P0CAA: Hybrid/EV Battery Temperature Sensor M Circuit Low

What does P0CAA mean?

P0CAA means the voltage at sensor M has dropped below its valid band. Sensor M is an NTC element supplied via a bias resistor on the board, so a grounded lead or a shorted element drags the voltage down and reads as an extremely hot spot. GM's 2018 HPC2 table 18OBDG02 fails the test once the converted temperature exceeds 160 °C in 8 of 10 readings taken every 100 ms, Type B, with more than 10.2 V supply. The 2011 Volt table set it above 87.5 °C, a raw count below 680, held for 1.4 s within 2 s over two trips. Mitsubishi's Outlander PHEV calls it a short of sensor [12]: below 0.12 V, roughly above 125 °C, for 2 s; the remedy there is a new battery module or cell monitoring unit. A real overheating shows as P0A7E instead.

The code tells you which monitor failed, not which part is broken. Test before replacing parts.

Symptoms

• Warning lamp with strongly reduced battery power
• Battery cooling running without reason

Common causes

• High Sensor M signal shorted to ground inside the pack
• Medium Thermistor M internally shorted
• Low Measuring input of the cell monitoring unit defective

Likelihood is a general estimate across vehicles; your make and model may differ.