1991 Honda Accord LX failed emission test on smog test. High CO and HC on 25mph

Check everything that is connected to a vacuum line, check the CAT and the injectors and last run a compression test.

Emission test by codes reader? or smog? Any codes?

Any codes? any problem tune up, catalytic convener, burning oil.

Hi Larry, I your position I believe I would take the vehicle for a diagnostic scan and then check the read outs. There could be a problem with any one of a number of sensors but I believe you'll end up with all odds pointing at the cat. Regards John

Jose, I don't know if this will give you the answer you are looking for because there are many things to consider here. The exhaust gas recirculation valve (EGR) is one of the main things involved in reducing NO emissions. This valve is controled by the power control module. (PCM) This is your vehicles computer. The PCM monitors the vehicles speed and will command the EGR to open when the vehicle attains a certain speed, which is why the ASM test being done is recorded at two different speeds. At 25 MPH your vehicle failed the hydrocarbon (HC) emissions limit and this could be for a number of reasons. When is the last time you car was tuned up? Old spark plugs, wires, clogged air filters, oxygen sensors, leaking injectors (personal experience) or the PCV valve could be contributing to the results of the test. I guess that the easiest way to go is to tune up the engine replacing the spark plugs, wires, air filter and the PCV valve and don't forget to have the oil changed at the same time. Only then should have the vehicle retested and see what the results are. Should it fail again you may have to consider having the EGR or the catalytic converter replaced depending on what portion of the test fails. If the NO fails I would consider the EGR. If the HC fails then I would consider the catalytic converter. Good luck with this.

the smog test looks like the engine is miss firing, that could cause the timing to be out of whack also, try a old fashioned tune up, new spark plugs, wires, and filters also replace the PCV valve. any time you see the NO that high suspect a miss fire.

High HC means moisture in the system. Try driving it longer then take it to the smog shop. Otherwise it could be hard like a head gasket or easy such as a PCV not functioning. Any ways you should always drive it about 20 miles before smog check, longer when the weather is cold.

How well does the vehicle run?

Assuming that the engine did pass the EGR function test on the emission test results, and the MIL (Malfunction Indicator Light) or check engine light is not on when the engine is running, and if you are not sure when the last time it was that the engine had a complete tune-up, with distributor cap, ignition rotor, spark plugs, spark plug wires, air and fuel filters, then a complete tune-up would certainly help, and if the engine oil and oil filter have not been changed in a while, then they should also be changed before an emission test because long used engine oil traps carbon and it will show up as higher CO on the emission test because the emission analyzer will be able detect the higher CO from the engine oil through the PCV valve.

However, it would seem from those HC readings that there is a vacuum leak, and carefully inspect all of the vacuum lines for any cracks or damage, (especially the vacuum line to the fuel pressure regulator) because cracks in the vacuum lines seem to like to hide underneath the lines.

The emission label under the hood should have the vacuum line routing diagram printed on it, and the vacuum lines should all be checked to be certain that they are all connected correctly.

Here is the firing order diagram for that vehicle to help assist you tune-up the vehicle.

Things to consider or replace...

If you are getting a Check engine Light let me know, it will make things easier to diagnose.

Catalytic Converter
Oxygen Sensor
EGR Valve

You can try some over the counter solutions per say. Some of the mechanics in a bottle. They sell some products This will probably be the cheapest router. If this doesn't work then you need repairs.

Pass your emissions test.