2000 Miata runs intermittently terrible

Replace plugs, wires, cam sensor. If that doesn't fix the problem, replace the coil pack. (Or replace the coil pack, plugs, and wires.) The first O2 sensor may also be bad. Use NGK "blue" plug wires. (Mazda dealers usually carry the NGK blues.)

99 and 00 models have a high coil pack failure rate. A "redesigned" coil pack is available from Mazda. Plugs ~$8 for 4. Cam sensor ~$60 (discount price) Coil Pack ~$250-280 Plug wires ~$40 O2 sensor Bosch universal ~$60-90. Bosch specific purpose O2 sensor with Mazda connector ~$280

The O2 sensor may have an odd symptom, in that it may work, with approximately the correct output voltage, yet be bad because of response time. This will generally cause a cat efficiency code to be set, rather than an O2 sensor related code. The cam sensor and the coil pack seem to have temperature related failure modes that are fairly common. Stock black Plug wires have high failure rates, and as a result should be changed to the "Blues", which are much better.

If the above does not fix the problem, the crank sensor is suspect. Finally, the various connectors and the ECU can occasionally be the culprit. In one case, on my own 99, "rough starting" was caused by, of all things, the battery. It seems that the battery output voltage was sagging excessively under starting load. The engine would "crank" slightly slower than normal, and ignition firing pulses to the plugs were absent or erratic. Evidently the ECU didn't like the lower voltage, and did not send the ignition pulses to the coil pack as it should.

(Been there done that, have the bad parts and scars to prove it)

I recently got a new Bosch direct-fit O2 sensor (with the Mazda connector) for my '02 for $140 (including tax+shipping) from

From my experience, no. The coil pack can cause intermittent misfire, serious enough to set the ECU light, yet not cause it to flash. Or, in my case, it never set a light, even though the 99 ran poorly at times. There would be misfire codes, usually random, set in the pending codes. At the time, the "revised" coil pack was not available, so I ended up replacing the ignition system with an aftermarket coil over system. End of that problem.

Next issue was a rough start issue that started to occur after we replaced the timing belt. Turned out that it was also extremely intermittent. When all was said and done, the cam sensor was failing. The only reason we found, other than temperature sensitivity, was that the cam sensor had been placed on a Snap-On shop tray that had strong magnets attached to the bottom, in order for it to "stick" to metal car body parts. Another problem was one that had occurred on a very intermittent basis for several years. It began with a very occasional cat efficiency code set in pending codes, and the even less frequent setting of the ECU light. The ECU light would generally set (if it was going to set) when the 99 was operating at low speed. The would usually occur the next day after the 99 had been driven on the interstate for an extended period of time. It didn't make a lot of sense, since the converter was operated at a high enough temperature and long enough time to clear out anything that might have remained from previous low speed operation. Logic said that the code must be setting based upon the driving cycle testing.

Again, when all was said and done, it turned out that the front O2 sensor had a respones time too slow fault. The ECU test compared the front and rear O2 sensor reading, and decided that the cat must be bad (Wrong). Just to add spice to the problem, a new cat would make the problem seemingly go away, at least until it aged in, and the efficiency dropped (as all do). The front O2 sensor voltage output was approximately correct, and the 99 A/F would oscillate around the correct numbers. Replacing the O2 sensor with a Bosch universal replacement solved the problem.

The only remaining issue seems to be temperature related, in that the 99 will occasionally run poorly (less than normal power & possibly "stumble" if the engine is not fully warmed up. Evidently the temp sensors used by the ECU do not always produce the correct information, and the ECU thus sets for incorrect mixture or timing. The intake air temp sensor was replaced, with no change, and the next potential victum is the temp sensor at the rear of the engine.