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Q: Check Engine Light on - P2187 and P0420 codes

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About 4 months ago I installed a catless downpipe on my car. I did not get a tune because I was told that I don't need it if I don't want it. My car idles fine but starting it is weird. I've had times where I start the car and it shuts down. I hooked up a code reader to my car and I got the codes p2187(system too lean at Bank 1). Then a evaporative emission system leak(P0442). Also catalyst system efficiency below threshold bank 1(P0420).

My car has 61000 miles.
My car has a manual transmission.

A: Hello. If I understand correctly, when you ...

Hello. If I understand correctly, when you say "catless downpipe," you mean that you have installed a pipe without a catalytic converter. If that is the case, you are always going to have the code for catalyst efficiency.

The car has two oxygen sensors, one upstream and one downstream of the catalyst. The controller is looking for a specific oxygen level going in and coming out of the catalyst to indicate that it is doing its job. Without the catalyst, the controller will never see the levels it’s looking for and will always light the CEL.

This won’t effect the way the car runs, but in most states it means you won’t be able to pass inspection. Additionally, if the CEL is always on, it won’t be able to alert you to any new problems. As for the system too lean and EVAP codes, they may have the same cause. It’s possible that you dislodged or damaged a vacuum line while you were doing the work. The resulting vacuum leak could cause the P2187 and if the leak is on the evaporative vent line it could cause the P0442. Look for a damaged or disconnected vacuum hose. If you don’t find one, you may want to enlist the help of a certified technician to diagnose the Check Engine Light and guide you with any repairs needed.

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