My car has been shaking on start-up. The check-engine light has been on/off periodically. When I originally took it in, my mechanic replaced a sensor (and a whole bunch of other stuff not related to this problem) and said i'd be good to go. But I still noticed the shaking and a few days later the light came on again.

When I took it in again, he said he got a code for an intake runner cooler or something but said to drive it around and take it in again when the light comes back on.

The shaking mostly happens in the morning, when I first start the car up, when it is moist. There was heavy dew this A.M. and when I started up, it was really bad and the car actually stopped running. I was able to start it up again immediately though.

I have had a suspicion that it is the spark plugs, could this be the case?

When the car did that this A.M. The light came back on and I took it to AutoZone to get a free reading. The only code that came up was for an oxygen sensor, which I just had replaced.

What could be causing this?

As a side note, the light went off as soon as I got back in after the reading, does getting a reading clear the code?

car is a 2005 mazda m3

  • It is normal to clear the codes when they are read. Also, there are at least two O2 sensors on an OBDII system (with up to four). One or two are located before the catalytic converter, with one or two after (or in), depending on the system, the number of cylinders, and the number of cats. The before ones will affect running, the two after only monitor the condition of the cat. Without the exact code, it's going to be hard to tell you if this might be causing the poor running. Aug 29, 2014 at 20:02

1 Answer 1


Most of the time on the Mazda3 the blame is defective engine mounts, they are too soft. Its the upper mount that's the prevalent one. There are aftermarket replacements available.

Now there are a few other things that it could be.

  1. Massive air leaks, check the EGR valve. It would be a good idea to do a smoke test on the intake and engine. The test will show any leaks.

  2. Clean the MAF and the throttle body (Do Not Use Throttle Body Cleaner on MAF), disconnect the battery first. Check the wires, with it running wiggle the wires. When restarting don't touch the gas pedal and leave all accessory items off (a/c and etc.), the car will run erratic until the ECU relearns timing and fuel maps. If car stalls restart until it stays running, without touching the gas pedal.

  3. Fuel pump issues. Electrical (a) and fuel pressure (b) test-

    a) Check pump fuse. Check the voltage at the fuel pump (determine whether or not the charge that's leaving the fuse is getting to the pump). Perform a drop test using a voltmeter (Check to make sure that the power wire shows the full voltage and the grounding wire shows that it's grounded properly. If you show more than one volt difference, that means you've got a problem with corroded wires, or that you've got issues with the circuit on either the positive or negative side).

    b) Check fuel filter, resistance should be minimal. Use a fuel pressure guage and hook it to the fuel pump fitting. Rev the engine while checking guage. Let the engine warm up slightly, then check the pressure both at idle speed and at the rated speed listed in your pump specifications. If the needle doesn't move then the pump is defective.

  4. Clogged purge solenoid valve. Sometimes it sticks open and lets unmetered fuel vapors into the engine. This causes the computer to adjust fuel trims which affect the idle. It is hard to check the purge solenoid without a scan tool, because they don't typically fail permanently. Plug the solenoid and line, check if idle problem gone.

  5. Non-iridium spark plugs. Replace with correct ones.

As to will the OBD reading clear the code, no it won't. It will have to be reset.


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