1999 Mazda Millenia Check Engine light

it is the first oxegen sensor after the cat converter yes easy to change unplug wire unscrew from threads and screw new one in

sounds like the water pump bearing is worn out and has detroyed the water pump shaft seal.
coolant is leaking thru the seal area ,due to the bearing worn out --hence throwing the water pump pulley out of alignment.
The water pump pulley runs of the timing belt.---hence the knocking --either from the pulley or belt chaffing against the timing cover or even throwing the engine timing a few degrees off.
Need to replace water pump and timeing belt and anything else that may be damaged.
That's why, when ever you do a timing belt always do the water pump too.

sounds like its a different sensor causing problem. could be TPS SENSOR OR MAP SENSOR .

you should check timinh tensioner. thats probably the problem...

go to autozone & check the code for free mazda millenia they all have problem with the check engine light problem even though you bought all new parts it will still show , but never replace the cat converter most cause the engine light is the sensor only.

TRY DISCONNECTING THE BATTERY CABLE SO THE PCM (COMPUTER) RESETS TO ORIGINAL DEFAULT VALUES. HOPE THIS WORKS FOR YOU. GOOD LUCK

ususlly its something small it also could be that the car has been sitting a while in the car yard you could try disconnecting the battary for 30mns and then reconnecting this some times unscrsmbles the ecu causing the problem good luck

Check engine light usually indicates an oxygen sensor malfunction. The switch is usually installed in the side of the exhaust pipe near the engine. In the winter the wire that is connected to it often breaks off. It is usually a simple fix for a mechanic.

yes the sensor and it will not go away on Mazda Millenia even though you replace that code it will show another one Mazda Millenia Sucks & Mazda Parts are Expensive too

I've had the same problem with my 1999 Mazda Protege, 1.6L, Automatic.

The problem was a damaged spark plug, resulting in a loss of compression in one of the cylinders.

Solution: replace all spark plugs and make sure wires are in good condition. I recommend NGK Platinum.