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  1. #1
    Join Date
    Nov 2009
    Location
    New England USA
    Posts
    62

    Default XC70 PO420 error code

    Hi,
    I get the engine error code: PO420 once every month (or every other month). Sometime the light just disappears, but other time I have to reset it.

    My mechanic said just ignore it.
    Any comments on this?

    XC70 2004 over 200K.

    Thanks!
    Orvieto

  2. #2
    Join Date
    Aug 2017
    Location
    Pleasanton CA USA
    Posts
    434

    Default

    This is the

    Catalyst System Efficiency Below Threshold (Bank 1)

    The two O2 sensors are upstream and downstream of the catalytic converter. The ECM continuously compares these two as a monitor on the efficiency of the cat at removing oxides. I've found that transient excursions in the fuel-air mix can trigger this warning. Also, it's more sensitive to these glitches driving at high-elevation (> 4000 ft.). Another condition that correlates with these messages is driving on cruise-control in a very stiff wind...the CC thrashes about trying to find the desired throttle level, but can't because external factors (wind) are constantly tweaking the car's velocity.

    If either O2 sensor is going, this could also explain this error.
    2013 XC70 T6 Flamenco Red (hers)
    2015 XC70 T6 Seashell Metallic (his)
    past: 13 Volvos going back to '74 242 sedan

  3. #3
    Join Date
    Dec 2009
    Location
    GA
    Posts
    1,368

    Default

    Some have used an oxygen sensor extender on the rear O2 sensor to keep that code from appearing. The extender moves the rear sensor partially out of the exhaust stream, fooling the ECM.

    You could easily check older style sensors (but not on a 01+) with a scope or voltmeter - the voltage from the front sensor would normally vary between ~.2 to 1.2 volts in a sine wave pattern (in relation to the free oxygen left after combustion) as the mixture varies from lean to rich. The rear sensor voltage, if the cat is working correctly, should be a steady almost zero. The catalytic process uses the leftover HC and O2, takes them apart to make CO2 and H2O.(hence the steam from your tailpipe on humid days, and water that drips from many)

    With a missing or damaged converter the rear sensor voltage fluctuates with the front sensor. ECM sees this and sets a cat efficiency code.

  4. #4
    Join Date
    Nov 2012
    Location
    Pacific NW
    Posts
    361

    Default

    O2 sensors if not outright defective, can become slow to respond as they age. If have found "lazy" downstream sensors on two cars. Replacement solved the problem, and the vehicle ran better!
    PDX-Vancouver, USA

    2005 XC70 140,000

  5. #5
    Join Date
    Dec 2009
    Location
    GA
    Posts
    1,368

    Default

    Quote Originally Posted by Scopeman View Post
    O2 sensors can become slow to respond as they age.
    Have seen that many times, and new sensors can solve a variety of check engine light or poor performance problems. But many times the cars are so old by the time they fail the customers can't afford a retail car repair solution. I have never put an extender in (it's not legal to modify the emission control system and i could not risk it) but I have suggested it many times.

    For reference, my M5 (years ago) had replacement of all 4 O2 sensors as part of the 100k service! (I sold it way before then)

  6. #6
    Join Date
    Dec 2019
    Location
    New York
    Posts
    9

    Default

    Hey new here, seems like allot of great info. Im fighting a p0030 code, bank one sensor one. I replaced the sensor and the mil light went out on its own after a couple days of driving, i thought i was good to go and the day of my emissions test the check engine light comes back on. I tried clearing it with my scanner, no good, disconnected battery for half hour and did 60 highway miles and light came back on restart. I checked for vacuum leaks and found none, i was told a proper tune up using volvo plugs and new coils might do the trick, 2004 xc70 2.5t has 130,000 miles. I bought last year and dont know its history but figured this couldnt hurt. Could the fuel im using have an effect? Running regular at the moment, not even sure what octane

  7. #7
    Join Date
    Dec 2009
    Location
    GA
    Posts
    1,368

    Default

    That code is for the O2 sensor heater, not the signal from the o2 sensor. There is a resistance heater in all modern o2 sensors that speed the warmup of the sensor. O2 sensors don't send a correct signal until they get very hot (think its about 700f)

    2 of the wires going to the sensor (i think the black 2) carry + and - 12 volts - easy to measure voltage at the connector. I don't have access to a wiring diagram for a few days but there should be a fuse for the heaters - (and something that caused it to blow) So check for continuity through the heater and check for 12 volts at the 2 black wires coming from the engine wiring harness.

    The ECM tests for that code when the car is first started and comes back if the resistance of the heater element is missing/incorrect.

  8. #8
    Join Date
    Dec 2019
    Location
    New York
    Posts
    9

    Default

    I never thought about a separate voltage source for the heater, thank u very much ill check it tommorow. As for a fuse that will take some looking. Thanks again!

  9. #9
    Join Date
    Dec 2009
    Location
    GA
    Posts
    1,368

    Default

    Quote Originally Posted by hoonk View Post
    there should be a fuse for the heaters -
    It's #4 in the b fusebox (under the hood in front of the driver)

  10. #10
    Join Date
    Dec 2019
    Location
    New York
    Posts
    9

    Default

    Thanks, found it this morning before work. 20 amp fuse was fine, im going to ohm the replacement sensor too make sure its not bad out of the box and go from there. Judging by the plug configuration im kinda guessing which wires are for the heater

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