1999 Camry P1135 A/F Sensor Heater

[jmborchers]

Use dealers guide to service. Intermittently 1135 code gets thrown.

Checked HTAF - 12V with ignition on. - OK

Checked sensor @ 65F = 1.4 ohms - OK

Car has 93K. So I replaced california spec A/F sensor on exh manifold - still have code thrown.

Replaced ECU - still have code thrown.

Sometimes when car is cold 66F coolant / 65F Air intake sensor light will come on intial morning startup, but not all the time. When it's warmer out it doestn' seem to do it. We haven't had every morning cold yet here in PA. Lately it's been in 50-60 range but sometimes code doesn't get thrown.

What the heck is wrong? Unloaded $1K to fix this problem and it's still not fixed.

Wiring all rings out okay.

[new echo owner]

Where did you check the heater resistance at? At the ECM? Have you check the ground connection?

N.E.O.

[jmborchers]

Between top two connectors with the sensor disconnected. both the old and new sensor measured the same at approx 65F.

Ground connection from the ECU was < 1 ohm.

[new echo owner]

Trying checking the resistance at the ECM, if there is higher resistance at the ECM, it would cause changes in current draw in the circuit, which is how the ECM determine if the circuit is functioning properly or not. Also check the ground circuit for the sensor heater circuit.

N.E.O.

[jmborchers]

This gets indirectly measured by measuring HTAF voltage. If the resistance of the wires were high the battery voltage would not be at the HTAF junction on the ECM @ 12VDC.

The ground circuit for the sensor heater circuit is in the ECM.

[new echo owner]

Did you check all the grounds for the ECM?

If the ground circuit wiring for the heater is ground to the body, it will still give you the good voltage reading, but the ECM may not. The voltage maybe correct, but the change of resistance will also change the current in the circuit.

BTW, have you check the pin tensions of the terminals?

N.E.O.

[jmborchers]

Yes, I did check the ground pin for the ECM for the heater part of the circuit. It was 0.2 ohms to the chassis.

I checked that none of the sensor wires was shorted to ground from the heater circuit.

The problem only seems to appear when it's cold outside. On most car starts HTAF goes from 12V to 3V on startup for a few seconds after sitting overnight and then pulses between 9-14 V. If the car is warm HTAF will never drop below 9. But sometimes if I start the car HTAF doesn't move. I don't know yet whether that was because the code was thrown or under or over current existed and the computer shut the circuit down and went into safe mode. It's been warm recently so I have not been able to duplicate.

What I have proven:

A) The ECU is grounded otherwise HTAF voltage would not change.
B) The pin tension is good because the voltage is being sunk to ground by the ECU drive circuit.
C) Because 12-14V exists on ECU pin it's not possible that the sensor is shorted to ground anywhere along until that point and the circuit is good through the ignition and ignition relay because the voltage exists.

This problem started suddenly. No other change/repair was made on the car when the problem started.

I'm wondering what sensor the computer uses to determine the duty cycle of the A/F heater circuit.

[new echo owner]

I meant All the grounds for the ECM, not just the A/F sensor heater circuit.

And checking the pin tension, I meant using a pin tension gauge, not just base on the reading of the meter.

BTW, are you working on a four or six? And have you check the current draw of the circuit?

N.E.O.