P0171 and P0170 1999 Honda Accord

Hey guys,

I recently bought a 1999 Honda Accord Ex 4 cylinder 2.3L. The car had a check engine light when I checked it p0141. I replaced the downstream o2 sensor and deleted the code. Everything was fine until the CEL came back during my road trip. I stopped at a gas station and checked for codes. This time it was p0170 and p0171.

After doing some research I checked out the freeze frame data and this showed up. I checked out the live data (warmed up the car) and my short fuel trim was around 8% during idle and when I revved up the engine and held it at around 1500 the STFT went up to 15%. I guess this problem only occurs when I’m driving not during idle. What can cause the SHFT to rise when I’m driving?

Currently at work but I’ll post pictures of the live data when I get off. This car doesn’t use a MAF sensor, it has a MAP sensor. How can I diagnose the issue

Thank you. Hey

your engine is running very lean, its starving for fuel... this could be as minor as a old or partly restricted fuel filter to a fuel pump.

you will have to do a pressure test of the fuel system to see what your fuel psi readings are at idle and WOT. Idle pressure should be around 33-37 WOT should be 10psi more than idle psi... while engine is at idle, you can pull the vacuum hose from the pressure regulator, the psi should rise 10psi the same as WOT. Note, you should not have any fuel present in the vacuum line, if you do, the pressure reg is bad which can cause the lack of fuel. If the fuel pressure does not rise, fuel pump is suspect.

Thank you. I will try this tonight

Did you notice any loss of power during your road trip? :huh: If not, I'd be immediately suspicious of the upstream O2 sensor. The sensor itself will get stuck lean and cause the trims to peg out positive (which is exactly what you saw in the freeze frame).

Did your freeze frame data have the O2 sensor voltage PIDs? Because those will be the key to catching this problem. With the upstream O2 falsely stuck lean and the fuel trims maxed positive, the downstream O2 sensor will be reading around 900mV. That's because the engine is now actually running extremely rich, instead of lean. Make sense? :silly:

If your freeze frame data didn't include those PIDs, you'll have to catch it in the act. Go for a test drive with the trims and O2 voltages available. When the trims go way positive, have a look at the O2's. Here's a shot I took from another vehicle with the same problem. The upstream O2 would only fail at highway speeds.



If you DID experience a loss of power during your road trip, follow a fuel delivery problem like scannerjohn described.

I wasn’t able to work on the vehicle yesterday due to getting off late but I’ll try today or this weekend. Thanks for the input. The freeze frame data didn’t have the o2 sensor data but I’ll post the pics when I can.