QuestionQUESTION: Realized I should have given you some more information. The car is a 1999 Miata sport with about 113000 miles.
Hi, I bought a used Miata last weekend, and after less than 40 miles of driving the MIL came on. Stayed solid for 5-10 miles, then flashing. Went straight home and it's been in the garage ever since. Flashes whenever I start the car now and continues flashing. Haven't driven it again, just started it to see what the MIL would do. While driving it, I've noticed no problems, but I honestly do not know how Miatas should feel as I've never owned one. On a few starts since I've bought it, it's been a little shaky -- engine rumbles oddly for 5 seconds or so and shifter shakes somewhat wildly but then idle returns to normal and is fine. Only did this a couple times. As I'm sure you know, the P0300 code is for a random misfire and the P0421 is for the cat. Another thing I've noticed, and I can't honestly say if it wasn't there before, is a slight gas smell in the interior after leaving it parked with the top up for a few days. I read that if the cat is bad or going bad, this could happen.
What do you think? I'm assuming I shouldn't drive it anywhere with the MIL light flashing.
ANSWER: Right, the emissions code is likely a result of the random misfire. Buy a new set of NGK spark plugs and NGK ignition wires, install them. Either have the codes reset by a tech with a scanner or you can disconnect the negative battery terminal, depress the brake pedal for 10 seconds them reconnect the battery to reset the ECU. If the MIL comes back on after 2 drive cycles and the second read shows the same code P0420, you may have a converter issue. Some 99-2000 models have had converter issues, though not a rash of problems there. Considering your miles it could be feasible.
Miatas are reliable, so stick with the basic maintenance items first. Off brand pugs and wires cause issues, old parts or even someone cleaning the engine and allowing water to get to the spark plugs recesses can be the cause of an ignition breakdown. Normal symptoms are low speed misfires that clear or appear to clear as you accelerate.
As for driving with the MIL on, as long as you have good oil level and pressure and the coolant level is correct and the temperature gauge reads normal then it is okay to drive with the MIL on. The ECU has built in redundancies and can operate on default settings and sensors in most cases allowing you to continue on your way.
service@miatashop.com
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QUESTION: Thank you for the very quick response. I noticed in your reply you mention error code P0420, but I am getting P0421. Does this make a difference? Also, the MIL isn't just on -- it's flashing. Does that make a difference in your experience? By all means, I am no expert, and know little about cars other than the basics. From what I've read online (I know, I know), it seems that I shouldn't be driving the car with the MIL flashing? Sorry to bother you again.
AnswerI don't see P0421 being on the '99 models flow charts. In either case an OBD2 read of P0421 or P0420 refers to catalyst efficiency under either code and means the primary #1 O2 sensor and the secondary #2 O2 sensor are at odds with the emissions reading from before and after the converter. In this case with a random misfire detection the most common real world cause is ignition failure of some level which will cause raw fuel to be passed into the exhaust stream and lead your O2 sensors astray. So before we wax intellectual on the OBD2 system or you end up cutting off the head to save the tail, try the NGK ignition parts. Good routine preventative maintenance even if by some chance you end up with a more sinister problem, the random misfire needs to be addressed first, the O2 sensors will not read properly otherwise. Miatas are simple and MOST (though not all) issues default to the routine.
service@miatashop.com
"if your good at what you do you should be well paid"
As for the flashing MIL, this almost always refers to a misfiring cylinder. P0300 is a default detection of a misfire, P0301, 302, 303 & 304 are specific to a particular cylinder respectively. P0300 simply means the ECU is unable to determine which cylinder is failing at this time.
