My xj 300 straight 6 has failed on exhaust emissions any suggestions on what to check

 

Oxygen Sensors, MAF, Coolant Temp Sensor... in that order

Does the engine run well

 

yes starts and runs fine, did think it was a tad heavy on petrol but never got round to doing fuel returns
the oxygen sensor is on the exhaust manifold, and the MAF on the inlet manifold, my apologies for asking but this is the first time I have tackled an engine with these sort of controls
tom

 

The oxygen sensors are further down from the exhaust manifold. It's on the actual exhaust pipe. If they are original to the car, then it's well past due to replace them. They owe you nothing more. I believe you should have two to replace.

MAF is well upstream of the intake manifold. Let's swap out the 02 sensors and see what happens.

 

If it has OBD II, use it to look at things such as fuel trims, OL/CL (open/closed loop) and plausibility of sensor readings.

 

JagV8 is right. If you aren't going closed loop, then that would cause the failure. The prognosis will likely be the same....oxygen sensors should only last about 50,000 miles, but people seem so reluctant to change them.

The Coolant Temp Sensor, a cheap $20 part will also prevent the car from going open loop.

After that it gets complicated with the MAF and the TPS.

 

I hesitate to suggest a simpler solution - have you taken it for a good old blast to get the engine and exhaust really hot, not warmed up but working hard for ten minutes. So stuff it into third and get up to 4,000 rpm say. You may find this tips the balance in your favour.

 

I think first you have to find out if the engine is getting into open loop. You'll need an OBD2 reader for that. I would recommend and ELM27 wireless plug (WiFi only for iPhone, WiFi or Bluetooth for Android)