I had been plagued by intermittent OBDII P0403 check engine light for months. I changed the EGR valve, had the harness replaced but to no avail. The workshop said it is the PCM and quoted a hefty price for it which I declined.

Feeling my engine "stumble" then the CEL come up I decided to give changing spark plugs a go. Changed it with NGK iridium spark plugs. To my surprise the spark plugs were standard Denso spark plugs. One cylinder had oil stains on the coil pack and spark plug. The valve gasket was replaced earlier so I presume this will not happen again.

After this the CEL never came back again. I wonder if I have really resolved the issue?

I still experience the following issues:

  • Intermittent engine vibration when accelerating at around 2100rpm

  • When idling my intake temp goes up quite rapidly from 55 to 70 deg Celcius.

Coolant is ard 98-100 deg Celcius.

Wonder if anyone has a clue?

  • 1
    I was also suffering from stumbling, rough idle and hesitation. The valve cover gasket was bad, and oil was leaking into the plug holes. Changed the gasket and the plugs and my problem went away. You can see picks of the plugs I took out on this other post: mechanics.stackexchange.com/q/21028/7132 Dec 23, 2015 at 17:28

1 Answer 1


P0403 is indeed the EGR system. The system includes actuators and electrical wiring and thier voltages, which all must be correct, not just the EGR valve assembly. Vacuum hoses and the EGR control solonoid are areas to check carefully. The PCV valve and system are also required to be checked carefully as they can affect the EGR valve if leaking air. The final part is a scanner check to see that all of the vehicles monitors have run - completed a drive cycle. If not why not. The Mazda 3 series of vehicles run very lean and are susceptable to this type of fault.

  • Finally got to change the PCV valve. Had to take out the intake manifold with the help of my mechanic friend. I do notice more power and smoother engine operation now.
    – Andrew Tan
    Jan 8, 2015 at 16:16
  • Scanner result is OK. All green (checked)
    – Andrew Tan
    Jan 8, 2015 at 16:16

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