CEL periodically turns on and off

The CEL on my 3.0l 93 4runner intermittently turns itself off and on. I have had it to a shop and got error code 25 and error code 26. (If the numbers are wrong the codes meant lean/rich mix. The first time this happened the shop replaced the O2 sensor. When the light came back on the shop said the problem was the “mass air flow (something)”. The shop ordered a new MAF and after testing the new one and the old one decided the MAF was not the problem. At this point I’m just ignoring the light and driving the vehicle. The vehicle does seem to run just fine.

Take your 4 Runner to Autozone or Advace and get the codes read for free and report back. This is the format of the codes.

P0025 “B” Camshaft Position - Timing Over-Retarded (Bank 2)
P0026 Intake Valve Control Solenoid Circuit Range/Performance Bank 1

Read more at: http://www.obd-codes.com/trouble_codes/

The error code 25 is for a lean mix condition, and a code 26 is for a rich mix condition. Changing the O2 sensor was a good idea. But, the diagnosis only gets expensive from there.With both of these codes turning on and off, the ECU is hunting for the right fuel mix for some reason.With the MAF sensor testing OK, my old Toyota manual suggests testing the fuel injectors, fuel line pressure, and the ignition system, including the coils, wires, and spark plugs. Don’t forget cap and rotor if this engine still has a distributor.


OP"s car is OBD1. It doesn’t have a 16pin DLC

Does Autozone read codes for free on older OBD1 vehicles?

I’m only bringing this up because those cheapo code readers (the ones which I would expect Autozone to have behind the counter) often don’t do OBD1

Sorry I did not look at the year. I don’t think the parts stores read OBD1.

Did anyone clean the MAF? You can clean it with a high purity alcohol or a MAF cleaner. The MAF may test good, but if the sensor part is dirty, it wont give the correct output.

I am more inclined to think you have a vacuum leak somewhere, possibly around the intake manifold gasket. That is causing the A/F mixture to be different from cylinder to cylinder, but a defective injector could also cause that.

Toyota’s of that vintage have a two pin connector under the hood somewhere that is used to check the O2 sensor. I believe it has a green cap on it. You can test the new O2 sensor at this connector with an analog voltmeter. Maybe the new sensor isn’t working or there is an issue with its wiring.

If both of these codes are present, the engine is at times running too rich and at times too lean. First things I would test are for vacuum leaks, then look at engine sensors. Your car technically does not have a Mass Air Flow sensor, rather it’s an air flow meter that is not subject to dirt contamination like a sensor. These sensors do fail but it’s going to take an experienced mechanic to find and repair it.

Have your mechanic check the intake boot between the air filter assy and the engine for cracks.

My early 90’s Corolla has those same two codes as I recall. In the manual I mean. Never displayed for me – knock on wood – yet. Unlike newer OBD II vehicles, there’s fewer gadgets involved on a 93 and only about 6-8 codes which makes things a tad easier to fix probably. So that’s a good thing.

Me, I wouldn’t have replaced the O2 sensor unless the diagnostic test outlined in the shop manual showed it wasn’t working. The reason is that this just introduces another variable into the mix. You now don’t know if the new one is working or not. But let’s assume it is. And it probably is.

All good comments above. I expect this will prove a bit of a puzzler. But it can be solved. If this were my car, first thing I’d do, after making sure all the routine maintenance is up to date, the engine timing is set, compression is ok, etc, I’d inspect all the rubber intake boots and vacuum lines very carefully. Clamps tight? Everything is there that should be right? Factory condition? Nothing added or deleted? There may be weird looking components that come off at right angles to the intake boots that look like they don’t do anything, but they are there to condition the air flow profile , necessary to correctly meter the fuel/air mixture. Remove all this stuff, one by one, inspect for any cracking in the walls. Do this systematically, one line at a time, use the diagram under the hood for reference.

Nothing of note? Connect everything back up, then one by one clamp off each vacuum hose connected either to the intake manifold or to the throttle body. I use one of those locking gadget-clamps that doctors use to clamp off arteries during surgery. When a tube is clamped, does it turn the CEL light off? Or any change in idle speed or quality? Could provide a clue.

Next, connect a hand held vacuum pump to the egr valve vacuum port. Putting vacuum on this during idle should stall the engine. Does it?

Next I’d use the same vacuum pump to see if any of the vacuum operated devices are leaking vacuum. Brake booster, egr, power steering VSD, etc.

Still nothing? Use a vacuum guage, make sure the intake manifold vacuum is around 20 at idle.

Still stumped? Ok, now it is time to check

  • Fuel pressure at rail.

  • Exhaust manifold for leaks .

  • Emissions canister and venting valves for proper operation.

  • Coolant temp sensor (the one used by the ecm).

  • Air intake temp sensor.

  • Cold start injector and thermal timer (if equipped).


Check cat for higher than normal backpressure.

If the vehicle runs fine and no emissions test is involved, drive on…Keep your wallet in your back pocket…

I’d also look for vacuum leaks after the MAF sensor. If the codes are reading sometimes rich, sometimes lean, what may be happening is un-metered air is leaking in, causing the O2 sensor to see a lean condition. The system tries to correct by dumping fuel, now it’s too rich. Lather, rinse, repeat. Air that isn’t being accounted for in a MAF sensor-controlled system will do this. An exhaust leak near the O2 sensor could possibly do this as well.

Thanks to everyone that contributed. At this point I will take Caddyman’s advise and keep my wallet in my pocket and wait for the problem to manifest itself in some way other than the CEL.