[MEADOWFERN]

Help. My 1997 3.0L lit a P0420 code. 152,000 miles and the engine runs fantastic, except for a transitory higher idle when fully warmed up (1,100 rpm), I'll address my attempts to fix that later.
Mainly, I just want the CEL off. Here's what I've done. 1) I ran DuraLube Catalytic Cleaner through the fuel system twice. Both times the CEL went off in less than 30 miles, then came right back on within a couple hundred miles. 2) I removed the Bank 1, Post Cat O2 sensor and installed two Spark Plug Defouler fittings to get the nose of that O2 sensor out of the exhaust stream. The CEL came back on in less than 100 miles. Note: I did NOT replace the O2 sensor, since it seems to be doing its job. My assumption is the original cat is not performing well, thus tripping the sensor and CEL.
Next: since the engine runs great, I assume the bank 1 pre cat sensor is doing its job with the fuel/air trim, true? Should I now just replace the post Cat sensor, and leave the Defouler fittings in place? Or, replace both bank 1 O2 sensors, pre and post cat?
Lastly, I read that a defective engine coolant temperature sensor, or even a defective EGR valve could be throwing the P0420 code...true or false?
Regarding my transitory high idle when engine is up to temperature...I replaced the idle air control valve, the EGR pressure valve, and EGR vacuum solenoid control valve, PCV valve. The test I did on the EGR valve itself indicated it was working, so I did not replace that.
So, what to do next?

 

[rjacket]

So, what to do next?

Use the diagnostic procedure in the workshop manual.

 

[chartmaker]

"What the P0402 code means. P0402 is an OBD-II generic code that the engine control module (ECM) has detected indicating the engine exhaust gas recirculation (EGR) valve is flowing too much recirculated exhaust gases when it is commanded to open the flow of gases into the intake manifold."
copy from internet.
1. Valve Stuck in Open Position
2. Dirty Air Filter
3. Bad Gasket

Best check these first.
Added cat plugged.
-chart-

 

[sheila]

Help. My 1997 3.0L lit a P0402 code. 152,000 miles and the engine runs fantastic, except for a transitory higher idle when fully warmed up (1,100 rpm), I'll address my attempts to fix that later.
Mainly, I just want the CEL off. Here's what I've done. 1) I ran DuraLube Catalytic Cleaner through the fuel system twice. Both times the CEL went off in less than 30 miles, then came right back on within a couple hundred miles. 2) I removed the Bank 1, Post Cat O2 sensor and installed two Spark Plug Defouler fittings to get the nose of that O2 sensor out of the exhaust stream. The CEL came back on in less than 100 miles. Note: I did NOT replace the O2 sensor, since it seems to be doing its job. My assumption is the original cat is not performing well, thus tripping the sensor and CEL.
Next: since the engine runs great, I assume the bank 1 pre cat sensor is doing its job with the fuel/air trim, true? Should I now just replace the post Cat sensor, and leave the Defouler fittings in place? Or, replace both bank 1 O2 sensors, pre and post cat?
Lastly, I read that a defective engine coolant temperature sensor, or even a defective EGR valve could be throwing the P0402 code...true or false?
Regarding my transitory high idle when engine is up to temperature...I replaced the idle air control valve, the EGR pressure valve, and EGR vacuum solenoid control valve, PCV valve. The test I did on the EGR valve itself indicated it was working, so I did not replace that.
So, what to do next?

P0402 or p0420??? If p0402, I'd change the dpfe (egr pressure sensor). If p0420, you've got bank#1 cat problems.

 

[MEADOWFERN]

P0402 or p0420??? If p0402, I'd change the dpfe (egr pressure sensor). If p0420, you've got bank#1 cat problems.

My bad, it is P0420. Sorry for the confusion. Given that error code, would it make sense to put a new O2 sensor on post Cat, along with the de-fouler spacer? The car runs great, so the cat is not clogged, so most likely just worn out. I thought the de-fouler on the post Cat would do the trick with the old O2.

 

[sheila]

My bad, it is P0420. Sorry for the confusion. Given that error code, would it make sense to put a new O2 sensor on post Cat, along with the de-fouler spacer? The car runs great, so the cat is not clogged, so most likely just worn out. I thought the de-fouler on the post Cat would do the trick with the old O2.

May try changing the bank#1 (firewall) pre-cat sensor (green elec. plug) B1S1. Sometimes on the post-cat sensor (blue elec. plug) B1S2, people have had to piggy back 2 non-foulers for success. May also have to drill out the 2nd non-fouler to accomodate the o2 sensor.

 

[JJacob]

Help. My 1997 3.0L lit a P0420 code. 152,000 miles and the engine runs fantastic, except for a transitory higher idle when fully warmed up (1,100 rpm), I'll address my attempts to fix that later.
Mainly, I just want the CEL off. Here's what I've done. 1) I ran DuraLube Catalytic Cleaner through the fuel system twice. Both times the CEL went off in less than 30 miles, then came right back on within a couple hundred miles. 2) I removed the Bank 1, Post Cat O2 sensor and installed two Spark Plug Defouler fittings to get the nose of that O2 sensor out of the exhaust stream. The CEL came back on in less than 100 miles. Note: I did NOT replace the O2 sensor, since it seems to be doing its job. My assumption is the original cat is not performing well, thus tripping the sensor and CEL.
Next: since the engine runs great, I assume the bank 1 pre cat sensor is doing its job with the fuel/air trim, true? Should I now just replace the post Cat sensor, and leave the Defouler fittings in place? Or, replace both bank 1 O2 sensors, pre and post cat?
Lastly, I read that a defective engine coolant temperature sensor, or even a defective EGR valve could be throwing the P0420 code...true or false?
Regarding my transitory high idle when engine is up to temperature...I replaced the idle air control valve, the EGR pressure valve, and EGR vacuum solenoid control valve, PCV valve. The test I did on the EGR valve itself indicated it was working, so I did not replace that.
So, what to do next?

It is recommended to check these first. If there is a problem after the replacement, it is better to replace it.

 

[MEADOWFERN]

May try changing the bank#1 (firewall) pre-cat sensor (green elec. plug) B1S1. Sometimes on the post-cat sensor (blue elec. plug) B1S2, people have had to piggy back 2 non-foulers for success. May also have to drill out the 2nd non-fouler to accomodate the o2 sensor.

I will follow up with replacing both pre and post Cat O2 sensors, and see what happens. I already did the stacked and drilled defouler fittings on the post Cat O2 position. Thank you.

 

[MEADOWFERN]

As a follow-up to my issues, the CEL is gone for P0402. The defouler adapter, post cat, worked. The light is now lit for a P0401. I replaced the DPFE sensor, cleaned the throttle body, tested the EGR, and cleaned the idle air control valve. This is a 1997 3.0 Vulcan. When I removed the throttle body, I did not have the air channel which accumulates carbon, mine was smooth. It seems on this model year they did not cast the channel that gets clogged with carbon. It appears the ports are in the side wall of the intake. Anyway, hopefully cleaning carbon off the TB plate with MAF sensor cleaner, this works to end the P0401 code an light. Thanks to all for your help.