Code 11, 12, 21, 56

I am having a bunch of timing issues when my jeep warms up.. these are the codes that it is showing... any advice?

Need year/engine and a more detailed description of the symptoms......

What do you mean by "timing issues"?

96' XJ 4.0L AT It runs fine when it is cold but after driving on the highway for 5 mins, the power cuts out and starts backfiring

It will run at full throttle though when this symptom starts

I had the same problems last year. It turned out to be the the O2 sensor. The O2 sensor is not read by the ECM when the engine is cold or when a WOT is detected.

techno1154 said:

I had the same problems last year. It turned out to be the the O2 sensor. The O2 sensor is not read by the ECM when the engine is cold

Click to expand...

11-camshaft signal or ignition signal
12-Memory to controller has been cleared (ignore as this is not a problem)
21-Oxygen sensor signal
56-no idea; not in my manual. Are you sure it's not a 55 which would signal the "end" of stored codes. Ignore if a 55

I agree with Techno. When engine is in open loop, computer uses default values (enrichened) for fuel/air mixture. When oxygen sensors reach a certain temperature and you reach closed loop, mixture is determined by the oxygen sensor input signal.

So, if your vehicle is not symptomatic when cold and symptoms start only when warm, gotta concentrate on things in play during closed loop. The #1 suspect there is the oxygen sensor itself. Upstream sensor is the one in play here. The downstream sensor is primarily to measure efficiency of the catalytic converter.