

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Wednesday, September 30th, 2009 AT 11:09 AM
Tiny
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The car seemed to hesitate more when it was hot but the hesitation is still there and it is significantly cooler. It just happens less often.

Until today I thought the steering wheel vibration only occurred after an ambient temperature startup. Going home for lunch today the steering wheel vibrated on the highway. I will make a point to notice for sure but I think the steering wheel only vibrates at the same time I feel a pulsation/vibration in the gas pedal. I do, however, feel a vibration in the gas pedal when the steering wheel is not shaking. Like I said before these conditions come and go. But these conditions are becoming more frequent.

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Wednesday, September 30th, 2009 AT 12:16 PM
Tiny
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I just drove home from work and the car performed perfectly. No steering wheel vibration or gas pedal feedback. I know the problem will come back in the morning. This is the weirdest thing. I cannot think of any problem that would come and go like that.

I have never run across a suspension, tire, drive-train or alignment issue that comes and goes like that. It is one thing for a vibration to vary in intensity, but to go away completely and come back the next day is perplexing. An intermittent engine misfire might cause this behavior but I have not noticed any difference in engine performance when this problem is occurring. I pulled the spark plugs and they look fine.

Any suggestions?

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Wednesday, September 30th, 2009 AT 4:51 PM
Tiny
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My thought is that this is most frequent before it goes into open loop. That is the symptom happens while the powertrain management is pushing to get the engine warmed up as quick as possible so it can run most effeiciently and cleaner.

A coolant temperature sensor for example can be giving the computer bad info and the a/f ratio goes out as it either richens or leans out. Either way it may be misfiring and thus the feeling in the accelerator pedal.

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Wednesday, September 30th, 2009 AT 8:08 PM
Tiny
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The hesitation problem has not gotten any better or worse. It is annoying but manageable.

The intermittent vibration issue is still there. I recently rotated the tires and that only made it more noticeable. The vibration is felt mostly in the steering wheel and gas pedal but I know when it's happening from the passenger seat. I have concentrated on the problem and there is no pattern to the behavior. If it only happened with the steering wheel pulled in one direction it might be an axle or steering linkage. Or if only on certain road types it might be bad tire(s). I have $180 new Bridgestones all around that have been balanced 3 times. If it was an engine misfire I would expect the vibration to go away when I let off the gas and pushed in the clutch but it does not.

The steering wheel sometimes vibrates by itself (my hand shakes like I've had 20 cups of coffee). Most of the time I feel a pulsation in the gas pedal when the steering wheel is vibrating. These vibrations occur about 30% of the time I am driving and come and go like the wind.

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Thursday, October 29th, 2009 AT 6:58 PM

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