Help me diagnose these emission #'s

[wbrian63]

'92 300SE. Runs fine but just squeaked by on my emissions test today.

Car was allowed to idle for an extended period - maybe almost 10 minutes (lazy inspector), so maybe that's why the readings are the way they are.

Outside temps were in the low-mid 60's.

I don't remember if he was this slow last year.

Last year's emission test results:
Low Speed 15mph
HC (ppm): 21 - max is 124
CO (%): .07 - max is .69
CO2 (%): 14.5 - not tested
O2 (%): .1 - not tested
NOx (ppm): 288 - max 937
Dilution (%): 14.6 - must be >6

High Speed 25mph
HC (ppm): 44 - max is 120
CO (%): .09 - max is .67
CO2 (%): 14.4 - not tested
O2 (%): .1 - not tested
NOx (ppm): 270 - max 848
Dilution (%): 14.5 - must be >6


This year
Low Speed 15mph
HC (ppm): 72 - max is 124
CO (%): .24 - max is .69
CO2 (%): 15.1 - not tested
O2 (%): .1 - not tested
NOx (ppm): 932 - max 937 - eek!
Dilution (%): 15.3 - must be >6

High Speed 25mph
HC (ppm): 112 - max is 120
CO (%): .24 - max is .67
CO2 (%): 15.5 - not tested
O2 (%): .1 - not tested
NOx (ppm): 637 - max 848
Dilution (%): 15.7 - must be >6

Something's definitely wrong since last year. I've put about 21K on the car. The plugs are new, and when recently changed, looked pretty good. The O2 sensor was replaced when the car failed to pass last year.

I've noticed the MPG is down somewhat from last year - about 1mpg less on average.

Any ideas?

I'll pull codes and see what's up - no check engine light on this car.