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I'm interested in realtime information for the MONITOR_VVT_B1 and MONITOR_MISFIRE_CYLINDER_(1|2|...) monitor codes.

I would poll them about once a second, maybe slightly more frequent.

Is it problematic to do that? Do these codes trigger some internal analysis in the engine, or do they just cheaply read out a buffer value stored on some ECU?

What about other monitor codes, are there some which should not sent to the OBDII interface for realtime feedback, but just, let's say, once a minute?

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They can be read as often as you'd like. Many monitoring tools let you see real-time values coming in. They're just reading a buffer; the ECU is always collecting values.

  • These are the "Mode/Service 06" codes, not the "Mode/Service 01" codes. Is that also valid for them? "Mode 6: Test results for continuously and noncontinuously monitored systems, supported" according to motor.com/magazine-summary/… – Daniel F Oct 22 '18 at 20:46
  • Oh, so these are the emissions test codes? Those are set less frequently, but there is no harm in reading them frequently. My 2003 Focus required the car to perform certain actions before the codes were all set. Things like idling for 5 minutes, driving at highway speed, driving stop and go, etc. – Spivonious Oct 23 '18 at 17:56
  • I doubt the misfires and "variable valve timing" (VVT) are emission related. I've also noticed that these don't change often. For me the issue is if asking for those codes triggers something in the ECU which initiates a test, which is involved with some extra work just for me, or if the only thing these codes do is to tell me "this is what I have in my buffer for that test, ask again at some later time to see if it has changed, but I won't start this test just because you are asking for it now". I hope you understand what I mean. I'm very new to OBD2. – Daniel F Oct 23 '18 at 18:03
  • I understand what you're asking. The car is constantly reading sensor values and running self tests when certain criteria are met. These values are stored in memory and are read by an OBD scan tool. There should be zero harm in reading them on a regular basis. The scan tool should not initiate anything (except for a code reset when initiated by the user). – Spivonious Oct 23 '18 at 19:48
  • Thanks, with code reset you mean fault code resets? As I understand it, Service 1 and 6 are not meant to reset anything; those are the only ones I want to use. – Daniel F Oct 23 '18 at 20:13

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