I donāt usually say anything critical, but in this instance, quite frankly - youāve bought a pig in a poke. Did you do any research before looking for a Mk3 2 litre car? If so, you would have learned that the 2 litre engine is thirsty on oil, and oil levels are critical. If not carefully monitored seized engines are not uncommon - thatās for starters. No-one in their right mind would contemplate buying a ānon-runnerā, without some closer examination first - you have no idea what you are getting into unless you do some basic checks.
Many car buyers today have no idea of what car maintenance really means, presumably you just bought one that illustrates that point. - Iāll give you one personal example, My son. Now in his forties, but this is only his second car, and maintenance is a word he doesnāt recognise. He bought a 2 litre Mazda CX5 last year (same engine as the Mk3, except this is a better version.) I warned him just after he bought it to keep a close eye on the oil level, and he just shrugged. Mentioned it at least twice since, and he muttered āThe car is sill under warrantyā. - That means nothing, if you donāt look after it. Same with tyre pressure, coolant levels etc. - they all need checking periodically. A āGood priceā can often be misleading - it often means āI really want to get rid of this wreckā
However, having got this off my chest, letās see how we can help you. It appears as far as electrics are concerned, you apparently are a beginner, pardon me if Iām wrong - no desire to insult you, as Eddie said above, we have no idea of what you can and cannot do.
Your first post asked if anyone could help you as the OBD port wasnāt working. Since then, youāve now acquired an OBD reader - does that now work? We need to know if the reader works - or not. Next step is to buy a multi-meter, you canāt check all the electrics without one. No need to buy an expensive one, Both Halfords and Amazon sell reasonable meters for around Ā£15 to Ā£20. You need one with a 12/24 volt (DC) scale, plus the ability to read Ohms (resistance) and preferably with a buzzer that you can use for fuse and wire checks, - for continuity. All Mazdaās are positive battery fed, with the battery negative lead connected via straps and some wiring to Earth - (The body of the car), also used as a main earth for local wiring. Any multi-meters will also read other voltages, either lower and higher DC volts, current flow - to a maximum or usually 10/12 Amps and other readings. Ignore the others for now, read the instructions that come with the meter, and you will understand (hopefully) how to use it. In that price range, you can either have an analogue (meter with scale and a moving pointer) or a digital - often recommended these days. Unless the meter works well, thatās the useful point. ā All meters require batteries, the secret is to avoid 9 volt ones (more expensive).
Next - go here: http://www.mellens.net/mazda/ This is an American website where you can download both mechanical and electrical diagrams for most MX5 cars, just pick the year closest to your model. Now, Mellens electrical diagrams are good, if you are a mechanic, not a lot if use if you are a dedicated auto electrician, as they appear to be colour coded, but these colours do not apply to any Mazda cars. Basically, itās a guide for mechanics, with some electrical knowledge. So Red indicates a āliveā (positive wire) and Black a negative. Other colours are for guidance only - they donāt relate to actual wiring colour is any car.
I would strongly suggest you print the electric diagrams, But avoid the blank pages in between, one for every page showing a circuit. A laptop does have itās advantages, but working on the car is not the ideal place for a laptop, printing is easier, better and safer.
At this point, we go back to school (if your school ever taught basic electricity) Take any battery, and with an equivalent voltage bulb and a switch, you connect the positive side from the battery to an open contact (pole) of the switch, connect another wire to the other switch contact which then goes to the lamp, and from the other side of the lamp you connect back to the negative side of the battery. Voila - throw the switch (not literally) and you have a working electrical circuit. Most cars built today follow this basic principal, except that where the actual load (multiple circuits or lights as an example) need more current draw than the actual lamp does at the end of the wire, so they use a 'relay ā instead. Relays are simple devices which are used to pass heavier loads than a normal switch does, even though a switch after the relay is not uncommon. Best part of a relay is itās easy to replace, though not always cheap as chips.
A relay is in effect, an electromechanical device that uses a coil as a magnet, which closes a contact to which live (positive) is connected and then allows the distribution of the live to other circuits. Youāll find a lot of these in any car. The coil is often energised by a switch on the dash, but can also be fed from other relays or devices in the car - like the ECU for instance. This is why modern cars are so electrically complicated.
O/K so far? Right, lets investigate your problem, best we can anyway. From what I said at the beginning, first step is check the oil level in the sump. I understand most Mk3 owners have trouble reading the original oil level stick, but AFAIK it will show some oil level - it should be full. If it isnāt then we start worrying. Next step is electrics - check ALL the fuses for any having blown, replace any that are so affected. This may not fix anything, but better safe than sorry. You can either do a visual check, but better to use either the buzzer or the Ohms scale to check they are O/K, visual checks can be misleading. Reading on the Ohms scale should be zero. (low resistance) - Buzzer is easier. One lead from meter to one fuse blade, the other to the other blade. (make sure you plugged into the meter Ohms socket!) When checking for voltage, hopefully you bought a meter with two separate leads, one with plugs for the meter, and the other end is with a needle point probe. The other set will have meter plugs and crocodile clips for the detecting end. If you only have the latter, use the red (positive lead) for voltage and clip it to a thin bladed screwdriver blade, so you have a circuit. Take the black croc clip and make sure it clamps onto something directly connected to the car body (Negative side) - like a screw, small bolt, or anything that fits into the car body. - Donāt forget to check the meter selection and the leads are plugged into a DC voltage socket!
According to Hoyle (who only ever wrote rules for card players, so is now a figure of speech) The live (positive) side of any fuse board should be the top terminal,but car makers make their own rules, so check with the red probe or the thin screwdriver that all top sockets in the fuse board are actually āliveā, if they arenāt check the other side. Once determined, remember it (if you can) it may not help immediately, but may do in the immediate or future sense. - Depends on how far you get. Logic says, if there is positive at one socket, then if the fuse is good, there will be positive at the other end, leading out from the fuse board.
Now - Engine start problem. Known variety of causes, each a question of trial and error. However, at least you can check all the wiring is good, and any relays also are in working order. Best way IMHO - is disconnect the battery negative lead, and using th buzzer or the Ohms scale, check each wire for a good reading (Buzz or low resistance) Relay coils generally are high resistance, as they pass little current for the coil, but check each coil to see if you can read resistance or not. Might be a short or open circuit. If in doubt - replace but check you are reading across the coil, and not one of the contacts,. Coil pins are generally easy to identify, they are two separate pins at one end if the relay base. If you are checking the main starter leads, be careful, make sure you donāt accidently connect the positive lead (heavy duty) to earth (car body) youāll get an almighty flash and a bang. - But not if the battery negative has been disconnected first. Bear that in mind while checking voltage anywhere! Also - remember that if you disconnect the battery negative, you cannot check voltage anywhere as thereās no negative circuit.
Rule of thumb, dis (disconnect) battery negative if conducting resistance or continuity checks, but connect if you wish to read voltage - at which point, be careful. Continuity checks on one thin wire to other end, even covering the whole car, should be low-ohms for resistance, and sound the buzzer if good, depending how you select the meter reading. Major task Iām afraid, but my immediate suspicions in order would be 1) seized engine 2) starter motor failure 3) fuse failure or 4) Other major problems.
If you canāt find the problem, try and get hold of a good mechanic or auto electrician, experienced on Mazda cars for sure. The car may even have to be trailered away. (Not to the scrapyard I hope)
PM me for further explanation of any of the above.
Good luck.