Dodge Caravan CNG Failed for High NOx

No timing control and no EGR... these two things are major high NOx culprits. Since your Dodge Caravan has neither, you'll need to look into proper fuel control, any and all vacuum leaks and possible high compression.

The simplest way to start is checking for vacuum leaks. First look for disconnected, broken and brittle vacuum line. Your Caravan has several vacuum line running underneath the intake manifold. You might need to use a shop mirror to inspect all the hoses. At a smog check repair station the way this is conducted is by first using visuals and then using propane. Propane is present via a small vacuum tube around the intake manifold and an observation of RPM increase is performed. Obviously take caution when performing this task. We don;t recommend anyone do this that has not had proper training in smog check repair and diagnosis.

Next would be checking for compression. Older engines have much carbon collected on the cylinder head. This causes high compression, high heat, and as a result high NOx. NO x is created when engine cylinder temps rise above 2500f. Check compress and ensure they are within specs. If too high, you may need to do an engine decarbonization. Look into a product called Sea-Foam.

After that the process gets complicated and requires some serious diagnosis knowledge and tools. As far as tools, mainly you'll need a multi-meter. The trick however is to know how to use the meter and where and what voltages to check for. You'll need to ensure your Caravan's oxygen sensor is working correctly, the MAP sensor is registering vacuum/pressure properly, and that the TPS (throttle position sensor) is sending the correct voltage to the engine control computer.

These are the three main things to start looking into.