O2 and air/fuel ratio sensors

What DTC were logged? It is quite unusual for a Prius to require new sensors at the odometer reading your car has.

Assuming that new sensors are required, then if you can DIY, I'd suggest buying the correct Toyota sensors so that you don't have to worry about fitting or splicing the correct wiring harness connectors.

For example, it looks like the oxygen sensor can be purchased for $112 while the air/fuel ratio sensor is priced at $162, here: Champion ToyotaWorld

I don't think the job will be that tough as long as you have jackstands and a hydraulic floor jack so you can get under the car. If you find that Chilton's doesn't provide sufficient detail, then you can download Toyota repair manual info at a subscription website: techinfo.toyota.com

 

air/fuel ratio sensor -- how to backprobe to test if working

My 2004 Prius (only 20k mi) set a CEL (intermittent). OBD2 scanner said upstream O2 sensor (I think). I tried to backprobe the blue wire on the (upstream) Air Fuel Ratio (AFR) sensor, but as soon as I power up the car I get 3.4 volts (blue probe from AFR to DMM + lead, DMM - lead to car ground). Is this wrong? I thought the blue wire was signal, white was ground, and the 2 black wires were heater + and -.

 

It would help if you could be precise regarding exactly what DTC was logged.

The Toyota 2004 Prius wiring diagram shows the following color coding:

Red: AF+ (sensor connection to Engine ECU)
Green: AF- (sensor connection to Engine ECU)
Yellow: Pulse width modulation for A/F heater
Black: +12V

The heater current is supposed to be > 0.8A and < 10A or else a DTC will be logged.

After the A/F sensor has warmed up, at idle speed the sensor voltage is supposed to fluctuate from < 3V to > 3.35V. If the voltage remains greater at all times, then the air/fuel mixture is lean.

 

I have a Prius factory wiring diagram CD-ROM and it also shows different wire colors than the actual AFR sensor leads (which are 2 blacks, 1 white, 1 blue). They're probably the harness wire colors. I believe my scanner DTC was "bank one O2 sensor 1" or something similar. I'll look for it. What I don't get is everything I've read on O2 sensors says the 2 leads that match are for the heater, and the other 2 are sensor ground and signal. I read somewhere (a Toyota truck forum?) the blue is signal, the white is ground, and 2 blacks are heater + and -. By your method above, are you saying the sensor voltage is measured between AF+ (red) and AF- (green)? What does it do before it's warmed up? Should I backprobe the blue and white wires on the AFR sensor and assume they're connected to the AF+ and AF- harness wires?

 

Patrick, I believe my DTC was P0031. I found a good diagnostics tree for P0031 on Google. Besides a bad AFR sensor, it could be wiring, the relay, the ECM, etc. I will start checking, starting with the AFR sensor resistance and voltage going to it from the relay.

 

Yes, DTC P0031 means that the A/F sensor heater circuit has a problem. The heater is consuming 0.8A or less.

Either the sensor heater is bad, or the wiring harness is bad, or the EFI M relay is bad, or the ECM. The first two are the most likely.

Your plan to verify sensor resistance and measure voltage across the sensor heater terminals makes good sense. Note that the voltage is going to be a square wave, PWM, so you may need to measure AC voltage. If your DMM measures AC frequency, pls see what it picks up.

 

Questions:
1) I will measure heater resistance, with AFR sensor unplugged. That seems straightforward.
2) Measuring current through the heater seems harder. It has to be in series with the heater? How can I break into the circuit to insert my DMM? Are you saying the current is a square wave? So, I use my DMM on AC current and that works?
3) Or, should I instead measure AC voltage at the heater leads, using my DMM set to AC volts? This seems easier than measuring current, since I won't need to break into the circuit to put the DMM in series with the heater.

Thanks again! Really appreciate the guidance! Will do this tonight!

 

you also need Oxygen sensor socket, ~20$ at Pepboys. I has a slot for wire.

Also find out which one (usually it is one on manifold) YMMV.

 

2 & 3: I think it would be sufficient for you to measure voltage across the heater leads. It should be both DC and square wave AC, so use your DMM to measure both the DC and AC voltages and see what you get. Again, if your DMM measures AC frequency pls also see what the frequency is.

If you can measure current by using jumper wires between the wiring harness connector, the sensor, and the DMM, then it would be interesting to see what current you measure. However that may not be necessary if you can get good voltage and resistance measurements. Then you can use Ohm's Law to determine the likely current flow.

 

air/fuel ratio sensor socket -- does the Pep Boys one require shaving like the Autozone one?

The AZ socket is only $10 but requires shaving it down to get it into the AFR sensor.

I was wondering if the PEP BOYS socket is thin wall enough to get in there w/o shaving the outside of the socket? Do you have a p/n?

 

Patrick, I'm wrong. The DTC was P2238 (I found it in my original post in March). Any advice for what to measure first? Thanks!

 

That DTC indicates a problem with the A/F ratio sensor or circuitry. Here is a full list of possible suspects:

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Open or short in A/F sensor circuit​

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A/F sensor​

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A/F sensor heater​

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EFI M relay​

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A/F sensor heater and relay circuit​

ECM

 

Re: O2 and air/fuel ratio sensors. Harbor Freight has the 7/8-inch crowsfoot socket tool for $6.

I measured my AFR sensor (disconnected from the car) heater leads resistance. It is 2.6 ohms. Is this normal?

I will measure the voltage (AC) at the heater leads while connected to the car and report what it is.

I saw the tool for the O2 sensor for sale at Harbor Freight for $6. It looks like the Autozone one ($10). I am guessing they both need grinding to fit into the collar on the car around the AFR sensor.

 

Re: O2 and air/fuel ratio sensors. Harbor Freight has the 7/8-inch crowsfoot socket tool for $6.

I haven't had occasion to measure this and the repair manual is silent on the resistance to be expected. However 2.6 ohms seems reasonable. If a steady 14VDC voltage was applied, >5A of current would be flowing, resulting in >70W of power to the heater.

Please measure voltage using your meter's DC and AC voltage functions and see what you get for each range.

 

Re: O2 and air/fuel ratio sensors. Harbor Freight has the 7/8-inch crowsfoot socket tool for $6.

I removed the AFR sensor connector from the car harness and measured the resistance across the heater terminals (sensor side). It is 3.7ohms. I plugged the sensor back in, and backprobed the heater terminals. DC voltage is 10.25-10.44 VDC (ICE on) and 13.3 VDC (ICE off). I switched the DMM to AC voltage and measured 5.48-5.5 VAC (ICE off) and 5.27-5.23 VAC (ICE on).

Would someone tell me if I'm measuring DC or AC? Is the voltage right? What should I check next? I have a DTC P2238 (I think; it's earlier in this thread).

 

Re: O2 and air/fuel ratio sensors. Harbor Freight has the 7/8-inch crowsfoot socket tool for $6.

Standard is between 1.8 to 3.4 ohms. It needs to be replaced.

 

Re: O2 and air/fuel ratio sensors.

I do not understand why you posted two different resistance measurements; however if the latter measurement is correct then it would be reasonable to replace the A/F ratio sensor now.

I don't know what the voltage spec is supposed to be but the values you posted seem reasonable.

 

Re: O2 and air/fuel ratio sensors.

I think the first resistance value was from memory, and the second value was from the piece of paper I scribbled it on when I took the actual measurement (and burned my wrist on the exhaust manifold). The second value is reliable. Sorry about the mix up. So, you believe 3.7 ohms means a bad sensor? Would you be willing to bet $142 on it?

 

Re: O2 and air/fuel ratio sensors.

I don't mean to be sarcastic, but that's a lot of money. Also, I heard those sensors should last a very long time (10+ yrs)? What could cause mine to go bad at 20k miles (car sat parked outside for 2 yrs without running or starting it up)?