So I have a 94 prelude Si H23A1 all stock. This code was stored in my ECU, it is not setting a check engine light but I get 4 long flashes and 3 short flashes when I jump that blue connector under the dash. Fuel filter was changed maybe 6 months ago. I was driving around without a fuel cap for a while, maybe that set it off? I have no idea where the O2 sensor is on this car, must be under the car somewhere, it's not in the header. No major performance issues, I sometimes notice a slight stutter when under heavy acceleration between 2500rpm and maybe 4000 or so, I think this is ignition related however its due for a cap and roter and plugs and wires. If its possible for no fuel cap to set this code does anyone know how to clear the codes so I can see if it comes back?

 

that code means the fuel trims have reached the limit, of either too far positive or too far negative. Most times when that code sets it means either the filter is clogged or the fuel pump is dying. You changed the filter already. The proper way to go about it would be to do a fuel pressure test with engine off, and with engine idling with the vacuum line on the regulator unplugged and plugged in, then go driving and watch what pressure does at various rpms and throttle positions. If there's a drop in pressure as rpms climb, bad fuel pump. Another possibility is the fuel pressure regulator is failing, which would be indicated by the fuel pressure at idle being too low.

 

A failing or lazy O2 sensor can send faulty correction instructions to the ECU and thus, the short term and long term fuel trims max out as motoxxxman suggested. First, inspect the exhaust manifold and exhaust piping for any exhaust leak between the engine and the O2 sensor... as well as about 18 inches behind the O2 sensor. If a leak is discovered, repair it and see if the code goes away... and the performance of the engine returns. If it does not... consider a new O2 sensor.

It could be caused by the fuel pump being weak, but before you pull the tank, this theory can be tested. Either disconnect the battery or remove the ECU and back up fuses from the under hood fuse box, wait a few minutes, then re-install them. Drive the car from cold start to well warm... say ten to fifteen minutes. If the performance of the engine degrades as it warms up, it can be related to the fuel pump because there has not been enough time for the long and short term fuel trims to significantly affect engine performance. If however, the car performs well for say the first 20 miles after resetting the ECU... it is unlikely that the problem is the fuel pump, and more likely an O2 problem like I have outlined above.

 

Thanks for the replies. Well i cleared the codes, drove about 100 miles and code 43 is yet to come back, maybe it was stored from before I did the fuel filter? I tried to notice a performance difference after resetting the ECU but I didn't notice much, if there was a degradation in performance after the car fully warmed up it was too subtle to notice. I don't have a fuel pressure gauge or any fittings but if the code comes back I may buy a kit, it would be nice to have anyway. I do have some idle issues, when turning the car back on after it was already warm I sometimes get a quite low about 500rpm idle until a rev up the car a bit or start driving, then it will idle normally around 900rpm. I believe my fuel pump has an access panel inside the car between the trunk and back seat, think I would still need to drop the tank if it were a bad fuel pump? By the way the car has 220,000 miles and as far as I know the fuel pump or anything related to the fuel system other than the fuel filter has never been changed.

 

yes your car should have an access panel to the fuel pump. driver's side right behind the back seat in the trunk iirc.