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Doug, ASE Certified Technician
Category: Mitsubishi
Satisfied Customers: 8609
Experience:  Mitsubishi employed and Factory trained ASE certified technician
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I have p0161 p0135 and p0421 codes came up

Customer Question

i have p0161 p0135 and p0421 codes came up
Submitted: 1 year ago.
Category: Mitsubishi
Expert:  Doug replied 1 year ago.


The P0135 is pretty straight forward... that is indicating a heater circuit malfunction on the bank 1 sensor 1 oxygen sensor. This is the firewall side upper sensor (nearest the engine). The fault is indicating that the circuit resistance went too high, which means an open circuit... since you aren't getting any other codes related to that sensor we know it is not unplugged, which leaves you with either a sensor failure as your only realistic failure point (unless you have a rodent issue or something weird like that).
Replacement of the sensor will resolve the issue; the vehicle is very particular about the sensors installed however. You can not use "Bosch" brand sensors or cut and splice/universal sensors. You can use of course Mitsubishi sensors or you can use "Denso" brand sensors labeled as "Direct Fit" or "First Time Fit". These are identical to the Mitsubishi sensors but a good bit less money in many cases.

The P0161 is indicating the same thing, an open circuit on an oxygen sensor heater, but in this instance it is on bank 2 sensor 2, which is the radiator side of the engine and it is the lower sensor (farthest from the engine). The same repair would apply here, a new sensor and you should be good to go. Heater circuit failures are a common weakness of the Denso control system.

The P0421 is not as friendly of a repair.... this is indicating that the firewall side pre-catalyst (part of the exhaust manifold) has internally depleted. These catalysts are filled with precious metals that perform chemical reactions when the exhaust hits them and as such they do have a finite life span. The Mitsubishi electronics are extremely precise in their monitoring of these catalysts (not like on a GM car etc where you see these codes pop up all the time from bad sensors etc).... if you see these codes on this era Mitsubishi you can pretty much guarantee the catalyst is depleted and will require replacement as long as you don't have any obvious circumstances to affect it like rust holes in the exhaust etc that would affect the reading.
If you live in California or a California emissions state/county, you need to get the manifold from the dealership and they are quite pricey (nearly $1000 often, they fluctuate with the value of their contents); if you do not live in a CARB following area then there are aftermarket options out there for much less (often around $300 from places like Rock Auto). And of course if you have no emissions testing at all where you live, you could leave it be... it will not harm anything being left this way, these are not prone to clogging or anything like that. The only caveat to leaving it be is that if you have another issue come up, the engine light is already on so you would not be made aware of the new issue so that is a bit of a concern with leaving it as is. The other thing to keep in mind with regard to replacing it, is that the other (radiator) side manifold has done just as many years/miles as this one has, so it is likely to be pending a failure in the future too.