P0420 intermittent engine light

MICAH K. BINGAMAN

MEMBER
2002 VOLVO S40
1.9L
4 CYL
TURBO
AUTOMATIC
192,000 MILES

My new to me car started displaying the check engine light intermittently, and of course the DSA light came on as well and not from hitting the button on the dash. Currently all indicators are off. However, I got an OBD at scan tool and it shows that the code P0420 was stored by the ECM. Also, I have got Volvo OEM enhanced codes of 91, 5A, and 82. Now I took a live data recording of 40 frames with engine rpm's ranging from Idle 703 to 2965 Frame 1 shows Fuel SYS 1 OL-Drive, O2S B1S1 is.780 (V) and 02S B1S2 is.630(V) and on the next frame fuel SYS 1 is now a closed loop and my LTFT B1is constant at 3.9(%) all 40 frames while my STFT B1 -5, 5(%) to -9.4(%) to 0.0(%) to 5.5(%) to 1.6(%). Well it is all over the place. Is all of this an indication that my O2 sensors are faulty or is my TWC needing replacement? Or is it something else like the ECM needs to be replaced using the newest gadgets out there to reprogram it? Plus there is the A/C cooling fan code 82 and the torque reduction fault code 91. Could you point me in the direction I should be headed towards cause? I understand cars and a lot of the repairs but when it comes to emissions and what not, I am just a novice.

1 Reply

WRENCHTECH

MECHANIC
20,761 POSTS

That code will almost never be caused by faulty 0/2 sensors. There are very specific strategies required to set this code and it is rarely a false code. The upstream 0/2's are a constantly changing reading, constantly switching from high to low and back. The downstream sensor on the other hand is more of a steady reading of what is coming out of the converter. What sets this code is when both the upstream and downstream sensors start to mirror each other indicating the converter is not changing anything. The only other thing that I have ever seen cause this code other than a bad converter is an air leak just ahead of the converter that skews the upstream 0/2 reading.