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45 - Atmospheric pressure sensor - Faulty sensor
49 - Air flow sensor - Use of improper air flow sensor

I'm getting these two codes on my EJ20G ECU. JECS MAF. I changed it to my EJ22T ECU with the hitachi EJ22T MAF and same thing. Recently when my car wouldn't start (turned out to be a loose fuel pump connector) and I cleared the codes, only the code 49 showed up and now that it's running, code 45 is present as well.

Where should I begin? I'll be going through the FSM and testing whatever it says to test and I'll post the results. The car runs fine, seemingly. I don't know what kind of difference, if any, these codes make in the operation of the ECU.

How does the ECU know the MAF is the wrong one? I think there must be a short somewhere, possibly one the wire that tells the ECU what MAF is being used. It's running with that MAF which must mean it's functioning. Right?

Kimo,

Yes there is a wire that goes to the MAF sensor that tells the ECU whether it's the right MAF or not. Check pins 3 & 5 of the MAF sensor. Here's a diagram and testing instructions.

http://www.main.experiencetherave.com/s ... sting1.jpg
http://www.main.experiencetherave.com/s ... sting2.jpg

I'd start with that and if you find out the problem, reset the ECU and see if 45 comes back as well.

In addition to checking voltage, check continuity of the wiring from the MAF sensor plug to the ECU and ground point.

Thanks Josh. Ill run through the diagnostics when I get the car back home from the shop.

Mike says sometimes codes can stay in the ecu even with the ecu unplugged. I have not tried the official code clearing method.

Yes, sometimes codes will linger even after the power has been removed. From what I have seen others experience this seems to be more related to the canister purge solenoid code, but can happen with others. The other hypothesis I have is that if the CA/Federal pin is pinned for California mode that the codes don't clear by disconnecting the battery, however I have not confirmed this.

Try the code clearing procedure on my site and see if that works. If not, try the diagnosis for the MAF sensor.

The car is still nearly an hour from my moms house where it needs to go. Its at my buddy's shop. Hopefully Wednesday ill go get it and bring it back for some serious work. Ill update after running through the diagnosis.