High HC Emissions Failure


Oct 26, 2018
I recently failed the Colorado Emission test due to High HC. Replaced Plugs, Oil, Cleaned all filters, filled with 93 octane gas before testing, Installed TiurboTweak Emissions Chip, Converter in place. Car was in a shop for a 1 1/2 years in a dirty environment. Passed in 2016. Only thing done to the car since 2016 is replaced AC delco plugs with NGK, Replaced fuel pump. Should I buy a new O2 Sensor (new in 2016) and
EGR (new in 2016). Only about 400 miles put on car in the last 2 years. I have attached the diagnostic report. DOnt want to throw money at the wrong things. Thanks!!

HC GPM: 1.3520 Limit 2.5 Pass
CO GPM: 14.2937 Limit 20.0 Pass
CO2 GPM: 467.9591
NOx GPM: 1.3414 Limit 4.00 Pass

HC GPM: 3.8172 Limit 2.5 Pass
CO GPM: 14.3584 Limit 20.0 Pass
CO2 GPM: 494.6008
NOx GPM: 2.4582 Limit 4.00 Pass


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Did you drive the car long enough to get the cat hot enough? Last time I went to get a smog I was in the car with the rpms up to keep the exhaust hot before they put the sniffer on it.
The gas cap is the easy part. LOL

Do you have a stock or emission chip? Is your catalytic converter intact? EGR working? It can be a real pain in the ass to get them to pass.

There has to be some low mileage, classic car, show car exemption in Colorado. No? I would check on that. Rather than go through a bunch bs.
High HC; here is the chart I use, taken from:


If this were my car I would:

-Check /change the air filter again (you said it was in a dirty environment)
-Change the O2 sensor
-Look for the vacuum leak.
-I would leave the EGR alone since that mainly affects NOx, but look for a vacuum leak around that area.
Good luck, let us know when you pass.
HC is unburned hydrocarbons. This is fuel that isn't being burned. I would have to guess the new pump is supplying more than the old one. You might consider lowering the fuel pressure if you have an adjustable fuel pressure regulator. Do you have the ability to have it checked multiple times or is it a one shot deal kinda thing?
I get a free retest within 10 days. I have lowered the fuel pressure some and I can clean the K&H filter again, but just cleaned and oiled it prior to the last test. Should I go ahead and replace the O2 sensor again? Its only 2 years old. Cant find any vacuum leaks.
I was running error code 23 on the scanmaster (MANIFOLD AIR TEMP (MAT) CIRCUIT (LOW TEMP) with the Emissions chip installed. Went away when I reinstalled the TT chip. Am going to clean the MAF, replace O2, PCV, Plug Wires and reclean K&N filters (crank case & Air filters) with lighter oil (might have had too much oil on them) and reinstall the emissions chip. Car seems to run much better with TT chip. Will retest this Friday. Will make sure it is good and hot. Any other recommendations?
K&N oiled filters? I would not clean and re-oil them again before the test . Freshly oiled filters are known to muck up the MAF. Definitely clean that MAF . IF you feel like re-cleaning the filters, clean them, dry them well, and don't oil; You can oil after the emissions test.

Code 23 is low temp on IAT (MAT) air temp sensor by your air cleaner. Is it plugged in? If so, is it dirty from the Air cleaner oil? Might be something to check.

Like you said, make sure it is hot; Run the AC if you have it before the test.

If you have E85 in your area, put some in the tank, it can drive down HC readings.

Good Luck!
that sucks thats why I live in FL
no check no cat???
no problem
only thing I can think of Is adjust timing to run a little leaner or fuel delivery like Texas T said
maybe ad can of octane boost or put some Sunoco race fuel in
I am currently replacing all of the vacuum lines, check valves (missing 2), capping the vacuum ports to the dash HVAC system, replacing the PCV valve and will lean it out and try again.