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SMOG/EMISSION inspections. Everything you need to know OBD1

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EVERYTHING you need to know to pass
SMOG and EMISSION inspections
Compilation thrown together by CivicOCD

Alright I wanted to contribute something that should prove very helpful to those that have to deal with
having to pass smog check every year/other year.  I've compiled a collection from several different sources,
condensed and re-written for Realhomemadeturbo.  The information contained here should give you a very thorough
understanding of how smog testing works and how to effectively diagnose and solve most any emission
related issue that you might come across.

I have tried to only put factual information and proven sound advice here while weeding all speculative, non proven theory out.

A section at the bottom will be dedicated for unorthodox/unconventional fixes or culprits behind various failed test experiences.
This post will concentrate on OBDI vehicles requiring tailpipe "sniffer" test while on a load test dyno.

Updates and corrections coming today 17 March 2009

If everything in your car is already in good working order, you have no CEL's, and you have a good sealing gas cap
here is what you can do to help pass smog the first time:

1.Tune-up to include Spark plugs, Wires if needed, distributor cap and rotor.
If cap and rotor aren't completely fucked looking, use small nail file to remove corrosion from contact points

2. Fresh oil change. (5w30 for Honda's, you can go a little heavier if you have shitty rings or valve stem/guide seals)

3. Have at least a quarter tank of premium gas (*under some circumstances NOx can be reduced by adding HEET, GTP, Denatured alcohol, or 97% Isopropal/ethal alcohol. see details below*)

4. Tighten your throttle cable to stock specs

5. Retard your timing to 14 degrees BTDC. Some states visually inspect or check your timing as a part of the test so timing can only be retarded within factory allowed tolerances.
(For Honda's this is + or - 2 degrees.)

*If your state doesn't check this then you can retard further than this, but I would not recommend any more than 5 or 6 degrees and that's only if you already know your going to fail on NOx.

6. Drive your car for 30-45 min prior to pulling into inspection station. This does not mean let it idle until it's warm..this means drive the piss out of it, up hills and fast. This is to thoroughly warm your engine and heat soak the catalytic converter.
The converter can then be "Super Heated" by revving up to and sustaining 3000+ RPM's for 2 to 3 minutes while parked  just prior to being pulled onto
 the dyno for testing.

 It's illegal in all states, but depending on which one you live in they may or may not visually inspect it all though all states are supposed to. I personally run a converter 1 day a year for the very purpose of passing smog.  Most of you are running turbos with custom tunes. Most of these tunes sit a little on the rich side especially if your on a basemap, you are going to want to put a converter on. Running a cat is also almost essential if you happen to own a Honda that does not have an EGR system.
(IMPORTANT: If your car doesn't have an EGR valve, ensure that you get a **3 WAY** converter if you buy a universal or aftermarket cat)

The only people that realistically should even attempt to pass without running a converter:

A. you know the guy or know they ain't gonna give a shit if you don't have one.

as well as

B. being very experienced/proficiant with wideband tuning and feel confident you can to make it run  and hold stoich AFR's across the board under load, and make the proper timing corrections.

(It's convenient to have a V-Band welded to each side of a good converter, and run a test pipe with the same size connections after your downpipe that is the same length. 5 minute change-out ftw)

If You have FAILED your inspection

Depending on where on the test and what you failed will dictate what should be focused on.

The three gases(pollutants)tested for in the sniffer test are:

1. HC (Hydrocarbons) measured in PPM (parts per million)
2. CO% (Carbon Monoxide)
3. NOx (oxides of Nitrogen)

Two other gases(Non-pollutant) that can be detected and recorded for diagnostic purposes are:

CO2 (Carbon Dioxide)
is the product of a good running engine. The better your engine runs, the more CO2 it produces
In a properly tuned vehicle it will be present at levels around 14-15% shown on your test results.

The amount of CO2 in the exhaust is directly related to the correctness of the air/fuel ratio.
as the fuel mixture approaches stoichimetric (14.7:1) the level of CO2 peaks. it decreases as the mixture becomes
richer or leaner.

O2 (Oxygen)

When there is a relatively great amount of fuel (rich mixture) all of the available oxygen is used up, resulting in LOW O2 readings in the exhaust.
As the mixture gets leaner, the amount of oxygen in the exhaust steadily increases, because less of it is being used in combustion.
In other words, when the relatively small amount of fuel in a lean mixture is burned up, there is still some oxygen left which exits through the
exhaust.  Therefore HIGHER Levels of O2 in the exhaust are a direct indication of leaner air/fuel ratios.

Although HIGH O2 can be caused by a lean air/fuel mixture, most of the time noticeably high O2 is caused by a leak in the
exhaust pipe allowing air to enter. An exhaust leak PRE-CONVERTER can cause high NOx and/or HC.
This due to the catalytic converter not being able to function due to over abundance of oxygen.

By understanding these results when you see them, you will be able to figure out exactly what is going on inside the combustion chamber.
If your area's smog check happens to contain the results for these, they are the first thing to be considered when
determining the cause of an emission failure.

What causes high HC (hydrocarbon)?

High HC is caused by incomplete combustion; in other words, the air and fuel that went in to the combustion chamber was not completely burned, now the unburned fuel has ended up in the tailpipe. Problems that can cause incomplete combustion include:

1.An ignition misfire. If a spark plug does not spark, the air/fuel mixture in the combustion chamber does not burn and then goes out the tailpipe.
2.incorrect air fuel mixture. If there is not enough fuel in the combustion chamber to ignite, the fuel that is there goes out the tailpipe.
3.Low compression. If the compression is too low the air/fuel mixture will not ignite and the unburned fuel will go out the tailpipe.
4.A bad catalytic converter. The converter lives in the exhaust pipe and burns unburned gas as it passes through.


What causes high CO (carbon monoxide)?

High CO is caused by a rich air/fuel mixture. The air fuel ratio should be 14.7 parts air to to 1 part fuel. Problems that can cause a rich mixture include:

1. Oxygen Sensor
2. Coolant temperature sensor
3. Air flow meter, MAP sensor, etc..
4. Fuel pressure. Fuel pressure that is too high can cause a rich mixture. (stock Honda regulator should put static pressure between 40-47 psi with vacuum hose disconnected.)
5. Fuel injector stuck open. (use a stethoscope and listen for clicking at each injector. resistance measured between stock injector pins should read between 10-13 Ohms)

Things to keep in mind:

CO forms when there is a SHORTAGE of oxygen
The Higher the CO % the RICHER the mixture
The Lower the CO % the LEANER the mixture

CO is a good indicator if the system is too RICH but is a poor indicator if the mixture is too LEAN.


NOx (Oxides of Nitrogen)

High NOx is the most common reason for emission failure.

IMPORTANT: A Honda WITHOUT and EGR, assuming everything else working properly, a LEAN condition is most likely the cause of NOx failure.
Check Timing, Cooling system(is enough air being blown through the radiator by the fans at the test facility?)then fuel delivery.

High NOx is caused by high combustion temperatures and pressures(aprox 2500 degrees). Some NOx is unavoidable however is reduced to reasonable levels by use of a 3 way catalytic converter with correct A/F ratio exhaust gasses passing through it, and in most cases the EGR(Exhaust Gas Recirculation) Valve.

Problems that can cause high NOx (Oxides of Nitrogen):

1. An inoperative EGR system(if applicable)
The EGR system was designed specifically to reduce NOx; that's it's only function. The EGR valve allows exhaust gas to enter the combustion chamber through the intake. The exhaust gas has a cooling effect on the combustion chamber.

2. Over advanced ignition timing. The more advanced the ignition timing, the higher the combustion chamber temperatures.

3. Overheating. If the engine temperature is too high the NOx emissions will go up.

4.Lean air/fuel mixture. If the air/fuel mixture is too lean the combustion temperature will go up.(Sometimes lean condition will only occur while under certain loads.)

5. Clogged or inoperative fuel injector(s)

6. Compression to high. Compression can increase as the engine is used. When carbon forms on the tops of the pistons, or a milled head has been installed. Timing can usually compensate for this.


If your positive that your catalytic converter is functional, and/or your vehicle is passing everything except for NOx,
Go through this checklist step by step.  Pay attention to detail and don't skip anything.

A) Make sure there are no PRE-converter EXHAUST LEAKS

B) Double-check CAM timing.  Visually verify TDC marks on crank and cam pulleys perfectly line up with marks on block.

C) Double-check IGNITION timing. Retard timing a few degrees from stock (16* BTDC)
*Before adjusting timing, ensure engine is completely warm and that the ECU service port connector has a Jumper wire attached.


1: Loosen 3 distributor bolts with a 12MM box wrench.
2:  Rotate Distributor Clockwise(Toward you if standing in front of the car) to RETARD the ignition.
3: You will see the TDC mark on the crank pully move closer to marks on the timing belt cover.
4: Re tighten one of the 12MM bolts making sure you don't move the distributor accidentally.

D) Clean PCV and the vacuum line going to it

E) Clean and test EGR. (if applicable).

F) Test system fuel pressure.
On stock Honda fuel pressure regulators, if static fuel pressure is in the mid 30's and only moves to 40ish psi area with vacuum hose disconnected, cap off intake manifold port and leave FPR disconnected.

G)Check for clogged or inoperative fuel injector(s)
Use a stethoscope and listen for clicking at each injector. resistance measured between stock injector pins should read between 10-13 Ohms.
this only checks for functioning of the injector, it doesn't indicate if it is clogged or not.

H) Thermostat and Cooling fan
Use a cooler range thermostat or for the sake of testing remove it all together.  To further increase cooling to the engine you can install a jumper clip between the two wires going into the fan switch connector going into the back of your thermostat housing. This will make your fan run constantly anytime the car is running.
**This is an especially good idea if your barely failing NOx during 50% load test but passing or really close to passing 25% load phase of the test.

I) Spark plugs
Run a new spark plug that is one heat range colder. For example:
stock d16z uses NGK BCPR5E-11 ----switch to--->BCPR6E-11
or BCPR7E-11 if you're engine currently uses 6's.

J). Compression check.
To high or to low will create obvious problems. make sure cylinder's are within a 28 psi (or your engine's specs)
range of one another.


5. ***Adding GTP (Guaranteed to Pass) according to directions, HEET brand gas line anti-freeze, denatured alcohol, ethyl or isopropyl alcohol to fuel.
This works by lowering combustion chamber temps/increases octane. This will have varying effects, and highly application dependent.

There is much debate over the proper mixing ratio for these various chemicals.  Success has been reported with as little as the directions on the back, and other
reported instances of using an entire gallon of denatured alcohol to an almost empty fuel tank.

Alcohol mixed with gasoline will in fact lower combustion chamber temps and reduce NOx.
How much of an effect it has however has yet to be determined.

If you have experience with this or plan on doing it feel free to take notes and post before and after results here.

6. Sea foam and BG44 (important information)

I put this and other fuel/egr/engine cleaning treatments in a category all by itself.  These are all the options that entail pouring or spraying chemicals in your engine while it's running, and driving it until it stops spitting nasty smoke out of the back of your car. Sea foam, BG44, and other commercial engine treatments work EXTREMELY well and there in lies the problem....

If you run these things through your 150,000 engine that has had 150,000 miles worth of carbon building up in it, then you can COUNT on the fact that a lot of that carbon actually helping to SEAL your old valve  seals...when you clean all the carbon off of them, oil is now leaking into the combustion start wondering
why the car now constantly blows blue tinted smoke and it didn't before......this is why.  You are now burning oil.

If you have used these kind of treatments regularly throughout the life of the motor, then it posses much less of a risk.

**NOTE: If any of these "pour in with engine running" are used, it is imperative that the spark plugs be changed afterwords, because they will be completely fouled and gunked up.


   PCV - Positive Crankcase Ventilation
This is the earliest system required on automobiles. It is required and all car from 1955 on. Prior to this, engines simply vented crankcase gasses to atmosphere. This wasn't very effective. The oil would get contaminated by the byproducts of combustion that would blow by the rings. The heart of this system is the PCV valve. It is essentially a calibrated leak between the crankcase and the intake manifold. On a new engine, air is drawn in through the crankcase breather tube and through the PCV valve into the intake manifold. On an older engine with excess blow by, the PCV valve cannot draw all the crankcase gasses. The excess gas vents through the crankcase breather tube and into the air cleaner to be drawn into the engine.

The PCV valve is both a restricted orifice and a check valve. Generally, if it rattles when you shake it, the check valve is still working properly. This check valve function is to protect the engine in the event of a backfire through the intake.

   EGR - Exhaust Gas Recirculation
This is probably the reason people believe smog equipment equals no horsepower. EGR introduces exhaust gas into the intake manifold
to reduce oxides of nitrogen (NOx) by reducing peak cylinder temperature and pressure. Unfortunately, high cylinder pressure equals power.
The heart of this system is the EGR valve. This is a vacuum actuated valve between the exhaust and intake. On a carburetor car it is usually run by
ported (venturi) vacuum. This will mean the valve is closed at idle and will open as the air flow through the carburetor increases. Later computer
controlled cars typically have a solenoid valve that cuts off the vacuum at WOT. This is a great addition since you can have lower NOx at part throttle without sacrificing full throttle performance.

  CAT - Catalytic Converters

A three-way catalytic converter has three simultaneous tasks:

 1. Reduction of nitrogen oxides to nitrogen and oxygen: 2NOx → xO2 + N2 (A two-way converter is missing this element therefore if put on a non-egr equipped car, there would be nothing to reduce NOx)
 2. Oxidation of carbon monoxide to carbon dioxide: 2CO + O2 → 2CO2
 3. Oxidation of unburnt hydrocarbons (HC) to carbon dioxide and water: CxH2x+2 + 2xO2 → xCO2 + 2xH2O

These three reactions occur most efficiently when the catalytic converter receives exhaust from an engine running slightly above the stoichimetric point. This is between 14.6 and 14.8 parts air to 1 part fuel, by weight, for gasoline. When there is more oxygen than required, then the system is said to be running lean, and the system is in oxidizing condition. In that case, the converter's two oxidizing reactions (oxidation of CO and hydrocarbons) are favored, at the expense of the reducing reaction. When there is excessive fuel, then the engine is running rich. The reduction of NOx is favored, at the expense of CO and HC oxidation


O2, CO, & CO2 Relationships:

a) High O2, low CO & low CO2 = lean
b) Low O2, high CO & low CO2 = rich
c) Low 02, low CO & high CO2 = OK (good)

Here is a chart to help interpret the results of your emissions test:

Hope this helps. I'll be fine tuning and editing this post as well as working on  flow-chart style step by step troubleshooting.

Good luck


Nice read lucky I live in Wyoming ;D

I must be one of those lucky ones.
They dialed in my car as a 1.5 SOHC Non-Vtec

T3/T4 turbo
2.5" exhaust
No Cat

Reason why I passed is because I had more 99% rubbing alcohol in my tank than of 92 gas.
Flying colors, including nox.
Idle was tuned to 14.7-15 but there was a leak at the down-pipe during inspection.



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