P0135 A/F heater fault solved on 2.4l 2003 5 spd

In this thread:
https://www.tacomaworld.com/threads/p0135-might-be-bad-ecm-2003-2rz-fe.410875/

I described the troubleshooting I did to try to find the cause of the P0135 fault.

So in this new thread I hope you will play along and see if you can determine the problems based on the evidence I presented in the other thread.

After enough people give good answers or ask good questions, I will provide additional evidence or "hints" to help guide you through to successfully finding the cause of the problem.

I will not offer further hints for shot-gun type guesses - please describe your reasoning so we can all become better at fixing our Tacomas.

From the previous thread:
1) No codes stored and the CEL not lit.
2) Adjusted valves - this involved removing the intake and valve cover.
3) Four valves were adjusted with new shims. Also cleaned the MAF, throttle body, and IAC.
4) Put everything together and started the engine.
5) The engine now runs well but the CEL is lit and P0135 is stored (A/F sensor heater circuit fault).
6) Measured the A/f sensor heater resistor - it is fine.
7) Load tested EFI relay - also fine.
8) Measured voltage on AFHT C4 - +14V so no current flowing through heated to ground.
9) Measured resistance between AFHT C4 and B+ on A16 which is connected to the EFI relay output as is the A/F heater + wire.
10) All the while my scan gauge is plugged in to the ODB2 port so I can check and reset codes.
11) Sometimes I also get P0036 code which indicates that the O2 sensor heater is also broken.


ENJOY!!!!

EJPHI

 

I had the same code - P0135 occur about 4 yrs. ago on a 2004 4 cyl Tacoma. I swapped EFI relay with another that had the same part number - cleared code and it popped up again.

The 3 most likely causes according to my '04 FSM were A/F sensor - EFI relay - ECM. EFI was eliminated. ECM? I doubt it. Bought new OEM A/F sensor from an online seller for $84 less than local dealer wanted. Replaced - cleared code.

Problem solved.

The vast majority of the time a P0135 is going to be a faulty A/F sensor.

 

Hi Mr. Lugnut,
Thanks for playing along. The 2003 FSM has a very nice troubleshooting chart for this problem:
1) Check A/F sensor heater resistance -- it was withing spec.
2) Check EFI main relay -- Load tested perfect.
3) Check voltage on ECM connector AFHT pin (they show the wrong pin on the diagram) with respect to body ground --
it was 14V. Furthermore, the connection between the AFHT pin, the A/F sensor heater and the EFI relay all showed low resistance so it is AOK.
4) At this point, the FSM directs you to check and replace the ECM and refers to IN-28 where you verify the wiring to the ECM. Specifically you check for broken wires and poor contacts. The resistance between the ECM ground pins and the frame ground near the ECM was very low. The resistance between the ECM B+ and the EFI relay is also very low. So The ECM has good power connections. If no other sensor input can trip P0135 then what else could it be?

I will repeat some hints:
Through all of this troubleshooting, my scan gauge was plugged in so I could monitor the fault codes and clear them.
I just completed adjusting the valves, cleaning the MAF, throttle body, and IAC valve.

 

From the free online 2003 Toyota Tacoma FSM:

 

Yup,

That is exactly the procedure I followed. Toyota does a good job writing these troubleshooting guides.

 

Trust the troubleshooting guide, before just replacing parts.

 

I salute your desire to trouble-shoot vs. throw parts at the problem.

Good luck and let us know how things work out.

 

Hey Lester,

The problem was solved although I was not as disciplined as I should have been.

As I mentioned, I just completed a repair that was unrelated to the P0135 code.

I should also mention that the FSM troubleshooting guide was spot on.

So if we can rule out the A/F sensor based on the FSM troubleshooting guide, what would you do next to find the problem given the hints I provided?

 

You have 2 threads going on related to this. If I can find the time to decipher each a little more than I have, I'll post a response. I'll also need to review the section of the FSM in detail that I put up.

I'm wondering how you can say you were not as disciplined as you should have been and then follow that with "the FSM trouble-shooting guide was spot on". Maybe to gained some discipline at a later time?

Later.

 

Sorry about the two threads. I will not reply to the other thread anymore.

The FSM was correct and as you say my discipline came late in the game just before I determined the problem and fixed it.

The whole point of drawing this out is so that other people will not throw parts at a problem but rather use the FSM and their heads to guide efficient troubleshooting. Thanks for trying to find the real problem. Once I get my head around what was going on, it did not take long to fix.

Another hint: The A/F sensor is AOK!!

 

Hey there, I know this is an old thread but I was wondering what you came up with on this One. I am stuck on a p0135 on my 03 2.4 went through everything you did on this thread not sure where to go next, don't like throwing parts at a problem, but I have hit a wall. Thanks.

 

I bought my 5spd 2.4 taco a few weeks back with this code. Haven't dug into it yet, but stumbled on this thread while looking for something else....so consider me subbed.

 

This is a good forum and I am glad to see people fixing these great trucks.

So, I had my scan gauge plugged into the ODB2 port and could start and run the truck with the CEL and fault code.

I just completed adjusting the valves and had the top of the engine compartment taken apart.

When I unplugged the Scan Gauge, I could not start the truck and got a weird beeping noise from the dash board.

As 98tacoma27 pointed out, there was a ground problem.

I forgot to reconnect the main ECM ground to the passenger side of the cylinder head and the only ground path for the ECM was through the Scan Gauge ODB2 connector.

Scan Gauge ties the signal and power grounds together on this connector so the ECM was making ground connection through the Scan Gauge!!

This ground connection is OK for engine operation except that the AF ratio sensor heater needs a 8A to operate correctly. If the ground does not have low resistance, the fault code is set because the ECM sees that the current is not high enough to ensure that the heater circuit is operating correctly.

So check the quality of the ground connect from the ECM to the passenger side of the cylinder head. There is a good reason for Toyota to put 5 wires in parallel between this cylinder head ground lug and the ECM. Without low resistance, you get the fault code and are tempted to buy new AF ratio sensors and new ECMs.

Stop and think before throwing parts at the problem.

EJPHI

 

If any of you guys are still out there, I just want to extend a huge thank you for this thread!

I recently replaced the valve cover gaskets (and a bunch of other stuff) on my '01 TRD. A week or so later the CEL came on while I drove to work. Truck was running fine, but I scanned it when I got home and it threw a P1135.

I dove into everything I could find about it, and went ahead and ordered a new set of Denso O2 sensors (the truck has 263k on it, and I got it with 241k, so I figured it wouldn't hurt to replace them). Long story short: the new sensors didn't do a thing, and just this evening I was on the verge of condemning my ECM after spending today ripping the dash apart and finding no issues with voltage drops in any of the connectors between the EFI relay, the ECM, or the sensor itself. The new sensors had passed their resistance checks, and the ECM passed its voltage checks, but I couldn't think of anything else that could be at fault besides the ECM. As a last chance I came here and found this post and, of course, I discovered that I hadn't tightened the ground screw on the drivers side of the engine, just beneath the OBD1 port, when I closed up my valve cover job.

Problem solved, and hundreds of dollars saved. Thank you so much! I hope everyone realizes what a resource a forum like this can be, given a little patience.