I had a bunch of maintenance work done at Chapel Hill Tire in University Mall in Chapel Hill. The following week the check engine light came on. I brought the car back in, insisted that they check what was going on for free (when they wanted to charge me), and they said after doing a computer engine analysis, codes 21 02 Sensor and 52 Knock Signal came up. The mechanics said it has to do with my fuel efficiency. The mechanic said it could be a fluke so he turned off the check engine light. Now it keeps coming on on its own, going off, coming back on. Any ideas why? Should I go to a separate mechanic? Is my car going to die if I don’t?
The first problem that you are going to run into is that the codes you are getting are OBD1 (probably) codes, not OBDII which Toyota phased in during the 1995 model year. All cars sold in the US since the 1996 model year use OBD2 – which means there are few cars on the road today that use older systems. That means that many mechanics are going to have no idea how to read or interpret codes for your car.
As far as I can tell 21 is probably an Oxygen sensor fault and 52 is indeed a knock sensor. I don’t know what 02 is. Might be a modifier for code 21 identifying which O2 sensor? Anyway, here’s a link to a list of codes for a 1993 Lexus which is about the closest I could come to a 1994 Corolla with Google in a reasonable amount of time. http://us.lexusownersclub.com/forums/index.php?s=0d7b220696d5922a134962c7eb188d01&showtopic=26394&pid=347747&st=0&#entry347747
As for what’s wrong. I haven’t a clue. Maybe somebody around here with more rapport with engines than I have can make some useful suggestions. If I had to make a guess, I’d think maybe timing, but you didn’t mention engine roughness or lack of power so very likely not timing.
If it keeps coming on, you need a older mechanic, one that knows that 1994 car better.
The codes are read by jumping the T1 and E2 connector on the diagnostic port under the hood, and reading the flashes on the ‘check engine’ light on the dash with the ignition switch in the ‘run’ position. Do not start the car.
The codes are for a fault in the circuit, not necessarily a bad sensor. Find a good Toyota mechanic in your area, and let him look at it. Both sensors are not expensive for this car if they do need to be replaced. If the Oxygen (O2) sensor was never replaced, or been past 5 years, it probably needs to be. A good Toyota mechanic will first look for proper operation of these sensors before replacing them. This repair should not cost you an arm and a leg. It should be a reasonable charge, as both sensors are easy to get to and check.
The car will not die. If the signal from either of these sensors fails, the computer will compensate by using a ‘default’ mode that will reduce your gas mileage, and increase the emissions to prevent damage from happening. Of course, in the long term, damage can still happen. Best to get it fixed.
Hello! Thank you so much for responding. That’s more or less what the mechanics said - what concerned me was that the check engine lights came on after I spent $853 at that mechanic the week before for a 100,000 mile tune up plus oil change. I wasn’t sure if I was getting ripped off cuz I’m a woman - it happens!!
Do you think I can wait until my next oil change due in May/June? I just don’t have the money to spend on it if isn’t urgent after all that I dropped last month. I don’t want to spend more than the car is actually worth 
Thank you again for taking your time to address this questions.
Jennifer
You just spent almost $900 on it just now. These repairs won’t get close to that. If the O2 sensor is bad, running it for another 2 months may cause a catalytic converter problem from running too rich. Plus, with 15 years and 100,000 miles on it, you need to keep up with stuff like this before other aged and worn components start to break. Just my 2 cents.