P0421 - Warm Up Catalyst Below Threshold Bank 1

[Randy M]

I have a 2001 Galant DE 2.4 liter with about 150,000 miles. The error started recently and a web search has left me a bit confused. Some say it's that catalytic converter, some say it might be the upstream O2 sensor, others say it could be the downstream O2 sensor. I can change the upstream sensor, but I'll have to put it in the shop for the other two. Is it worth changing one or the other, or both, of the O2 sensors? Or should I bite the bullet and have the cat replaced? Alternately, would a couple of heavy doses of B-12 or Seafoam help?

[CHICAGO_ONE]

I've gone through this and from what I've heard, changing the cat to a universal one will not help (at least if your car is cali spec) & will be expensive.

There's a $5 fix for this, which I've done and it completely works. I was able to pass emissions afterwards and the code has not come back since. It's pretty simple to do. You're basically putting a spacer on the o2 sensor, which is underneath the car. Your hardest battle will be getting the sensor off.

Check this out: http://www.club3g.com/forum/performa...t-5-bucks.html

[Randy M]

I've gone through this and from what I've heard, changing the cat to a universal one will not help (at least if your car is cali spec) & will be expensive.

There's a $5 fix for this, which I've done and it completely works. I was able to pass emissions afterwards and the code has not come back since. It's pretty simple to do. You're basically putting a spacer on the o2 sensor, which is underneath the car. Your hardest battle will be getting the sensor off.

Check this out: http://www.club3g.com/forum/performa...t-5-bucks.html

Thanks, but I can't work under the car anymore and don't have a lift. I'll have to put it in the shop for a modification like this and most places won't tamper with emissions devices.

[GSpeed]

I purchased the following items for 2002 Mitsubishi Galant ES-V6 after receiving error code P0421 (Bank 1 below threshold / check engine light illuminated);

MR507386 : GALANT; 3.0L; FRONT O2 sensor; CALIFORNIA; RIGHT
MR507387: O2 sensor GALANT; 3.0L; REAR
MR507385: Oxygen Sensor, Left, Pre cat

Would anyone be able to confirm location of which sensor is Bank 1 (is it between valve cover and firewall on passenger side)? Would Mitsubishi part MR507385 be correct sensor for Bank 1?