June 23, 20205 yr Will a bad thermostat throw a code 21? Check the ohms on the sensor, I get 2800 cold and 550 hot. Unplugging the sensor spikes the idle up, so the ECU is reading it. Didn't know if it would throw the code if it didn't get up to temp.
June 23, 20205 yr I haven't seen a code 21.... I've had a bad CTS sensor that was intermittent and caused the idle to be all over the place. You want to watch the ohms while it cools down, it should change smoothly. A loose or corroded connection could be messing with the ECU also. Note, since I have seen some confused by this - the CTS is the 2 wire sensor on the thermostat housing, that the ECU uses to know the engine temp. It has nothing to do with the 1 wire sensor for the gauge, if you have one.
June 23, 20205 yr 1 hour ago, Ionstorm66 said: Will a bad thermostat throw a code 21? Check the ohms on the sensor, I get 2800 cold and 550 hot. Unplugging the sensor spikes the idle up, so the ECU is reading it. Didn't know if it would throw the code if it didn't get up to temp. no. unplug the engine connector and check resistance from the engine connector to the connector terminals of the CTS. probably need to replace the sensor or fix the wiring. corrosion can creep back up the wiring underneath the insulation unseen. peel back the insulation and see what the wiring looks like. repair if needed, cover back up when done.
June 23, 20205 yr Code 21 (on my '87) is for the coolant temperature sensor, passenger's side, behind the intake manifold. Either the sensor is going bad or the harness has corroded, probably some of both, based on my experience. You can buy replacement electrical connectors, it's the same style plug used for the fuel injectors on a MPFI EA82 engine.
June 24, 20205 yr Author I am checking the sensor. I'm just not getting quite to the 300 ohm spec, but the temperature is also not reaching 193. Didn't know if the computer thinks it's supposed to reach 193, but the sensor never reaches it, it would throw code.
June 24, 20205 yr They are not precision, so it can be off by a bit. The ECU is not super high tech, so I doubt it would set a code for something like that. I know it won't for a 180 degree thermostat. The OEM ones are around 190.
June 24, 20205 yr The ECU temp sensor doesn't trip a CEL based on temperature, all it's doing is telling the ECU that the engine is up to temp. If you're getting a Code 21, it's because: A) the sensor has failed, or, the harness is damaged/corroded.
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