'97 accord is throwing two codes.
P0170 Fuel trim Maufunction (Bank1)
P0420 Catalytic converter below threshold

I presume that this means I need a new cat to address the second code, but what could be causing the first code? it has 220,000 and runs great. Stock 02 sensor, could that possibly be the problem? When I clear it it usually comes back after a few days. Thanks

Jim

 

Scheduled Maintenance would dictate a new OEM, NGK or Denso O2 Sensor. I would have O2S activity looked at before tackling the Catalytic Convertor, a lean mixture will not allow the Cat to operate properly, fix these one at a time but I recommend a shop with a portable scope to go on a drive with you to look closely at the O2S first and take car of that issue first.

 

X2

You might want to check for vacuum leaks, they would also create a lean condition.
If not 220,000 miles the catalytic converter could be going bad.
Has it ever been replaced?

 

Generic Scan Tool
I'd be more interested in what the REAL code is, the code the generic scan tool is calling P0170.

 

This should get you started:

The P0171 is caused by a lean condition that the ECM can no longer adjust for. The condition is caused by the deterioration of a fuel metering component or system that has occured over time. Over time the PCM has been making adjustments to fuel trim to compensate for the deterioration of the fuel metering system until now when the need to adjust (compensate) has exceeded the ECM's ability to do so.

the cause could be a deteriorated O2 sensor but that will usually cause a code P0136 for the O2 sensor range or response fault. More likely it is a mechanical problem such as low fuel pressure, a restricted fuel filter or a worn fuel pump.

Misadjsuted valves can cause this condition as well. On the 1997 the valve adjustment is critical and at 190k you shoutld start with the valve adjustment if you can't remember when you had it done last.

Once the valves are adjusted use the car to see if the code sets again. If it does, then have a fuel pressure test done; if the pressure is low check the filter and lines before condemning the pump.

You need a digital oscilloscope or scan tool to properly test the O2 sensor. I think that's a possibility but its much further down the list than you would think.

here's the possible causes in most likely order

Valve Clearance needs to be adjusted
Fuel Pump insufficient flow/pressure
Fuel Filter clogged
Fuel Injector clogged,
Fuel Feed Line clogged, leaking or bent
Fuel Pressure Regulator stuck open
Primary HO2S (Sensor 1) deteriorated
EGR System malfunction (too much flow) (egr hung open)
Exhaust leak before the O2 sensor (exhaust manifold cracked or leaking manifold gasket)

P

 

Thanks for the advice, guys.
The cat nor the 02 sensor has ever been replaced, so a failure of either/both is possible. I will be sure to get the 02 sensor checked.
Valves haven't been adjusted ever.
I replaced the fuel filter along with EGR cleaning and full tune up @ 210,000 so I doubt that is clogged, but as for injector, line, or FPR problems, I'll look into them. And I hope that its not the fuel pump. I can't detect any vacuum or exhaust leaks, but once it stops raining, I'll check better. Gas mileage is still he same (28mpg) and the car performs fine and doesn't have any lack of power. I'll do some diagnostics and get back to you. Thanks again, guys.

Jim

 

Can you get a free loaner fuel pressure gauge set from Autozone, do a fuel pressure test. Can you check the voltage back to the computer from the MAP sensor, it must be very close to 1 VOLT DC, you guys help out with a pin number please. That voltage will let us know if you need to do a valve adjustment, check it with a fully warmed up engine with a steady idle, no accessories on, no fan either.