DTC P0420 "Catalyst System Efficiency Below Threshold" - the catalytic converter is not working. Although it is possible one or more of the O{2} sensors has failed, a more common fault is a catalytic converter that was poisoned by either excessive oil, an oil additive, or fuel additive. Check the message archives but one poster replaced their catalytic converter for about $200-300. But this is not going to be an effective solution if the engine is still 'burning' oil or emitting something that poisons the catalytic converter. What is the engine oil consumption? How far do you go between oil changes and how much oil is added between oil changes? I typically go ~7-8,000 miles (11-13,000 km) between oil changes and add no oil between changes. Is the tail pipe 'carbon' coated? A normal Prius tail pipe has at most a very, very light dusting. One burning or tossing oil will have a heavier coating. Another step is to remove the spark plugs and inspect for carbon deposits. If they are oiled or show excessive carbon build-up, the engine needs a ring job which might as well include a complete engine overhaul: bearings, valves, and rings. Daryl's 2001 Prius also has the same problem but it had over 250,000 miles (400,000 km.) I checked his exhaust pipe and it had significant carbon build-up. However, Alabama has no vehicle inspection system. Now if someone in the past uses a leaded gasoline, gas or oil additive, after the source is gone, a catalytic converter replacement makes sense. Of course the easy thing is to replace one or both O{2} sensors in a hope that one of them is bad. But the cheaper solution is to monitor the O{2} sensor voltages, making sure they vary. If both vary together, swapping O{2} sensors is not going to do any good. Now in the USA, modification of a car to defeat the emissions detection system is illegal, a Federal fine. However, it may be possible to spoof the O{2} signals. Check your local vehicle codes to determine the risks. Bob Wilson
Carbon deposits on a spark plug indicate a rich air/fuel mixture. Ash and/or oil deposits indicate excessive oiling of cylinders. Oiled spark plugs do not necessarily indicate piston ring failure. A malfunctioning PCV system or an overfilled crankcase can lead to excessive cylinder oiling. Finally, catalytic converters do age, and can deplete their catalysts enough to trigger a P0420, regardless of engine condition.
I have a GenI (2002) Prius with 136k miles, and I have P0420. I confirmed that the cat converter is "exhausted" by looking at the live data on an Actron CP9145 scanner. The O2S11 and O2S12 sensor voltages were similar... indicating that the cat converter was not working. I replaced the cat converter with Eastern Catalytic #40544 (about $200). I recommend ordering the flange gasket at the same time (FelPro 61357; about $13). The flange gasket is uncommon, and if it is grotty (mine was) then a replacement is in order. The 40544 cat converter is a couple inches longer than the OEM converter, it's a close snug fit. It has a port for the trailing O2 sensor, so the distance between the two O2 sensors in the system is correctly maintained. Procedure: Elevate & secure the car (ramps or jackstands). Remove the post-converter O2 sensor (22mm open end wrench). Cut the OEM pipe just behind the O2 sensor. (I used a 10cm fine tooth metal-cutting sawzall blade on an electric sabre saw; took 2 minutes to cut the pipe). Disconnect the cat converter by removing the 2 flange bolts (14mm socket & a couple of extensions). Remove old cat converter. Remove old flange gasket; install new one on manifold (if you're replacing the gasket). Fit the new cat converter; may require some amount of wiggling about and moving the pipes until the old oem pipe fits inside the exit port of the new cat converter pipe. (It may require some muffler cement on the oem pipe to establish a gas-tight fit). Once the components are roughly in place, reinstall the flange bolts and springs. Then clamp the cat converter fitting over the stub of the oem pipe with a 1 7/8 in muffler clamp. Start the engine and check for exhaust leaks. Muffler cement may be necessary if the fit between oem pipe and cat exit port is not gas tight. I checked operation by looking at the live signals on the OBDII live data stream; O2S12 was 0.05v(approx), O2S12 was 0.80v(approx). Use the OBDII tool to clear the P0420 code. Lower the car from supports. Drive it for about 10 miles, let the IM Readiness tests complete, and the check engine light for this code should stay off. Total time on a nice early spring day: about 2 hours.
Sounds good. What was the condition of the exhaust system downstream from the catalytic converter, with regards to rust and corrosion?
