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Discussion Starter · #1 · (Edited)
*This post is purely for entertainment purposes and is not to be considered advice or endorsement of any action. Any discussions are for off-road or track use only.

I posted a bit of this info in another thread, but wanted to do a separate, more detailed post on this subject as I think it could be helpful to others. I know some have struggled with this problem and maybe there are others who still suffer in silence. I myself had to seek professional help. And no this does not involve a blue pill.

This is about the Desolation Of Smog! Otherwise known as the OBD2 readiness monitors. In 2015, California started requiring model years 2000 and later to no longer use the tailpipe sniffer test, but instead have a scanner plugged into the OBD2 port. This communicates back to the state mothership and checks the Readiness Monitors on your car. Well over half of the states in the US now implement a similar type of smog test though the model years affected and how many or which Readiness Monitors they require to be “Ready” can vary.

Readiness Monitors are tests that the computer runs to confirm that the smog equipment is operating within spec. However, for these Readiness Monitor tests to run certain conditions have to be met and often in conjunction with a specific driving method (drive cycle). The states of the Readiness Monitors are usually either “Not Ready” meaning the test has not run yet or “Ready” meaning that they have run and everything is okay. There is also an N/A state for not applicable, but if it’s not applicable then you don’t have to worry about it as it won’t affect your smog test. For the Gen 3 Viper (in particular my 2006), there are 4 Readiness Monitors that revert to “not ready” state when you disconnect the battery or clear a Diagnostic Trouble Code(DTC) or Malfunction Indicator Lamp(MIL) aka check engine light via a scan tool:

1- Catalyst Monitor – confirms the condition of the catalytic converters.
2- Evap System Monitor – checks Evaporative Emission Control System (gasoline vapors)
3- O2 Sensor Monitor – checks for functioning oxygen sensors
4- O2 Sensor Heater Monitor – confirms working heater circuit in oxygen sensors

For CA Smog, the Evap Monitor is the only one allowed to show as “not ready”. If any of the other 3 remaining monitors show as “not ready” you will not pass the smog test. During my research on conditions and drive cycles for setting OBD2 Readiness Monitors on Vipers, I came across a lot of conflicting and some just plain wrong info. Don’t listen to the smog guy when he tells you to just drive the car more. Don’t just go out and aimlessly put on 100 miles on the odometer. Every manufacturer has a specific set of conditions coupled with a drive cycle to cause each Readiness Monitor to set to “Ready”.

A PDF of the drive cycle procedure I used on the Viper is attached at the bottom of this post.

Some guys may have tried other methods and got it to work for them, but I can confirm that I tried this Chrysler Drive Cycle Procedure and got my monitors to set. In my experience, the O2 sensor monitor and the O2 heater monitor set fairly easily. It seems that the Catalyst Monitor is the one where most people have trouble with. It set pretty easily for me in one go after I followed the conditions and drive cycle outlined in the PDF. In fact, I actually ran it with a 100% full fuel tank and it still set to “Ready”... go figure. As I noted in my other post, my old PCM tune was preventing my cat monitor from setting. The professional help I mentioned earlier that I sought was none other than resident Viper Hero Todd Abrams @ A&C Performance. He kicked my old PCM tune to the curb and developed a whole new SCT/Dyno tune for my setup. This took care of me right quick! Picked up a little power as a bonus and even better drivability too. Thank you Todd!

Now a word about modified Vipers as they often involve headers, relocated catalytic converters, or no cats…of course this is all for off-road/track use only. Getting the cat monitor to set for modded Vipers can add an extra layer of complication. To understand some of the problems we need a refresher on Upstream and Downstream O2 Sensors. Here is a look at the Gen 3 Viper’s stock exhaust and O2 sensors layout (a lot of this applies to later Gens as well).


The Upstream O2 sensors are integral to managing the engine’s air/fuel ratio and they tend to sit closer to the cylinder heads and before the catalytic converters. The Downstream O2 sensors sit after the catalytic converters and their main job is to monitor the cat’s condition/efficiency. On a side note, you can see that the Viper has 4 catalytic converters. The Downstream O2 sensors are located right after the initial cats. Further down the chain are the secondary cats which sit in the Viper’s side sills. Their only job is to help clean up any residual hydrocarbons….helpful during the days of tailpipe emissions. Notice also that the secondary/sill cats have no sensors so the computer has no idea about these cats or even if they are functioning or missing…again this was just about cleaning up hydrocarbon smells from the side pipes. Both Upstream and Downstream sensors monitor air/fuel ratio by detecting if the mixture is rich or lean. Lean mixtures produce a low voltage signal and rich mixtures produce a high voltage signal. However, the PCM only uses readings from the Upstream O2 sensors to calculate and adjust the air/fuel ratio for the engine. On startup, the readings from the Upstream and Downstream sensors will look similar, flipping back and forth between rich/lean. Once the cats are warmed up and fully working the Downstream O2 sensors should settle down to a more steady voltage. The PCM takes the measurements between the Upstream and Downstream O2 sensors and determines if the catalytic converter is working properly. It does this by checking the variance between these outputs taking into account transition rates, switching voltage, etc. If the readings continue to mirror each other with no variance then you will get an error code.

Here is a look at a set of American Racing Headers for the Viper and the layout for the O2 sensors (this applies to most header designs for the Viper).


The long tube design necessitates moving the Upstream O2 sensors further away from the cylinder heads. There are only two catalytic converters in this setup and they reside in the side sills. The Downstream O2 sensors are located behind these cats which mean they live inside the side sills. Herein lies the problem. Viper engineers never intended for O2 sensors to reside in the side sills. With only 2 cats in this header setup it becomes a very inhospitable location for a sensor to live. The main reason? An overwhelming amount of heat. The process where a cat converts hydrocarbons to more inert gases requires a lot of heat around 1,000 degrees. The more pollutants a cat has to clean up the harder it works and the more heat it produces. In the Viper’s stock setup of 4 cats, the initial cats take the brunt of the heat and the secondary cats in the sills run cooler. This is because by the time the gases hit the secondary cats a lot of the hydrocarbons have already been stripped away so the secondary cats do not have to work as hard. Modded cars tend to produce more hydrocarbons and now with only 2 cats they work harder and the cat temps run a lot hotter. Now imagine trying to keep a Downstream O2 sensor alive in the cramped confines of the side sill. The sensor wiring still has to run right past the catalytic converter with mere inches to spare. Preventing that wiring from melting or burning out can be a real challenge. And if that Downstream O2 sensor circuit shorts out (at least on the Gen 3s) it can blow a fuse. The very same fuse it shares with the reverse lockout solenoid on the transmission. After that trying to shift into reverse becomes a muscled, two-handed affair.

It can be a frustrating situation. Back in the old days you could just give up and tune out the rear O2 sensors or have them plugged in, but just tied up in the open. The problem with those ideas are that they will not set the Cat Readiness Monitor. So your options are to very carefully heat wrap the Downstream O2 sensor and its wiring and possibly even the Catalytic Converter…companies like Heatshield Products are worth a look…OR (and again this is all for off-road/track use) go on eBay and look up “O2 sensor catalytic spacer”. Weld an O2 bung anywhere you can fit one for each side(before the cat is fine), screw in the cat spacer, then screw in your Downstream O2 sensor into the spacer, plug it in and Bob’s your uncle! You’re all set. These spacers move the Downstream O2 sensor out of the path of the exhaust gases and what gases do get in pass thru a mini-cat element first. This essentially tricks the sensor into thinking it is sitting behind a perfectly working cat. The readings the sensors output also match what the PCM expects to see. It is good enough to set the Cat Readiness Monitor. Some guys have reported luck with spacers that did not have the mini cat element. I can only confirm what I have seen work and to me the mini-cat element is an additional layer of assurance. Your mileage my vary so pursue at your own risk. And with that the cat is outta the bag!


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