P3355 — Mixture adaptation region 3, CNG/LPG mode, rich limit exceeded

P3355 is stored when the ECU sees below threshold on the circuit named in the code. That describes how the circuit is behaving, not which part has failed — the wiring, the connector and the component itself can all produce it, which is why the checks below start with the cheapest.

Common causes

The cause is almost always one of a few things. Here they are — P3355 rarely comes from anything outside this list:

• The monitored part has degraded rather than failed
• An upstream fault is loading the part beyond its design
• Sensor used to judge the threshold is itself inaccurate
• Normal wear reaching the end of its service life

Where to start

In the order a sensible mechanic would work — the cheap checks first, the expensive replacements only after:

1. Fix any other stored codes first — this class is very often a consequence, not a cause
2. Verify the sensors that judge the threshold before replacing the expensive part
3. Check for leaks or restrictions that would make a healthy part read as failed
4. Compare readings before and after the component
5. Replace the component once it is proven to be the one that has degraded

That is what holds for P3355 in general. From here it depends on your engine — tell me which car it is and what it does, and I will give you the steps for it: real procedures, torque figures and diagrams instead of general advice.