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2002 Mazda 626 2L 4 speed Winter Performance Issues.

#895612
Nightflyr *
Richard Kirshy
Participant

P0172
Combustion engines run most efficiently when they maintain an air-fuel mixture ratio of 14.7 parts air to 1 part fuel. When the upstream oxygen sensor detects there are less than 14.7 parts air to 1 part fuel in the air-fuel mixture, a rich condition exists. To keep the engine running properly, the powertrain control module (PCM) tries to compensate for the rich condition by injecting less fuel to the mixture in an effort to maintain the proper 14.7:1 air-fuel ratio. When these adjustments become too large, code P0172 is triggered.

Going by your description your miss-fire may well be dumping fuel causing the O2 to see a rich mixture.
You state you have low compression on one cylinder.
I would first do a compression test on all cylinders and see if your miss firing cylinder has a issue with compression.
Then suggest you hook up a scan tool and see what your fuel trims are doing.
Also might do a injector flow rate test to see if all the injectors are functioning equally.

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