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Oxygen Sensor Codes


February 25, 2018
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Year, Model & Trim Level
1999, Sport 2WD, SOHC

I just replaced my starter last week and the check engine light came on this week. I've got three codes coming up:
P0171 -System too Lean (Bank 1)
P0174 -System too Lean (Bank 2).

Where should I start for the leans?

1. Vacuum leaks
2. Restricted fuel filter or bent/pinched fuel system lines
3. Incorrect input from other sensors, such as the Mass Air Flow Sensor, which may not always drop a separate code
4. Engine misfire
5. Bad O2 sensor

Thank you.

C1234 is easier, try to clean off the right front wheel speed sensor. If the problem remains, examine the cable for breaks, the cable connector for corrosion, but might need a new sensor.

With both banks being too lean, the first easy thing I'd try is cleaning the MAF sensor.

Next I'd check the fuel pressure. If low, could be a clogged filter, or fuel pump, or fuel regulator. IIRC, your '99 is the first year the regulator switched to the in-tank type instead of on the fuel rail. At almost 20 years old, unless you're trying to do the cheapest repair possible, "IF" it is the fuel pump or regulator then I would replace the entire sending unit w/regulator and the pump while the tank is dropped.

I doubt the fuel lines are bent or pinched, unless you have some reason to suspect it like rock crawling, and even then they're up in the frame rail most of the way, IIRC. I would sooner suspect a pinhole rust leak than that, but surely you would smell gasoline leakage with it parked and idling.

I doubt an O2 sensor. Doesn't a '99 SOHC have both left and right bank sensors? In that case it would seem unlikely both would fail simultaneously, and the O2 sensors probably still work to some extent or you'd get an O2 sensor code. Misfires you should get a pending code for cylinders if not set code for them yet.

Vac leak, sure the intake manifold gasket is a common leak, or any other hoses/etc. If your fuel pressure is good, MAF sensor is clean, then I'd start hunting for vac leaks. It might help to have an OBDII scan tool capable of showing live data like long term fuel trims, then as you pinch or pull and plug vac lines you can see if it makes a difference in fuel trim dropping down closer to 0.

Does it seem to run worse in any particular situation, RPM or cold vs hot or ?? Vac leak tends to run worse at idle.

Thank you so much for the reply. You've given me much information to digest. In terms of "Does it seem to run worse in any particular situation, RPM or cold vs hot or ?? Vac leak tends to run worse at idle." It's not running poorly, it's just those codes that won't go away.

^ Then I would expect the leak is minor enough that the ECM can still compensate for it, should show high fuel trims from scanner live data. That's more often a vac leak but a little low fuel pressure or MAF reading off some could still cause it.