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The car is a 2006 Opel (Vauxhall, Holden, Chevy) Vectra 2.2 direct (Z22YH) with 220,000 km (140,000 mi) on it. ECU is a Simtec 81.

With the engine warmed up, driving at highway speed, I recorded this with Car Scanner Pro:

Car Scanner Pro recording of upstream and downstream O2 sensors

The upstream O2 wideband sensor value (or rather its interpretation by the ECU) oscillates around 14.7. The downstream O2 sensor voltage oscillates between 0.2 V and 0.8 V at about the same frequency.

With the car stationary, engine warmed up, revving at about 1500 rpm I have captured this downstream O2 sensor voltage:

Oscilloscope Reading of downstream O2 sensor

Downstream O2 sensor voltage oscillates at a frequency of a little lower than 1 Hz.

BTW: I had an emission test done (by German “Abgasuntersuchung” standard) and the vehicle passed. Test report lists “CO 0.133 %vol” and “Lambda 1.009”.

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  • If your car passed an emissions test why do you think there's anything wrong at all? Is there an issue you are dealing with?
    – GdD
    Commented May 16 at 11:42
  • I'm actually surprised by the downstream sensor. I'd expect it to be a pretty much flat line if the cat is doing what it's supposed to. On the last image, It's hard to tell the reference of waveform (ie: what does it equate to; what are the high/low values).
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented May 16 at 11:59
  • Just thinking about it, the upstream reading for the wide band seems weird, too. While 14.7:1 is stoic, vehicles are usually set to run around 12.5-13:1 at idle. When driving down the road at very little load, they can sometimes go up to 16:1 towards the lean, but 14.7-ish is pretty lean for idle.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented May 16 at 12:44
  • @GdB The actual issue is an intermittent DTC P0420.
    – devopsix
    Commented May 16 at 18:29
  • @Paulster2 The first image is from driving at constant (highway) speed with light load. The second image is not directly related to the first one. In the second image low values are about 0.1 V, high values are about 0.8 V. I captured the raw downstream sensor voltage just to confirm it's really oscillating (because I do not trust the OBD adapter and scanner app 100%).
    – devopsix
    Commented May 16 at 18:50

1 Answer 1

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Yes, this downstream O2 sensor signal indicated a bad catalyic converter.

After determining that the outlet temperature was about 50 °C lower than the inlet temperature, I have replaced the catalytic converter. Now, the upstream and downstream O2 sensor signals look like this (engine warmed up, cruising at highway speed, approx. 10 seconds shown):

Upstream and downstream O2 sensor signals

The upstream O2 wideband sensor value still oscillates around 14.7. The downstream O2 sensor voltage now is steady at about 0.65 V.

Here is a longer trace (approx. 4 minutes covered) of accelerating, cruising at highway speed and slowing down:

Upstream/downstream O2 sensor signals and throttle position

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