I'm getting a code 21 on my 94 XJ.

I also have a cracked manifold and/or (leaking manifold gasket) and get the code 21 with both cold and warm engine. Question is if the computer somehow stores the info from driving and therefor give me the 21 or if I have both a cracked manifold and a faulty o2 sensor?

Haven't tested it yet but is has about 130K on it (if from stock). It did give me the same code a while back when I trouble with a CPS (camshaft pos. sens.) but then went away until recently.

 

About the testing, (not sure if this is how it works and how it is written in the US?)
I was in for a yearly vehicle inspection today and the values looked ok (in the eyes of the inspector); got 0 on all CO2 and regular 1 on the lambda (which I think is the same as o2 sensor?). All numbers makes sense with an exhaust leak somewhere.

Lambda = 1,01
1,00 CO = Low
HC = Low
CO2 = Low

 

You should resolve that manifold issue FIRST. It could very likely be the root cause of your code.

 

Yep, I already ordered a new header – but was considering getting a o2-sensor in the same shipment (due to intl. postage pricing, they go for around 160USD here). Do you know if the ECM stores info from the O2 sensor (no CEL when driving)?

 

Yes, the computer stores codes. If you have had a check engine light, you should have a "stored" code for retrieval with a code reader or a scan tool.

 

Ok, thank you