P1153 & P1157 with poor MPG

Questions specifically about a Petrol powered Citroen C3 (usually engine or fuel related problems).
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My Name: Ozvtr

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Posts: 997
Joined: Wed Jul 10, 2019 3:31 am
Model: C3 2002-2005, Original shape model
Year: 2003 (53)
Engine Size: 1.4 (8v)
Fuel Type: Petrol
Mileage: 80000
Trim Level: Other
Gearbox: Automatic PRND
DPF: No
LHD or RHD: RHD
Engine name: TU3 (75 PS)
Location: Brisbane, Australia.
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You can identify some problems with an engine by looking at the fuel trims.

You have to be careful interpreting what is happening. This is what the engine ECU "thinks" it needs to do to get the engine running correctly. If you have a faulty O2 sensor the engine ECU "thinks" the engine is running lean or rich whereas in reality it's running fine. The sensor is just lying, causing the ECU to compensate (trim) for something that isn't happening!!!
My Name: Jjb0800

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Posts: 38
Joined: Wed Nov 11, 2020 7:37 pm
Model: C3 2002-2005, Original shape model
Year: 2006 (06)
Engine Size: 1.4 i
Fuel Type: Petrol
Mileage: 71200
Trim Level: SX
Gearbox: Manual 5 speed
DPF: No
LHD or RHD: RHD (UK)
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Yes my cheapo scanner does have like voltage figures but no live graph, but I will take a photo of the test on both bank 1 sensor and sensor 2
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My Name: Ozvtr

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Posts: 997
Joined: Wed Jul 10, 2019 3:31 am
Model: C3 2002-2005, Original shape model
Year: 2003 (53)
Engine Size: 1.4 (8v)
Fuel Type: Petrol
Mileage: 80000
Trim Level: Other
Gearbox: Automatic PRND
DPF: No
LHD or RHD: RHD
Engine name: TU3 (75 PS)
Location: Brisbane, Australia.
Has thanked: 40 times
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The output of sensor 2 should be flat at just under 0.5 Volts. If it oscillates more than a few dozen millivolts the engine ECU will throw an "ageing" catalyst or catalyst low performance error. But from what you are saying sensor 2 doesn't seem to have a problem.

My bet is that sensor 1 is almost constantly at just under 1 Volt and doesn't oscillate at all.

It's hard to read just values of numbers, a graph is better but we don't have that luxury.
My Name: Jjb0800

Top Contributor
Posts: 38
Joined: Wed Nov 11, 2020 7:37 pm
Model: C3 2002-2005, Original shape model
Year: 2006 (06)
Engine Size: 1.4 i
Fuel Type: Petrol
Mileage: 71200
Trim Level: SX
Gearbox: Manual 5 speed
DPF: No
LHD or RHD: RHD (UK)
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Here are my readings

Mininum and maximum voltage readings are from bank 1 sensor 2
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User avatar
My Name: Ozvtr

Moderator
Posts: 997
Joined: Wed Jul 10, 2019 3:31 am
Model: C3 2002-2005, Original shape model
Year: 2003 (53)
Engine Size: 1.4 (8v)
Fuel Type: Petrol
Mileage: 80000
Trim Level: Other
Gearbox: Automatic PRND
DPF: No
LHD or RHD: RHD
Engine name: TU3 (75 PS)
Location: Brisbane, Australia.
Has thanked: 40 times
Been thanked: 285 times

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Your downstream (post cat) O2 sensor is oscillating a lot (0.039V to 0.933V), but you don’t have a catalytic converter error (yet). Hmmm. This shows 2 things. 1) The sensor is working, as it is reporting something. 2) The engine ECU is switching rich to lean and then lean to rich, which is what it is supposed to do.

The upstream O2 sensor is dead or there is an open circuit in the wiring to the sensor. It’s just sitting at 0.449 Volts, which is the default bias voltage. From the downstream sensor output voltage we know the engine ECU is swapping between rich and lean mixtures but the upstream O2 sensor is not reporting this. It doesn't budge from 0.449V!

The engine will not be running optimally as it does not know what the actual air fuel ratio in the cylinders is! It's just guessing.

You have 2 choices. Replace the upstream O2 sensor on spec, or swap the sensors to check the wiring. I seriously doubt it’s the wiring.
My Name: Jjb0800

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Posts: 38
Joined: Wed Nov 11, 2020 7:37 pm
Model: C3 2002-2005, Original shape model
Year: 2006 (06)
Engine Size: 1.4 i
Fuel Type: Petrol
Mileage: 71200
Trim Level: SX
Gearbox: Manual 5 speed
DPF: No
LHD or RHD: RHD (UK)
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Thank you I will replace my upstream sensor. Shall I replace the downstream sensor as I heard that you are supposed to replace in pairs of possible? Also, you said something about my cat does this car not have a cat?
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My Name: Ozvtr

Moderator
Posts: 997
Joined: Wed Jul 10, 2019 3:31 am
Model: C3 2002-2005, Original shape model
Year: 2003 (53)
Engine Size: 1.4 (8v)
Fuel Type: Petrol
Mileage: 80000
Trim Level: Other
Gearbox: Automatic PRND
DPF: No
LHD or RHD: RHD
Engine name: TU3 (75 PS)
Location: Brisbane, Australia.
Has thanked: 40 times
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Er... it should have a CAT. If you have a look at my rants over in the "Citroen C3 club" area (G'day) I have chronicled some of my (mis)adventures with my C3's. Including the fact that an aftermarket catalytic converter had been installed and it had no catalyser in it!!! It was empty!!
So it should have a CAT...I cant say that it does have a CAT! LOL!

The downstream O2 sensor only monitors the performance of the catalytic converter. It does not affect the performance of the car at all. Being further down the exhaust system it is not subjected to as much stress and tends to last longer than the upstream sensor.
I would not replace the downstream O2 sensor.
My Name: Jjb0800

Top Contributor
Posts: 38
Joined: Wed Nov 11, 2020 7:37 pm
Model: C3 2002-2005, Original shape model
Year: 2006 (06)
Engine Size: 1.4 i
Fuel Type: Petrol
Mileage: 71200
Trim Level: SX
Gearbox: Manual 5 speed
DPF: No
LHD or RHD: RHD (UK)
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I have just replaced my upstream sensor. Here are my readings.

I have seen an improvement in mpg from 25 up to 38-39. However it did drop down to around 35. Also when i start up the car it does feel like a rough idle could this be time for new spark plugs? Ps I have no fault codes too
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User avatar
My Name: Ozvtr

Moderator
Posts: 997
Joined: Wed Jul 10, 2019 3:31 am
Model: C3 2002-2005, Original shape model
Year: 2003 (53)
Engine Size: 1.4 (8v)
Fuel Type: Petrol
Mileage: 80000
Trim Level: Other
Gearbox: Automatic PRND
DPF: No
LHD or RHD: RHD
Engine name: TU3 (75 PS)
Location: Brisbane, Australia.
Has thanked: 40 times
Been thanked: 285 times

Post

Umm I’m not sure what is going on.
According to the readings…nothing has changed!?

However, the improvement in fuel economy and no DTCs show the engine ECU is happy with things.

The diagnostic codes seem to be correct but the live data is not?

I can only assume there is some problem with interpreting those readings or it is not displaying what I think it is.

What do you mean by it dropped to 35? Permanently or temporarily?

It might take a short period of time to get the fuel trims right. The engine ECU will be re-learning the optimum values. This might explain the rough idle.

I always recommend removing spark plugs and inspecting them periodically. There is a lot that you can glean about an engine from the look of spark plug electrodes. While even regular spark plugs can last 100000Km they need to be gapped from time to time and even a small increase in the gap can degrade the performance of an engine. Cleaning, gapping or replacing ‘plugs can make a significant difference to the performance of an engine.
My Name: Jjb0800

Top Contributor
Posts: 38
Joined: Wed Nov 11, 2020 7:37 pm
Model: C3 2002-2005, Original shape model
Year: 2006 (06)
Engine Size: 1.4 i
Fuel Type: Petrol
Mileage: 71200
Trim Level: SX
Gearbox: Manual 5 speed
DPF: No
LHD or RHD: RHD (UK)
Has thanked: 1 time
Been thanked: 1 time

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Update my mpg which showed considerable improvement after replacing the upstream sensor has now gone back to its normal reading of around 26-28. I just dont get it with this car... its not had a service since 2019 but its only done 11k miles since then. Could this be a factor?
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