Ignition misfire and Lean diagnostic codes?

[shorod]

On an OBD-II car has anyone experienced or heard of codes for lean cylinder banks (P0171, P0174) being set or pending due to an ignition misfire (bad CoP, cracked spark plug, etc.) assuming everything else is functioning properly (no faulty MAP or MAF sensor, no plugged air filter, fuel pressure is good and injectors are working)? Assuming it's possible or even likely, is there a pretty reliable way to determine if the codes are truly lean conditions or phantom (monitor fuel trim numbers, etc). To make things easier, we'll assume the V6 or V8 car will start, run, and drive, albeit roughly due to a misfire.

-Rod

[curtis73]

I've seen it plenty of times. O2 sensors read free oxygen. If you have a cylinder not firing, the oxygen isnt getting used and it shows up as lean.

[shorod]

Thanks for the data point. I've not seen and am struggling to distrust the OBD-II diagnostics. Another member has seen this but I needed some confirmation before adding this to my memory bank for future reference.

-Rod

[curtis73]

OBD2 codes can be frustrating, but its almost always a bonus when you get two codes. For instance if you had "vehicle speed sensor" and "transmission component slipping" but didn't notice anything wrong with the tranny, it would point to a faulty VSS. But if you just had "transmission component slipping" by itself you might worry that its time for a rebuild.

Its also different for every vehicle. For instance, Ford increased the threshold limit on coil failure before CEL because it was causing constant check engine lights. Its very possible to get a lean code for a misfire long before you get a misfire code. Chevy did the same for MAF codes in the mid 90s. In fact, my 96 Impala had complete failure of the MAF without ever setting a code.

That same Impala broke a rotor in the distributor and would barely run. I limped it home for three miles and never got a code or a CEL.

But God forbid I don't properly tighten the gas cap on my 05 Scion and it lets me know in a few miles.

[shorod]

Since you mention the dual code concept, is there a rule of thumb with OBD-II codes like I recall there being for pre-OBD-II? My recall is that with the old system you would start with the highest code number and troubleshoot towards the lower code numbers. Since the P1xxx codes are manufacturer-specific, a person would likely want to start with those and they will tend to be less generic, but if you have a P1356 and a P1104 (I just randomly typed numbers, don't try to look them up) would you want to start troubleshooting the circuit related to the P1356?

-Rod

[maxwedge]

OBD11 does not work that way, in fact if an engine misfire code appears, even though a lean exhaust mixture would occur the pcm is programmed not set the o2 codes as it knows the misfire is the culprit. On the other hand the pcm in some cases has to see a certain condition for preset ign cycles before the code is set, sometimes a misfire will not set immediately.

[curtis73]

OBD2 is sorta "call it like you see it." Its far more complicated, but not necessarily more intuitive. The first fault it sees is the first code it displays, but depending on the manufacturer (and their own protocols for when the fault is recognized) its a bit of a learning curve.