Testing a 4-pin pre-cat oxygen sensor.

Question

G'day Scott 🙂

I have a Ford Fiesta 2008 (mk6) 1.6 petrol model with a fault code P0030. The description on my scanner is

Heated oxygen sensor (H02S) Heater Control - Circuit Malfunction (Bank 1 Sensor 1)

I'm assuming that this is the pre-cat sensor correct? Assuming that it is, I have already carried out a number of tests on it, but I'll need your confirmation that the sensor is definitely dead and needs replacing.

Firstly, I am getting a good 12v at the source connection from the cars wiring at ignition on. Then, the next testing procedure I did is I warmed up the car and kept it idling while the sensor is disconnected, using a multimeter set at 200ohm, I have tested every pin on the sensor in every combination and returned no signal. Or should I try reving the car to 3,000 rpm while testing the pins?

Could you also advise if there's any other quick tests I should try to verify before I go out buying a new one as they're expensive components and I want to be sure before replacing it.

Thanks so much I highly appreciate it.

Moe

jack62 · Accepted Answer

The P0030 code is telling you that the bank 1 upstream heated O2 sensor (HO2) isn't heating up fast enough.

So the issue is with the HO2 sensor's heater/heater circuit.

By confirming 12 volts at the connector you've already diagnosed 1/2 of the heater's circuit to be good.

The other 1/2 of the heater's circuit is the ground wire/circuit for the sensor's heater.

The computer controls that to switch the heater On/Off.

So in the example below (your connector terminals may be arranged differently), terminals 1 and 2 of the HO2 sensor connector are for the heater side of the HO2 sensor

You've already tested terminal 1 and found 12 volts.

During KEY ON ENGINE OFF and warm up, the computer will provide a ground to terminal 2 of the connector to turn the heater on.

(When the HO2 sensor begins outputting a voltage signal at terminal 3 ((fluctuating between 0.1 and 0.9 volts [ideally])) the computer switches off the ground at terminal 2 which turns off the HO2 sensor's heater)

So if you "backpinned" the connector at terminals 1 and 2 (while connected to the HO2 sensor) and attached the positive probe of multimeter to terminal 1 and the negative probe to terminal 2, you should read that same 12 volts (during KEY ON ENGINE OFF and warm up) telling you that both the power circuit and the ground circuit to the HO2 sensor's heater is good.

In that case, Replace the HO2 sensor and Clear The Codes.

yaser · Answer

Just change the sensor.