Scotty
A 2001 Toyota Echo showing P0141 bad o2 sensor heating element (and now bad evap canister code too — which is new). Was showing P0135 bad sensor heater for front sensor and P0141 bad sensor heater for rear sensor and P0121 coolant temp issue. So I replaced all these with Denso parts BUT the P0141 code remained. So I figured a defective rear sensor, and replaced again, but same thing, a P0141 code. So I figure a bad fuse or relay? Read on the forum where you told a guy with similar problem “pray the computer is okay.” I cannot find a fuse for this car that goes to the sensor heating circuit — maybe you know of the fuse and/or relay I should check? (Odd that the front o2 sensor heating element issue cleared up and yet the rear sensor heating element issue continues — on different circuits/fuses/relays?)
I'm pretty sure they share power from the EFI relay so that's probably not the problem.
The P0141 code is saying that the rear HO2 sensor is taking too long to heat up. It's a performance code for the HO2 sensor's heater performance.
The computer "performs" performance tests during specific engine/driving conditions.
Everything from testing the time it takes for HO2 sensors to heat up to playing around with your EGR valve (under certain driving loads and rpms) to test if it has control to completely Close it (0%) and completely Open it (100%).
In some cases the parameters are programed into the PCM.
In other cases, such as the EGR performance test, it records voltages in it's KAM (Keep Alive Memory) and compares those previously stored voltages with the voltages it receives during the performance test.
Anyway, it's not saying that your downstream O2 sensor isn't working. The PCM monitors how long it takes the sensor to heat up and start sending an adequate signal. Your HO2S2 is taking too long to heat up to around 700 degrees.
So understand, the HO2 sensor will eventually heat up from the hot exhaust and start functioning correctly if the sensor isn't faulty.
But because of stringent emission control regulations, they want the engine going into Closed Loop as quickly as possible and heating the O2 sensors facilitates this.
So with that new OEM HO2 sensor, why isn't it heating up within the specified time period and throwing that P0141 performance code?
If you wanted, you could graph your HO2S2 (Live Data) voltages with a cheap scanner and likely confirm that when the engine has been running for a little while, that downstream HO2 sensor is working fine.
On yours I think the rear HO2 sensor connector is under the seat or under the center console inside the car.
You're using OEM and you've already entertained the possibility of a faulty new HO2S2 by replacing it again.
It's time to check the power and ground at that HO2 sensor's connector because there's no reason that OEM HO2 sensor isn't heating up within spec if the heater is getting Battery Voltage and Ground.
So you should have a 4 wire connector on that Echo.
On those older 1.5's I think the 2 white wires are for the HO2 sensor's heater. On the 1.8's I think they were the 2 black on some and the Pink and Black on others so go to one of the Echo forums on the internet and confirm the wire colors
One of them needs to have battery voltage (around 12 volts) Key ON.
The other white wire is the Ground being provided by the PCM to switch the HO2 sensor's heater on/off.
The way it should work is when you turn the key to ON battery voltage is provided to one of the white wires at all times.
And like we've discussed, the PCM, using voltage feedback from the HO2 sensor, provides Ground on the other White wire to turn the heater on until it heats up enough and starts getting an adequate voltage signal from the sensor. Then the PCM switches off the Ground to the heater and the hot exhaust keeps the sensor at operating temperature.
But here's the thing. You want to test that battery voltage on the HO2 sensor side of the connector (by backpinning) to check for voltage as well as voltage drop and the easiest way to do that is while the computer is providing Ground to the HO2 sensor and the heater is operating and the circuit is under load.
Voltage drop can be caused by corrosion, a short, or a loose connection. You want to measure the voltage being received by that new HO2 sensor's heater on its side of the connector while under load.
Here again, it may be handy to have a scanner (Live Data) to confirm you're in Open Loop. Once it goes into Closed Loop it won't be long for the computer to shutdown that Ground and turn off the heater (and the load on the circuit)
Whenever dealing with a computer provided Ground or even Computer provided Power, I like to use an LED test light ($15). You want to take an abundance of caution not to "overload" the computer ground and an LED test light won't do that.
You're trying to determine which of those two HO2 sensor heater wires are which.
Measure for power on one of those White wires first. You'll do this by attaching the alligator clip of the LED test light to a Ground and probing the Back Pin (a pin from your wife's sewing basket) which you stabbed into the back of one of the White wires on the HO2 connector (Key ON).
Even if you guess wrong, putting Ground to the Computer Ground won't hurt anything because, They're Both Chassis Grounds. If the LED doesn't light, just stab the other White wire at the back of the connector with the pin and check for power there.
Once you figure out which heater wire is which, hook up your multimeter (DC 20 volts range) to confirm the heater is receiving Battery Voltage while under load. Use the computer provided Ground as the contact for the multimeter's Negative Lead. Just be sure your multimeter is set to DC volts. ... check that twice. You can't hurt your PCM in DC Volts.
