Instant cross counts upon startup

Mark E.

I am working on a car getting ready for a smog check. I have helped with this car for years.

I noticed upon startup the 02 cross counts start immediately. 0-255 in about 5 seconds over and over.

When it warms up and the 02 lights off the CC's go to a more normal count.

I do not recall any other TR doing this.

Is this abnormal?


Staff member
cross counts are the count of the o2 reading going over and under 450mv

now why yours is flipping would indicate that its reading low o2 from an unfired cylinder and then going high again from the cylinders that fire , this could cause a cross count increase to happen with every 2 rpms

Mark E.

What gets me is that the cross counts start immediately. Usually it takes a minute to warm up and start changing state.

Mark E.

Thanks for the replies. I had not noticed this happen before and thought I had another problem to fix.



Well-Known Member
One thing to check is the proximity of the O2 sensor signal wire to spark plug wires. Too close and that can cause the instant x-counts.

Also, the O2 sensor ground is via the body of the sensor to the exhaust and then on to the engine block. Then to the ECM from the grounds on the back of the head. Any 'looseness' of one of these connections can also cause noise that is seen as x-counts.


Mark E.

I do not think any plug wires are nearby. Will double-check. I had thought about the grounding. I secured the battery ground to the front of the block. Also tightened the grounds on the back of the head. Then reseated the ecm connector also. I usually give the ecm connector a shot of wd40 but I was out of stock. Will get some today. Also may try another ecm just for kicks. Will be retesting this afternoon.

Thanks! Mark

Mark E.

Well I am having another issue with this car and I think they are related.

It has low BLM's just driving 25 mph. Down to 90.

This O2 issue is the only thing I really see wrong. The constant crosscounts when cold and lower max reading than my cars.

I put a wrench on the grounds on the back of the head and both are tight now. Probably 1/4 turn or so.

Swapped out ecm, maf, chip, new stock GM O2, tightened battery ground at front of block.

I drilled out the return line restriction at the front frame. Now fuel pressure drops with vacuum properly. No change.

Thinking O2 purple wire to ecm or O2 gnd from head to ecm. Also O2 body to header to battery, but that seems unlikely.

Going to continue some more tomorrow afternoon.