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P0170 code on 1996 Mazda Protege

I am wondering what I should do with a check engine light, scanguardII code Po170 reading on 1996 Mazda, over 100,000 miles on car. The same reading oc cured in 2004 and my mechanic replaced a fuel filter to “correct”

The check engine light does not go off and on, but stays on contantly.

What should I do?

Is it crucial?

How much will it cost?

Car seems to be running fine.


DTC (Diagnostic Trouble Code) P0170 is for Fuel Trim on bank 1 (the side of the engine with #1 spark plug). Your mechanic needs to use the scan tool to see what the Fuel Trim percentages are, and use that information to decide what to do next.
The fuel trim means that the PCM (engine computer) has learned to add a little bit more fuel, or a little bit less fuel to keep the engine running smoothest. The percentages are percentages of richness (more fuel), or of leanness (less fuel), the PCM feels is the needed amount of fuel adjustment.

You might try cleaning the the MAF sensor to see if that helps. Be careful with the sensor if you do that. You might also check the coolant sensor for the engine to see if that is doing its job. There is a red/blue wire tied to pin 3G of the ECU and it ties to the sensor. By checking the voltage on the pin while the engine is warming up you should see a change in DC voltage. If not, the sensor is bad. I’m not sure if the voltage will go up or down but I suspect it will drop as the engine warms up. The main thing is you need to see a change in voltage as the coolant temp changes.

Thanks to both for responding!