Engine Coolant Sensor

Code P0128 - Possible faulty ECT Sensor. Where is it and can I reaplce it, or is it a Dealer issue?

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.

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").

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?

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.

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?

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.

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?