Oxygen / Lambda sensor P0030 HO2S11 on 1.25 2008 Fiesta

[Gazzamit]

Hello,

This error keeps reappearing and the car failed emmisions test. Sensors seemed OK, so I thrashed it for 20 minutes and it restesed OK.

The images show readings from cold start (40 degress) to hot (102 degrees).

The readings for sensor before minicat oscillates from 0V to 0.82V. The second downstream sensor changes from 0.4V to 0.6V as engine temperatures rises.

This all looks OK to my inexperienced eyes, howeer, the DTC returns every day now.

Do you have any advice or other troublshooting tips?

Regards,

Gary.

[TomsFocus]

The heater has failed inside the lambda sensor.  Will work fine once heated by the engine but take it's time to get there.  Needs a new sensor I'm afraid.

[Gazzamit]

Ah right, thanks thats really helpful. Presume this the sensor after the cat (the 0.4V to 0.6V and accessed under car)? Presume it should it read 0.6v from cold start?

Do you think the one before cat is OK (oscillates from 0v to 0.8V and accessed under bonnet)?

Regards,

Gary.

[TomsFocus]

52 minutes ago, Gazzamit said:

Ah right, thanks thats really helpful. Presume this the sensor after the cat (the 0.4V to 0.6V and accessed under car)? Presume it should it read 0.6v from cold start?

Do you think the one before cat is OK (oscillates from 0v to 0.8V and accessed under bonnet)?

Regards,

Gary.

No, it's the pre-cat sensor that's faulty.  It does appear to work ok from your graph (already warm?), but the ECU is flagging up a fault with the heater circuit instead.  You can test the heater easily enough with a multimeter, it's just a simple resistance check.

 

[Gazzamit]

3 hours ago, TomsFocus said:

No, it's the pre-cat sensor that's faulty.  It does appear to work ok from your graph (already warm?), but the ECU is flagging up a fault with the heater circuit instead.  You can test the heater easily enough with a multimeter, it's just a simple resistance check.

 

The car was cold started in the morning. It oscillates right away (tested three days in a row first thing). It stops oscillating as soon as I rev the engine and starts again imediately on idle. I'll see if I can work out the pins and do a resistance check.

[Gazzamit]

I took a few more Forscan readings and am very confused by what's happening.

• The red trace is ECT.OBDII - Engine Coolant Temperature
• The Green trace is O2S11- Heated Exhaust Gas Oxygen Sensor (bank1, Sensor1) (upstream).
• The Blue trace is O2S12 - Heated Exhaust Gas Oxygen Sensor (bank1, Sensor2) (downstream).
• The yellow trace is HTR11 - Heated Exhaust Gas Oxygen Sensor Heater (bank1, Sensor1).

 

After cold start the car reached 100 degrees in about 15 minutes and only then did the yellow trace toggled to on (HTR11). Shortly after the green trace (O2S11) starts to oscillate (first image).

Later on when driving the Yellow trace can be seen to toggle on/off. When its 'on' there are corresponding O2 sensor oscillations (second image).

Is this normal behaviour? The O2 sensor seems to only oscillate when the Heater is activated or am I misinterpreting the data? I thought the heater element would be on immediately after a cold start and would only toggle to off when operating temperature is reached.

Additionally, I checked the resistance of the heater element (white wires) and its open circuit when the car is cold.

Any recommendations or advice?

[TomsFocus]

Open circuit heater will be what's triggering the fault code.

Possibly a poor connection somewhere which allows it to come on later...either way, if there's no resistance through the heater and there's no obvious wiring break between the plug and sensor then the only fix is a new sensor.  Don't overthink it too much.