Hey guys. My 97 Formula with 105k has just thrown me two Check Engine Lights I'm a bit lost on and can use some help with.
I've got codes: P0100 Mass Airflow Insufficient Signal Activity
Also a P0420 Catalyst System Efficiency Below Threshold, Left Bank.

Now the car runs just fine with no issues or loss in gas mileage so I'm looking for some advice on how to diagnose the exact problem and go about fixing these issues. Any help or advice I'd appreciate. Thanks.

 

P0100 indicates the signal from the MAF sensor is static. This can be caused by a faulty sensor, or loss of 12V power to the sensor. The 97's seem to have a problem with the pink wire that supplies 12V. First, (key on) check the voltage at the MAF harness connector between the pink wire and the black/white wire. If you don't have 12V, its likely you have a break in the pink wire. I don't know is anyone has determined exactly where the problem typically occurs on the 97's, and it usually takes running a new wire to the MAF.

When the MAF signal fails, the PCM defaults to speed-density mode, and you won't notice the difference, unless your engine is highly modified.

P0420 sets when the after-cat O2 sensor does not respond correctly, based on comparing it to the pre-cat sensor. The intent is to detect a faulty cat. But it may be an O2 sensor problem, or a wiring problem. Swap the two after-cat O2 sensors side to side. If the problem follows the sensor (P0430) its a faulty O2 sensor. If the problem stays on the left bank, its a dead cat or faulty sensor wiring.

Engine will run fine with this code. It just telling you the cat has a problem.

 

Thanks. I checked the MAF voltage with key-on and was reading 11.23V. I "jiggled" the harness and connector to try to get a short but it was steady at 11.2V, fuses also looked good. So not sure if I didn't catch the short or if thats not really the issue.

Is there any other things to look at in regards to the MAF or P0420 code? I can try swapping the O2 sensors around, how hard is that to change them and remove them? Any tips on doing that job.

 

The O2 sensors can be difficult to change. Best way to do it is with an "O2 sensor socket", that has a slot to fit over the wires. Also easiest to do it with the exhaust hot. Avoid spraying rust penetrant on the sensors. You can contaminate the outside of the sensor thimble.

As far as the MAF, if you have 12V, it may be the sensor, or it may be an intermittent break somewhere in the harness that is hard to find. Did you verify the harness pins in the connector and MAF are not damaged?

Clear the codes and see how fast them come back. Best way to clear the codes is with key off, pull the "PCM BAT" fuse for 30 seconds. I think in 97 the PCM BAT fuse is in the box under the hood. If the MAF code comes back right away, and you have verified ~12V on the pink wire, it may be the sensor. Two things to check it - 1) swap in a known good sensor; 2) use a frequency meter to measure the signal output of the MAF (yellow wire). Should be in the 2,500Hz range at idle, and should increase significantly if you rev the engine. If you have a scanner, you can read the MAF directly from the PCM. Should see 6-10 GPS at idle, and the reading should increase as you open the throttle and rev it.

If the scanner shows the MAF reading as 471 GPS all the time, you have an open circuit on the harness wire, or the MAF is likely deceased.